Creatinine Clearance vs eGFR: Why Two Kidney Numbers Disagree

One blood test, two different answers

People who look up their kidney function sometimes find two figures that do not match. One comes from the lab report and is labeled eGFR. Another comes from a pharmacist, a drug reference or an online calculator and is called creatinine clearance. Both are estimates of the same thing, the rate at which the kidneys filter blood, and both start from the serum creatinine level in the same blood sample. They disagree because they take different routes from that number to an answer.

Creatinine itself is a waste product from muscle that the kidneys remove, so a higher blood level suggests the kidneys are filtering less. MedlinePlus notes that creatinine comes from muscle and that reference ranges differ between men and women. Because the level also depends on muscle mass, neither equation can be treated as a precise measure, and the details of each equation decide how it handles that problem.

eGFR: the number used to stage kidney disease

The estimated glomerular filtration rate, or eGFR, is the figure on most lab reports. It is calculated from serum creatinine, age and sex using the CKD-EPI equation, and the current version, published in 2021 in the New England Journal of Medicine, replaced an earlier equation that included a race coefficient. The National Kidney Foundation and American Society of Nephrology task force recommended adopting the race-free 2021 equation, and it is now the standard in US laboratories.

The result is expressed per 1.73 square meters of body surface area, a standard reference body size, so that it can be compared between people of different sizes. This standardization is deliberate: eGFR is designed to describe the function of the kidneys relative to a typical adult body, which suits staging a chronic condition across a population.

KDIGO, the international guideline body, defines chronic kidney disease by an eGFR below 60 mL/min/1.73 m², or by markers of kidney damage such as albumin in the urine, persisting for more than three months. It groups eGFR into categories G1 to G5, with G1 at 90 and above and G5 below 15, and pairs them with albuminuria categories. A single result is not a diagnosis, and a lab report that flags a low eGFR is a prompt for repeat testing and a urine albumin test.

Cockcroft-Gault: why a 1976 equation is still in use

The Cockcroft-Gault equation estimates creatinine clearance as 140 minus age, multiplied by body weight in kilograms, divided by 72 times serum creatinine in mg/dL, with the result multiplied by 0.85 for women. It was published in 1976 from a study of 249 patients, and the National Kidney Foundation publishes the equation as a reference for clinicians.

It has persisted for a practical reason. Pharmaceutical companies tested many drugs in people whose kidney function was described with Cockcroft-Gault, and the dosing recommendations on the label, such as reducing a dose when clearance falls below a certain value, were calibrated to that equation. Swapping in a different equation means that a threshold was set against a different number, so many references continue to specify Cockcroft-Gault.

It also has known weaknesses. The equation predates the standardization of creatinine assays, and modern laboratory methods tend to give slightly lower creatinine values than the ones used in the original study, which tends to make the equation read higher than the true clearance. And the use of body weight raises an unsettled question: whether to use actual, ideal or adjusted body weight depends on the body size, the reference being followed, and the clinician’s judgment.