Reading Your Kidney Test Results — A Sieve You Cannot See, What Stays Behind and What Leaks Through

The Kidney Section of Your Report

When you pick up a test report after a health checkup or a clinic visit, a long line of blood test items follows. Among them, a couple of lines of numbers with English abbreviations are the kidney section. In the urine section, marks like "Negative," "±," or "1+" are written instead of numbers. Next to them, arrows or asterisks may be attached, or there may be no mark at all.

Kidneys are bean-shaped organs, one on each side of the lower back, that filter the blood and send waste products and excess water out as urine. They are also called renal organs in medical terms. The kidney section of the report contains values estimated from blood and urine regarding how well this filtering work is being done.

This article traces how to read those numbers. It covers what each number measures, what makes them fluctuate, and what they fail to tell you. It is written based on international guidelines and research from the United States and Europe. Even after reading it all, you will not be able to diagnose your own report by yourself. Instead, you will come to understand why a doctor does not draw a conclusion from just that single number, and why they ask to look at more things.

The Invisible Sieve

Inside the kidneys are many tiny units that filter blood. A nephron is the smallest functional unit of the kidney that filters blood to make urine, consisting of a filtering part and a narrow tubule through which the filtered water passes. That filtering part is the glomerulus. The glomerulus is a tangle of fine blood vessels at the front of the nephron that filters water and small substances out of the blood.

When describing how hard the kidneys are working with a single number, this filtered amount is used. The glomerular filtration rate is the volume of water from the blood that the kidney's glomeruli filter out in one minute, serving as the representative number indicating how well the kidneys are working. It is considered the best indicator for gauging overall kidney function (PMID 27887750). A representative setting where this number is used is chronic kidney disease. Chronic kidney disease is a condition in which kidney damage or decreased kidney function lasts for three months or longer. The glomerular filtration rate is used to diagnose this disease, stage it, and determine medication dosages (PMID 34518032).

However, this amount is difficult to measure directly. The measured glomerular filtration rate is a value directly measured by drawing blood at set intervals to see the speed at which a marker substance introduced into the body passes out through the kidneys. A marker substance must be injected into a blood vessel and blood must be drawn multiple times, which incurs costs and involves strict procedures, so it is performed only in specialized institutions (PMID 40332971). Therefore, in the clinic, this amount is usually estimated from the concentration of substances in the blood (PMID 34518032).

Comparing this to a sieve, it looks like this. The kidney's sieve is inside the body and cannot be looked at. So, we guess at it using two clues. One is how much of what is supposed to pass through the sieve is left on top of the sieve, and the other is how much of what is supposed to be caught by the sieve has leaked below it. The former is looked at by a blood test, and the latter by a urine test. This analogy has limits. The actual kidneys are made up of numerous nephrons, and they also perform the task of taking back the necessary substances from among what was filtered into the blood. That work is followed in "How Do the Kidneys Filter Blood? — A Sieve That Takes Almost Everything Back".

Because the amount filtered by the kidneys is difficult to measure directly, the numbers on the report are values estimated from what remains on top of the sieve and what leaks below the sieve.

What Is Left on the Sieve: Creatinine

The first line that usually appears in the kidney section of a blood test is creatinine. Creatinine is a waste product made mainly by muscles, and because the kidneys filter it out, its concentration in the blood is used to gauge kidney function. Because it is fast, simple, and inexpensive, it is the most widely used measure for estimating kidney function (PMID 39858438). If the sieve fails to filter properly, the creatinine left on the sieve increases, raising its concentration in the blood.

The problem is that how much is left on the sieve is not determined by the state of the sieve alone. How much was poured in to begin with also plays a role. Most creatinine is produced by muscles, and that amount varies depending on age, physical activity level, high-protein diets, sex, medications taken, and race (PMID 39858438). It is heavily influenced by age, sex, muscle mass, body composition, and heavy chronic diseases, which can place a person in the wrong category or lead to incorrect judgments (PMID 34345408). This value can also be inaccurate in people with excessive fluid buildup in their bodies (PMID 25195004).

A range is usually written next to creatinine on a report. The reference range is the middle 95% of values gathered from healthy people, with the top and bottom 2.5% trimmed away. Older adults or people with little muscle mass may have creatinine stay within the reference range even though their kidney function has already declined, which is why early stages where intervention is still possible are frequently underestimated (PMID 29322215). There is also the opposite direction. An educational review covering special populations notes that in bodybuilders with a great deal of muscle, kidney function estimated by creatinine can appear lower than it actually is (PMID 29218435).

Because creatinine depends on both the amount filtered by the sieve and the amount produced by muscles, simply being within the reference range does not mean the kidneys are fine.

The Value Estimated by a Formula: eGFR

Right below the line for creatinine, there is often eGFR. eGFR is an estimated value of the glomerular filtration rate, calculated by plugging information such as age and sex into the concentration of creatinine in the blood. Because creatinine is affected by muscle, the formula includes information to stand in for muscle mass. Past formulas used things like age, sex, race, height, and weight for this (PMID 32393465).

The unit next to eGFR is often written as mL/min/1.73㎡. It is a value adjusted to a standard body surface area of 1.73 square meters, representing how many milliliters are filtered in a minute.

International guidelines draw the line for declining kidney function at 60 using this number. Chronic kidney disease is defined as a state where the glomerular filtration rate is below 60, there are signs of kidney damage, or both, lasting for at least three months (PMID 27887750).

For an individual, eGFR is a rough estimate. The European laboratory medicine community holds the view that values estimated from creatinine are not precise at the individual level for people whose muscle mass is unusual (PMID 37950562), that any formula is merely an estimate, and that it is especially rough for any single person (PMID 36279207).

The formulas themselves have also changed over time. Around 2009, it was concluded that the newly introduced CKD-EPI formula matched better and predicted risk better than previous formulas (PMID 20393287). This formula included an item asking whether the person was Black or not. This was because, in the study that created the formula, Black people had a directly measured glomerular filtration rate that was 16% higher on average even when age, sex, and creatinine were the same. A 2020 review noted that the reasons for this were only partly understood, and urged caution, stating that simply removing this item would systematically underestimate the function of Black people. The same review noted that there are limits to using race in formulas as well, and concluded that if there is a better alternative with solid evidence, it is better not to specify race, and that finding a more accurate method without race is the goal of research (PMID 32393465).

In the United States, this item was ultimately removed. A committee formed jointly by the National Kidney Foundation (NKF) and the American Society of Nephrology (ASN) recommended that a formula re-adjusted without the race item be used immediately in all laboratories in the United States (PMID 34563581). A 2024 review notes that the new 2021 formula eliminated the problem of systematically overestimating function in Black patients, and that individual errors are far greater than the benefits gained from the race item (PMID 38366867). Opposing views remain. Another 2023 review estimates that instead of placing 434,000 Black individuals newly into treatment targets and moving 584,000 into a heavier category, this formula would remove 5.51 million non-Black adults from the chronic kidney disease category and move 4.59 million into a lighter category, and notes that keeping separate formulas for each race is the least biased approach for both sides (PMID 37701164).

Europe chose a different path. In 2023, the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) recommended that European laboratories not use the 2021 formula, but instead continue using the 2009 formula without race correction (PMID 36279207). There is a summary noting that the 2021 formula did not fit European white people as well as the 2009 formula, and that both formulas overestimated the function of adults aged 18 to 30. The same summary notes that even in the U.S. population, the 2021 formula was no better than the 2009 formula, overestimating values for non-Black individuals and underestimating them by the same magnitude for Black individuals (PMID 37664574).

This means that even if the same blood is drawn, the eGFR can come out differently depending on which formula the laboratory uses. The eGFR on your report sheet is an estimated value produced by the formula chosen by that laboratory.

eGFR is an estimated value calculated by plugging information like age and sex into creatinine, the formula has changed across eras and regions, and it is a rough estimate for any single person.

Another Marker Less Tied to Muscle: Cystatin C

Some people also have a line for cystatin C on their report. Cystatin C is a protein freely filtered by the glomeruli, serving as another marker—much like creatinine—to gauge kidney function through its concentration in the blood. Comparing it to a sieve, the material poured onto the sieve is less tied to muscle. It is barely affected by muscle mass (PMID 37509111), dodging factors like diet and muscle mass that disturb creatinine (PMID 26066476).

That is why cystatin C is used for confirmation. In its 2012 guidelines, Kidney Disease: Improving Global Outcomes (KDIGO) recommended using cystatin C to confirm a diagnosis of chronic kidney disease when an eGFR estimated from creatinine suggested it (PMID 36514729). A U.S. committee also recommended more frequent use of cystatin C to confirm eGFR in adults who have kidney disease or are at risk for it. This is because using creatinine and cystatin C together is more accurate than using either one alone (PMID 34563581).

Cystatin C has limits of its own. It is affected by inflammation or changes in the rate at which the body breaks down protein (PMID 29218435). Cystatin C equations from around 2010 were not accurate across all populations, including people with reduced muscle mass or chronic illness who were expected to benefit more from cystatin C (PMID 20393287). Evidence also indicates that equations using only cystatin C are no better than those using only creatinine, and that improvement comes only when both are included together (PMID 37950562). It is also not yet a test readily available in every laboratory (PMID 37509111).

Not a few people show a wide divergence between the two values. Among some 820,000 outpatients who had both markers measured on the same day, 11% had an estimate based on cystatin C that was more than 30% lower than the estimate based on creatinine, and among hospitalized patients, the figure was 35%. When outpatients were followed for an average of 11 years, those with this wide divergence—compared to those whose difference between the two values was under 30%—experienced more all-cause deaths, at 28.4 versus 16.8 per 1,000 person-years, along with more cardiovascular events and kidney failure (PMID 41202182). Because this comes from an observational study, it does not mean the divergence itself causes those outcomes. Why the two values diverge remains a question that needs further clarification (PMID 37950562).

Cystatin C is less influenced by muscle and is used to confirm creatinine-based diagnoses, but it has its own limitations, and a large divergence between the two values is itself a signal to look closer.

What Leaked Through the Sieve: Albumin in Urine

Now for the other side of the sieve: urine. Blood also contains large substances that the kidneys should not filter out but instead keep hold of. Albumin is a protein made by the liver and sent out into the blood, making up the largest share of the proteins in the blood of a healthy person. If a large amount of this ends up in the urine, it is a sign that something that ought to have been caught by the sieve has leaked through. A small amount of albumin also appears in the urine of healthy people (PMID 37787795).

Albuminuria is when albumin—a protein that should not be filtered out—leaks into the urine in amounts greater than normal. Proteinuria is when protein mixes into the urine in amounts greater than total normal levels. In most kidney diseases where protein leaks, the main component of the protein in the urine is albumin (PMID 21782130).

The urine section of a lab report usually comes from a urinalysis. A urinalysis is a test where urine is collected and examined for various components using a reagent-coated dipstick and a microscope. Because it places no burden on the body, is simple, and is inexpensive, it is well suited for screening, but the dipstick test is a method that reads amounts roughly by how much the color changes, so it must be read with its limitations in mind (PMID 29322215). Markings like "Negative", "±", or "1+" represent these rough categories. A typical dipstick test catches only albumin and does not catch non-albumin proteins very well (PMID 23773939).

There are also things that the dipstick test misses. In an analysis pooling roughly 910,000 adults who had both a precise urine albumin test and a protein ratio or dipstick test measured on the same day (though sometimes not on the exact same urine sample), among people leaking albumin above the threshold (30mg/g), 62% were caught as "±" or worse by the dipstick test. Among people who did not exceed the threshold, 88% were correctly read as "Negative" by the dipstick test (PMID 32658569). This means that nearly four out of ten people whose albumin leakage exceeded the threshold came back as "Negative" on the dipstick test. How to read these numbers is covered in "What a Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability".

A more precise method of measurement is a ratio. The urine albumin-to-creatinine ratio is the value obtained by dividing the amount of albumin in a single collected urine sample by the amount of creatinine in that same urine sample. Because urine becomes diluted or concentrated depending on how much water you have drunk, dividing by the creatinine in the same urine sample irons out that fluctuation. The 2003 guidelines from the National Kidney Foundation recommended that adults at high risk use a spot urine sample to measure this ratio and find signs of kidney damage (PMID 12859163), and in routine clinical practice, the ratio from a single collected urine sample is used more often than urine collected over 24 hours (PMID 21782130). The unit is commonly mg/g, and some places write it as mg/mmol. 10mg/g is roughly 1.1mg/mmol (PMID 20483451). When only a dipstick test or protein ratio is available, you can also use a method that converts those values to estimate the albumin ratio, but the preferred measurement is the albumin ratio (PMID 32658569).

The framework of international guidelines for staging chronic kidney disease divides this ratio at 30 and 300. There are three categories: under 30, from 30 to 299, and 300 or higher (PMID 32658569).

The dipstick test also includes a block for occult blood. Occult blood is the section on the dipstick test that reacts to whether blood components are present in the urine. Checking repeatedly through dipstick tests and microscopic examination to see whether blood components and protein are caught becomes a sensitive clue for finding kidney diseases and whole-body diseases. However, because the dipstick test is a method that reads amounts roughly, it must be read while taking the limitations of the test into account (PMID 25804777). What a one-time positive marking means is sorted out by doctors by retesting and combining it with other tests.

Albumin in the urine is a sign that something that should have been caught by the sieve has leaked, the dipstick test shows only rough categories, and more precise measurement is done with the urine albumin-to-creatinine ratio.

Reading on Two Axes

Now we place two traces side by side. The definition of chronic kidney disease includes both "damaged" and "declined in function". The declined-in-function side is shown by the glomerular filtration rate, and the representative trace of the damaged side is albumin in the urine. International guidelines define the disease if either an estimated glomerular filtration rate below 60 or signs of kidney damage persist for three months or longer, regardless of the cause (PMID 27887750). Sorting out chronic kidney disease begins with looking at eGFR and albuminuria at the same time (PMID 37016948).

Even if an eGFR is above 60, if albumin is leaking, that person can fall into the damaged kidney category. Conversely, even if albumin is not leaking, if an eGFR stays below the guideline line for three months or longer, they can fall into the declined-function category.

The two axes each tell of risk separately. In an analysis of some 100,000 members of the general population, compared to a person with an eGFR of 95, the risk of death from all causes was 1.18 times at an eGFR of 60, 1.57 times at 45, and 3.14 times at 15. The urine albumin-to-creatinine ratio showed greater risk the higher it was without a threshold, and the two values independently increased risk. The authors concluded that an eGFR below 60 and this ratio of 10 mg/g or higher each separately predict mortality risk (PMID 20483451). In an even larger analysis pooling data from 114 cohorts, a low eGFR and high urine albumin were each associated with ten adverse outcomes, including kidney failure, cardiovascular disease, and hospitalization, even in the mildest categories (PMID 37787795). People with diabetes had a higher baseline risk, but the degree to which risk increased according to eGFR and the albumin ratio was nearly identical whether diabetes was present or not (PMID 23013602). These are associations from observational studies.

However, reports indicate that the rate at which the urine albumin ratio is measured regularly in actual clinical practice is very low. As a result, many people at high risk in the early stages remain hidden, missing the chance to receive treatments that reduce albuminuria and improve kidney and heart outcomes (PMID 37016948). Albuminuria is both a marker that forewarns of risk and is now treated as a target that can be reduced with therapy (PMID 37016948).

The kidneys are read through two axes—eGFR on the blood side and albumin on the urine side—and the two each tell of risk separately.

Why It Isn't Decided by a Single Number

The definition of chronic kidney disease includes a three-month condition (PMID 27887750). This means the disease is not decided by a number measured just once. Three months is the condition that defines chronic kidney disease. A sudden drop in kidney function over a few days that raises creatinine is a different problem called acute kidney injury, so it does not fit into the framework of waiting three months. Both acute kidney injury and chronic kidney disease greatly increase illness and death, and both are diagnosed with numbers like creatinine and urine volume (PMID 25195004). Acute kidney injury is followed up in "What Is Acute Kidney Injury? — A Tap That Suddenly Runs Dry, and Three Places to Look".

There are several reasons for waiting and watching over three months. First, simply seeing protein in the urine does not settle its meaning. Proteinuria can be completely harmless or may suggest a problem with the glomeruli, which is why a review article recommends sending someone with persistent proteinuria to a nephrologist (PMID 23773939). Next, the blood-based estimate is rough for any single person (PMID 36279207), and creatinine fluctuates with the body's condition on that day. Creatinine varies not only by age, sex, and muscle mass, but also depending on the amount of water in the body and the flow of blood to the kidneys (PMID 22888113).

Treatment can also move the numbers. In heart failure, a disease where the heart cannot pump enough blood, a slight drop in kidney function numbers after starting guideline-recommended treatment often comes from a change in blood flow and is not associated with bad outcomes, which was the conclusion of a controversy conference held by KDIGO in 2024 (PMID 41791738). There is also a summary noting that changes in creatinine after using diuretics or certain blood pressure medication classes in people with heart failure do not necessarily lead to worse outcomes (PMID 22888113). Both discussions are about heart failure patients, and which way that change points and what to do with the medication are judged by the prescribing medical team along with other information.

That is why we look at the trend rather than a single number. There is a proposal to look at eGFR together with other assessments like urine volume, confirmatory tests, and the trend of eGFR over time (PMID 35736781). When the estimated value may not be correct, it is measured directly. People with very large or small builds, people whose estimates from creatinine and cystatin C disagree with each other, people with cirrhosis, and people intending to donate a kidney are cited as such cases (PMID 34345408).

Chronic kidney disease is judged by whether it lasts for three months or more, and we look at the trend over time and other information together rather than a single number. A sudden rise over a few days is a different problem outside that framework.

Two People's Report Sheets

Let's apply what we have seen to two report sheets.

One person is a 30-year-old man who exercises a lot and has a lot of muscle. His creatinine came out above the reference range, and the eGFR calculated from that value came out low. When someone has a lot of muscle, the function estimated by creatinine can come out lower than it actually is (PMID 29218435). However, whether it is because of muscle or whether the filtering power has actually decreased cannot be distinguished by the two numbers of creatinine and eGFR alone. Creatinine is greatly swayed by muscle mass and body composition, which can place a person in the wrong box, and people with very large builds are cited as cases where measured glomerular filtration rate is used directly (PMID 34345408). Cystatin C, which is less affected by muscle, can also be used for confirmation (PMID 26066476), and a formula including both is more accurate than one using either alone (PMID 34563581). However, a formula using only cystatin C is no better than one using only creatinine (PMID 37950562), so distinguishing which is the case is done by a physician using confirmatory tests.

The other person is a 75-year-old mother who is thin and has little muscle. Her creatinine was within the reference range. In older people with little muscle, creatinine can stay within the reference range even if kidney function has already declined (PMID 29322215). Although eGFR partially offsets this point by factoring age and sex into the formula (PMID 32393465), the value estimated by creatinine is not precise at the individual level in people with unusual muscle mass (PMID 37950562), and creatinine can be inaccurate in people with little muscle (PMID 25195004). Therefore, even if the eGFR comes out above 60, it cannot be assumed that the blood side is sorted out, and a physician may add confirmatory tests like cystatin C (PMID 34518032). Apart from the numbers on the blood side, the urine side remains as well. Traces of kidney damage can exist separately from eGFR, and the definition in international guidelines views this disease based on those traces alone (PMID 27887750). Because a dipstick test's "negative" misses quite a few people who leak albumin (PMID 32658569), a physician may ask to measure the urine albumin-to-creatinine ratio.

Neither person's conclusion is settled by a single number. We measure again to see if unusual values persist. And what those numbers mean is read by a physician who knows the person's medical history, medications, and other tests.

Because the meaning of the exact same creatinine can vary depending on muscle, distinguishing that difference is the job of confirmatory tests and a physician, and the urine side is looked at together alongside the numbers on the blood side.

What the Report Cannot Tell You

The numbers on the report let us guess how well the kidneys are filtering and what is leaking, but they cannot tell us why. The leading causes of chronic kidney disease in high- and middle-income countries alike, and in many low-income countries as well, are diabetes and high blood pressure (PMID 27887750). Which cause brought about a given person's numbers is uncovered by medical history and other tests together.

This is also why these numbers matter. Many people have no symptoms, or experience only vague symptoms like low energy, itching, or loss of appetite, and the diagnosis often comes by chance on a screening urine dipstick or blood test, or only after symptoms have become severe (PMID 27887750). The 2003 US National Kidney Foundation guidelines noted that there is evidence that finding and treating the disease early can prevent or delay some of its bad outcomes (PMID 12859163).

Because of this, the kidney section of the report does not close in either direction. It does not close and say everything is fine just because it shows a mark within the reference range, nor does it close and call it a disease based on a single number below the line. The two axes are read together, over time, by a physician along with other information.

The kidney section of the report shows the degree of filtration and traces of leakage but cannot tell the cause, and its meaning is read over time by a physician along with other information.

Where to Look Next

The principle by which the kidneys filter blood and take back what is needed starts in "How Do the Kidneys Filter Blood? — A Sieve That Takes Almost Everything Back". The stages, causes, and ways to slow down a disease defined by two axes and three months are covered in "What Is Chronic Kidney Disease? — Lamps Going Out One by One, and the Load on Those Still Lit", while cases where creatinine suddenly rises within a few days are followed up in "What Is Acute Kidney Injury? — A Tap That Suddenly Runs Dry, and Three Places to Look". The debate over whether to call the kidney function that declines with age a disease is handled in "How Do the Kidneys Filter Blood? — A Sieve That Takes Almost Everything Back".

The same approach to reading report sheets is also found in Reading a Spirometry Report — One Point Among Your Peers, Two Graphs Drawn by a Breath and What Do Blood Sugar Tests Measure? — Three Numbers, and the Lines Each Organization Drew. The two main causes of chronic kidney disease can be seen in What Is Diabetes? — The Letter That Never Comes, the Letter That Isn't Read and What Blood Pressure Measures — Two Numbers, and an Answer That Changes With Where You Measure.

How to read the share of what tests miss—just like the 38% of people leaking albumin above the threshold whose dipstick tests came back "negative"—is covered in What A Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability. Evidence is accumulating that changes in urine albumin can be used as a surrogate indicator for kidney function loss in clinical trials of kidney disease with high albuminuria (PMID 41198855, PMID 30635225), and how to read such substitute numbers is in Better Numbers, Better Patients? — An Introduction to Surrogate Endpoints.

How to handle the race item in the eGFR formula and which formula to use are questions to which the United States and Europe are still giving different answers.

Source Papers

• "National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification." (2003) — PMID 12859163 — U.S. National Kidney Foundation guidelines: detecting kidney injury using the albumin-creatinine ratio from a random spot urine sample and estimating the glomerular filtration rate with a creatinine formula, with evidence that some poor outcomes can be prevented or delayed through early detection and treatment.
• "Chronic Kidney Disease." (2017) — PMID 27887750 — Glomerular filtration rate below 60 or markers of injury present for 3 months or more regardless of cause; diabetes and high blood pressure are the main causes; often asymptomatic or with non-specific symptoms, with diagnosis commonly found incidentally on screening; glomerular filtration rate is the best overall indicator of function.
• "Measurement and Estimation of GFR for Use in Clinical Practice: Core Curriculum 2021." (2021) — PMID 34518032 — Glomerular filtration rate is used for diagnosis, staging, and drug dosing; measuring clearance using exogenous markers is difficult in routine practice, so it is usually estimated with creatinine formulas, and confirmatory testing may be necessary.
• "Is Iohexol a Possible Method for Estimating Glomerular Filtration Rate?" (2025) — PMID 40332971 — Iohexol clearance is measured by blood draws at set intervals following intravenous administration; it has high precision, but cost and strict procedures limit it to specialized institutions.
• "[Assessment of kidney function : Creatinine is not the whole story]." (2018) — PMID 29322215 — Depending on age and muscle mass, creatinine can remain in the normal range even when function has declined, underestimating early stages; radioactive measurements are cumbersome and 24-hour urine collections are prone to errors; urinalysis is semi-quantitative, so it must be interpreted with its limitations in mind.
• "The Metabolism of Creatinine and Its Usefulness to Evaluate Kidney Function and Body Composition in Clinical Practice." (2025) — PMID 39858438 — Creatinine is mostly produced in muscles and is affected by age, activity, high-protein diets, male sex, medications, and race; it is fast, simple, and cheap, making it the most widely used.
• "Assessment of kidney function: clinical indications for measured GFR." (2021) — PMID 34345408 — Creatinine is heavily influenced by age, sex, muscle mass, body composition, and severe chronic illness, which can lead to misclassification; direct measurement is used in cases of extreme body composition, discrepancies between creatinine and cystatin C, cirrhosis, and living kidney donation.
• "Biomarkers of renal function, which and when?" (2015) — PMID 25195004 — Creatinine can be inaccurate in patients with low muscle mass or fluid overload.
• "Educational review: measurement of GFR in special populations." (2018) — PMID 29218435 — (Educational review on special pediatric populations) Creatinine is affected by muscle mass, and cystatin C is affected by inflammation and changes in protein breakdown; individuals with high muscle mass may have their creatinine eGFR underestimated.
• "Kidney Disease, Race, and GFR Estimation." (2020) — PMID 32393465 — All creatinine formulas contain proxy information for muscle mass such as age, sex, race, height, and weight; in development studies, measured GFR in Black individuals was on average 16% higher at the same age, sex, and creatinine level; simply removing the race term causes a systematic underestimation in Black individuals, suggesting a cautious approach.
• "Diagnostic standard: assessing glomerular filtration rate." (2024) — PMID 37950562 — Creatinine eGFR is inaccurate at the individual level due to muscle mass and other factors; cystatin C formulas work better when included alongside creatinine; discrepancies between the two values are linked to risk, and research on their definitions and determinants is needed.
• "The new, race-free, Chronic Kidney Disease Epidemiology Consortium (CKD-EPI) equation to estimate glomerular filtration rate: is it applicable in Europe? A position statement by the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM)." (2023) — PMID 36279207 — European laboratories are recommended not to adopt the 2021 equation and to maintain the 2009 equation without race adjustment; all formulas are estimates and are particularly rough at the individual level.
• "Advances in glomerular filtration rate-estimating equations." (2010) — PMID 20393287 — The CKD-EPI formula has better performance and risk prediction than the MDRD formula; the cystatin C formula is not accurate in all populations, including people with low muscle mass or chronic illness.
• "A Unifying Approach for GFR Estimation: Recommendations of the NKF-ASN Task Force on Reassessing the Inclusion of Race in Diagnosing Kidney Disease." (2022) — PMID 34563581 — Immediate adoption of the re-fitted CKD-EPI creatinine equation without a race term in all U.S. laboratories; expanded use of cystatin C to check eGFR in patients at risk for kidney disease (combining the two markers is more accurate than either alone).
• "Clinical Impacts of Implementing the 2021 Race-Free Chronic Kidney Disease Epidemiology Collaboration Estimated Glomerular Filtration Rate." (2024) — PMID 38366867 — Differences in eGFR performance between individuals far outweigh the benefit of race coefficients; the 2021 equation eliminates systematic overestimation in Black patients.
• "Retaining Race in Chronic Kidney Disease Diagnosis and Treatment." (2023) — PMID 37701164 — Formulas without the race term are estimated to eliminate chronic kidney disease in 5.51 million non-Black adults and move 4.59 million to a milder stage, with counterarguments that keeping race-specific formulas is the least biased approach.
• "New and old GFR equations: a European perspective." (2023) — PMID 37664574 — Both the 2009 and 2021 CKD-EPI equations overestimate in ages 18–30; the 2021 equation performs worse than the 2009 equation in European whites; the EKFC equation provides continuity in the pediatric-to-adult transition; no equation is perfect.
• "Should Cystatin C eGFR Become Routine Clinical Practice?" (2023) — PMID 37509111 — Cystatin C is a protein freely filtered by the glomeruli and is not affected by muscle mass; widespread use requires increased laboratory accessibility.
• "Cystatin C as a biomarker for estimating glomerular filtration rate." (2015) — PMID 26066476 — Cystatin C has fewer determinants other than the glomerular filtration rate compared to creatinine, avoiding the limitations of diet and muscle mass; it can be useful for determining drug dosages or as a confirmatory test when diagnosis is uncertain.
• "Advantages, Limitations, and Clinical Considerations in Using Cystatin C to Estimate GFR." (2022) — PMID 36514729 — KDIGO 2012 guidelines recommend using cystatin C to confirm chronic kidney disease diagnosed by creatinine eGFR.
• "Discordance in Creatinine- and Cystatin C-Based eGFR and Clinical Outcomes: A Meta-Analysis." (2025) — PMID 41202182 — Cystatin C eGFR is more than 30% lower than creatinine eGFR in 11% of outpatients and 35% of inpatients, associated with poor outcomes such as all-cause mortality of 28.4 vs 16.8 per 1,000 person-years over an average 11-year outpatient follow-up.
• "Assessment of proteinuria." (2011) — PMID 21782130 — In most proteinuric kidney diseases, albumin is the main component of urine protein; spot urine protein- or albumin-creatinine ratios are preferred over 24-hour urine in routine practice.
• "[Assessment of proteinuria]." (2013) — PMID 23773939 — Proteinuria can be benign or a sign of glomerular disease; persistent proteinuria should be referred to a nephrologist; standard dipstick tests detect only albumin.
• "Conversion of Urine Protein-Creatinine Ratio or Urine Dipstick Protein to Urine Albumin-Creatinine Ratio for Use in Chronic Kidney Disease Screening and Prognosis : An Individual Participant-Based Meta-analysis." (2020) — PMID 32658569 — 919,383 adults with albumin ratio alongside protein ratio or dipstick tests measured on the same day (sometimes not the same urine sample); dipstick 「±」 or greater has 62% sensitivity and 88% specificity for detecting an albumin ratio above 30, stages A2 (30–299) and A3 (300 and above); albumin ratio is the preferred measurement, and when unavailable, converted values from protein ratios or dipstick tests are helpful.
• "Association of estimated glomerular filtration rate and albuminuria with all-cause and cardiovascular mortality in general population cohorts: a collaborative meta-analysis." (2010) — PMID 20483451 — 105,872 individuals with albumin ratio measurements; all-cause mortality risk relative to eGFR 95 is 1.18 at 60, 1.57 at 45, and 3.14 at 15; albumin ratio increases risk without a threshold; the two indicators are independent and multiplicative; eGFR below 60 and albumin ratio of 10 mg/g or higher are independent predictors.
• "Estimated Glomerular Filtration Rate, Albuminuria, and Adverse Outcomes: An Individual-Participant Data Meta-Analysis." (2023) — PMID 37787795 — 114 cohorts; low eGFR and high albumin ratio are each associated with ten adverse outcomes, including the mildest categories.
• "Associations of kidney disease measures with mortality and end-stage renal disease in individuals with and without diabetes: a meta-analysis." (2012) — PMID 23013602 — Relative risks according to eGFR and albumin ratio are nearly identical regardless of diabetes status.
• "Albuminuria and cardiorenal risk." (2023) — PMID 37016948 — Chronic kidney disease is confirmed by simultaneous evaluation of eGFR and albuminuria; regular albumin ratio screening rates are very low, leaving early-stage high-risk patients unexposed; albuminuria is shifting from a risk predictor to a target for treatment.
• "Kidney disease and heart failure: recent advances and current challenges: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference." (2026) — PMID 41791738 — March 2024 KDIGO controversies conference: small decreases in kidney function after starting heart failure treatment are often hemodynamic and not associated with poor outcomes, so discontinuation is usually unnecessary.
• "The role of the kidney in heart failure." (2012) — PMID 22888113 — Creatinine depends on age, sex, muscle mass, fluid status, and renal hemodynamics; changes in creatinine following diuretics or ACE inhibitors are not necessarily associated with poor outcomes.
• "Kidney Function Assessment in African American Patients: A Narrative Review for Pharmacists." (2022) — PMID 35736781 — Suggests viewing eGFR alongside other assessments such as urine volume, confirmatory tests, and eGFR trends.
• "[Urinalysis in the 21st century: anything but obsolete!]." (2015) — PMID 25804777 — Urinalysis, including dipstick and microscopic examination, is a non-invasive, simple, and economical screening tool; repeated detection of hematuria or proteinuria is sensitive for discovering kidney and systemic diseases; because it is semi-quantitative, it must be interpreted considering diagnostic and technical limitations.
• "Global Prevalence of Chronic Kidney Disease - A Systematic Review and Meta-Analysis." (2016) — PMID 27383068 — 100 observational studies; stage-specific eGFR boxes (90 and above, 60–89, 30–59, 15–29, below 15) and prevalence (sources of the boxes in figure notes).
• "A meta-analysis of albuminuria as a surrogate endpoint for kidney failure." (2026) — PMID 41198855 — 48 randomized trials with 85,681 participants; every 30% reduction in 6-month albumin ratio in the treatment group is associated with an average 19% reduction in clinical endpoint risk, further supporting the use of surrogate endpoints.
• "Change in albuminuria and subsequent risk of end-stage kidney disease: an individual participant-level consortium meta-analysis of observational studies." (2019) — PMID 30635225 — A 30% reduction in albumin ratio over 2 years is consistently associated with subsequent reduced risk of end-stage kidney disease, supporting the use of surrogate endpoints in progression trials where albuminuria remains high.