Can eGFR Be Wrong? Factors Affecting Your Test Results

eGFR can absolutely be wrong, and it is wrong more often than most people realize. The test estimates how well your kidneys filter blood, but the number on your lab report is calculated from a blood marker, usually creatinine, that is influenced by far more than kidney health alone. Your muscle mass, what you ate the night before, medications you take, your hydration level, and even which formula your lab uses can all push eGFR up or down independently of your actual kidney function. In some cases, these non-kidney factors shift the result enough to reclassify you into a different stage of kidney disease or give you a clean bill of health when your kidneys are struggling.

Why eGFR Is an Estimate, Not a Measurement

Your kidneys’ true filtration rate can be measured directly by injecting a tracer substance and tracking how fast your kidneys clear it from the blood. That procedure, called measured GFR (mGFR), is the gold standard but is expensive, time-consuming, and impractical for routine screening. So labs estimate GFR from serum creatinine, a waste product generated by your muscles and cleared by your kidneys. The logic is straightforward: if your kidneys are filtering well, creatinine stays low; if they are struggling, creatinine rises. An equation then converts your creatinine level into an eGFR number, adjusting for your age and sex.

The problem is that creatinine is a messy proxy. Anything that changes how much creatinine your body produces or how it gets into the blood will change your eGFR result without changing your actual kidney function. The standard equations assume you are a person of roughly average body composition, eating a normal diet, in a stable physiological state. When those assumptions break down, the estimate breaks down with them.

Muscle Mass Is the Biggest Confounder

Creatinine comes from the breakdown of creatine in skeletal muscle, so people with more muscle produce more creatinine. This means a very muscular person can have a creatinine-based eGFR that looks lower (worse) than their kidneys actually warrant, while someone with very little muscle, such as a frail elderly person or someone with a wasting disease, can have an eGFR that looks deceptively healthy. One study comparing eGFR against directly measured GFR found that eGFR was falsely reduced by about 6 mL/min for every additional 10 kg of lean muscle mass, with no effect from fat mass at all.¹ The correlation between lean mass and eGFR error was consistent across different eGFR formulas and across all CKD stages.²

Lean mass also drives creatinine independently of physical activity or protein intake. A study that controlled for both meat consumption and exercise levels found that lean mass was significantly related to serum creatinine even after those adjustments, while cystatin C, an alternative biomarker, was not influenced by muscle at all.³ The effect is also sex-dependent: muscle mass has a stronger impact on creatinine in older men than in older women, which makes sense given that men tend to have more skeletal muscle and lose it at different rates with age.⁴

In practical terms, this means a competitive athlete or someone who does heavy resistance training might get flagged with a borderline-low eGFR that does not reflect any kidney problem. On the flip side, a person with muscle-wasting conditions, sarcopenia, or severe illness might appear to have normal kidney function on paper while already losing filtration capacity. Creatinine-based eGFR essentially punishes people for having more muscle and rewards people for having less, with no way to correct for it from the blood test alone.

Diet and the Cooked-Meat Effect

Eating a large serving of cooked meat can raise your serum creatinine enough to change your eGFR classification. In one study of people with diabetes-related kidney disease, a standardized cooked meat meal caused creatinine to rise significantly, and six out of sixteen participants who had been classified as stage 3a kidney disease were reclassified into the more severe stage 3b after the meal.⁵ The effect disappeared after 12 hours of fasting, which is why some guidelines recommend fasting or at least avoiding a heavy meat meal the evening before blood work.

Beyond a single meal, this dietary effect also shows up in day-to-day variation studies. In people without kidney disease, the natural fluctuation of creatinine over 24 hours was higher than in people with CKD, largely because of the more pronounced impact of meat consumption when someone’s baseline creatinine is low.⁶ If you are right on the border between two kidney disease stages, the timing and content of your last meal can genuinely tip the result one way or the other.

Medications That Raise Creatinine Without Hurting Your Kidneys

Several common drugs cause creatinine to rise by blocking the way it is secreted from the blood into the urine, not by damaging the kidneys. About 15% of creatinine is actively secreted through transporter proteins in the kidney tubule, and drugs that inhibit those transporters cause creatinine to back up in the blood.⁷ The result is an eGFR that drops without any real change in filtration.

The most well-known offenders include trimethoprim (a common antibiotic used for urinary tract infections), cimetidine (an older heartburn medication), and certain antiparasitic drugs like pyrimethamine.⁷ The creatinine bump from these medications is real and measurable, but it does not mean anything has gone wrong with your kidneys. If your doctor does not realize you are on one of these drugs when interpreting your results, they might unnecessarily worry about kidney decline or adjust other medication doses based on a falsely low eGFR.

Hydration and Exercise

Being dehydrated before your blood draw can also skew results. When participants went without fluids overnight, their measured GFR dropped from an average of about 108 mL/min to 97 mL/min, and their serum creatinine rose, even though standard hydration markers in the blood did not change enough to flag dehydration.⁸ That is a drop large enough to push a borderline result into abnormal territory. Current guidelines recommend staying hydrated before kidney testing, but many patients show up for fasting blood work having avoided all fluids, not just food.

Strenuous exercise is a related concern. Creatinine commonly rises after hard physical activity, which does not necessarily mean the kidneys are injured.⁹ Short bouts of exercise have minimal impact, but prolonged exercise with significant dehydration can lower eGFR substantially. One study found that after prolonged exercise with about 3% body weight lost from sweating, cystatin C-based eGFR dropped from roughly 118 to 103 mL/min.¹⁰ If you ran a marathon or did a long hike in the heat the day before your blood draw, your eGFR might look worse than it actually is.

When Liver Disease Masks Kidney Problems

People with liver cirrhosis present a particularly dangerous scenario: their eGFR tends to look better than reality. Cirrhosis reduces the liver’s ability to produce creatine (the precursor to creatinine), lowers skeletal muscle mass through wasting, and increases the tubular secretion of creatinine. All three effects push creatinine down, which makes eGFR look artificially healthy.¹¹ A person with cirrhosis can have moderate to severe kidney impairment while their creatinine-based eGFR still reads in the normal range. This matters enormously for medication dosing and for decisions about liver transplant timing.

Amputation and Lost Muscle Mass

A limb amputation removes a chunk of muscle, which lowers creatinine production and artificially inflates eGFR. In one study, amputation was associated with a creatinine drop of about 0.09 mg/dL and a corresponding eGFR jump of roughly 6 mL/min, enough to mask declining kidney function.¹² A study of military amputees found that their average creatinine was significantly lower than that of a comparable general population, confirming that creatinine-based estimates overstate kidney function after major limb loss.¹³ Cystatin C and other non-muscle-dependent markers did not shift after amputation, which points toward cystatin C as a better option for these patients.

Pregnancy Changes Everything

During pregnancy, blood volume increases dramatically and the kidneys filter faster than usual, a phenomenon called hyperfiltration. As a result, creatinine drops and eGFR rises. One study found a mean eGFR of about 123 mL/min in the first trimester, climbing to roughly 130 mL/min by the 24th week.¹⁴ The standard eGFR equations were not designed for pregnant women and cannot account for these changes.¹⁵ Applying a normal eGFR cutoff to a pregnant patient could miss real kidney problems, since a “normal” eGFR in pregnancy might actually represent a decline from the expected pregnant range.

Age and the Problem of “Normal” Decline

Kidney filtration naturally decreases as you age, typically starting around age 30 to 40 and accelerating after 50 or 60. This decline is part of normal organ aging and comes with structural changes in the kidneys. The difficulty is that current guidelines use a fixed eGFR threshold of 60 mL/min to define chronic kidney disease without adjusting for age. An 80-year-old whose kidneys have declined at a perfectly normal rate may end up labeled with CKD even though their kidney function is appropriate for their age.¹⁶ This has led to criticism that CKD is overdiagnosed in older adults, generating many false-positive diagnoses that can trigger unnecessary referrals, anxiety, and medication restrictions.

The Race Equation Controversy

For years, the most widely used eGFR equations included a race coefficient that assigned higher eGFR values to Black patients, based on the assumption that Black individuals have higher average muscle mass and therefore higher creatinine. This was controversial because it meant the same creatinine level would yield a different eGFR depending on how a patient’s race was recorded, potentially delaying diagnosis and referral for Black patients with real kidney disease.

In 2021, a major effort led to new race-free equations. The updated CKD-EPI 2021 equation uses only age and sex, dropping the race variable entirely. In validation testing, the older equation with its race adjustment overestimated measured GFR in Black patients by a median of about 3.7 mL/min, while simply removing the race coefficient without redesigning the equation underestimated GFR in Black patients by about 7 mL/min. The new 2021 equation represented a compromise, and equations that combined creatinine with cystatin C were the most accurate, with smaller differences between racial groups.¹⁷ The switch has changed who gets diagnosed with CKD: the newer race-free creatinine equations increased CKD prevalence estimates among Black adults compared to the old equation, while yielding similar or lower prevalence among non-Black adults.

Obesity and the Body Surface Area Problem

Standard eGFR is reported per 1.73 square meters of body surface area, a normalization that dates back nearly a century. But in people with obesity, whose actual body surface area is well above 1.73 m², this indexing systematically lowers the reported eGFR compared to what the kidneys are actually doing.¹⁸ The result is a higher prevalence of eGFR values below 60 mL/min and more CKD diagnoses in overweight and obese patients, some of which may not reflect true kidney impairment.¹⁹ Clinicians working with obese patients sometimes calculate an absolute (non-indexed) GFR for medication dosing to avoid this distortion.

Cystatin C as an Alternative, and Its Own Flaws

Cystatin C is a small protein produced by virtually all cells in the body, and it does not depend on muscle mass the way creatinine does. That makes it a useful alternative for people in whom creatinine is unreliable: the very muscular, the very frail, amputees, and those with liver disease.²⁰

But cystatin C has its own blind spots. Thyroid hormone levels affect cystatin C concentrations: hyperthyroidism can raise it and hypothyroidism can lower it, independently of kidney function.²¹ Inflammatory states, corticosteroid use, and conditions that ramp up the immune system can also push cystatin C higher, which makes eGFR look worse than it is. There have been reports of falsely elevated cystatin C in critically ill patients with active infections and high-dose steroid treatment.²² So while cystatin C neatly sidesteps the muscle problem, it introduces its own set of confounders.

Combining Both Markers for Better Accuracy

The most accurate eGFR estimates come from equations that use creatinine and cystatin C together. In a pooled analysis of over 3,400 people with CKD, an equation that combined both markers along with age, sex, and race had about 89% of its estimates fall within 30% of measured GFR, compared to roughly 81-85% for equations using either marker alone.²³ A Swedish cross-sectional study confirmed this pattern: combined equations had more stable performance across different ages, BMI levels, and GFR ranges.²⁴ A useful rule of thumb from that study: when the creatinine-based and cystatin C-based eGFR values agree closely (within about 10%), the combined equation reaches about 95% accuracy. When the two single-marker estimates diverge by 40% or more, accuracy drops, which is itself a signal that something non-kidney is probably distorting one of the markers.

Lab Methods Are Not All Equal

Even if everything about you is controlled, the lab itself introduces variation. Two main methods exist for measuring creatinine: the Jaffe method (a chemical reaction) and the enzymatic method (a more specific enzyme-based assay). Both are now calibrated to the same international reference standard, but they still do not always agree. In one study comparing the two methods in healthy subjects and people with diabetes, the creatinine values were similar on average, but in the diabetes group only 40% of paired measurements fell within 10% of each other, compared to 63% in the healthy group. The enzymatic assay showed better agreement with measured GFR in diabetic patients.²⁵ If your lab switches methods or you get tested at different facilities, your eGFR might shift for purely analytical reasons.

Critical Illness Breaks the Model

Standard eGFR equations assume your kidney function is stable. In critically ill patients whose kidneys are rapidly deteriorating or recovering, creatinine has not yet caught up with the real-time filtration rate. Creatinine can take a day or two to rise after an acute kidney injury, meaning eGFR looks reassuringly normal while the kidneys have already failed. In these settings, the usual equations perform poorly, with high biases and unacceptable error rates.²⁶ Clinicians in intensive care units generally do not rely on eGFR and instead use other methods to gauge kidney function in real time.

Natural Day-to-Day Variation

Even without any of these confounders, eGFR bounces around on its own from normal biological variation. In people with CKD, the within-person biological variation of eGFR runs around 5%, meaning two blood draws taken days apart under identical conditions can differ by that much purely from random fluctuation.²⁷ In people without CKD, creatinine variation is even higher, around 6.4%, partly because of the outsized impact of meals on their relatively low baseline creatinine.⁶ This means a single eGFR reading should never be treated as definitive. Trends over multiple tests are far more meaningful than any individual number.

When eGFR Looks Fine but the Kidneys Are Not

One scenario that trips people up is when eGFR reads normally while kidney damage is already underway. Early kidney injury often shows up as protein leaking into the urine (albuminuria) before eGFR drops at all. Damage to the kidney tubules can cause albuminuria even when the filtering units themselves still have a healthy GFR.²⁸ This is why kidney disease screening guidelines generally recommend checking both eGFR and urine albumin. A normal eGFR with elevated urine albumin still qualifies as kidney disease and still carries risk for progression. Relying on eGFR alone misses these cases entirely.