Kidney Function Equations

Kidney function is estimated, not measured directly, by several equations. The two that matter most are Cockcroft–Gault, which estimates creatinine clearance for drug dosing, and the race-free CKD-EPI 2021 equation, which estimates eGFR for CKD staging. If you only read one page, read CrCl vs eGFR: which to use and when.

Why Kidney Function Is Estimated, Not Measured

The true gold standard for kidney function is measured GFR — the clearance of an external marker such as inulin or iohexol. That test is accurate but slow, costly, and impractical for everyday care. Estimating equations exist because they turn a single, cheap serum creatinine result, plus a few patient variables, into a usable number in seconds. Creatinine works as a filtration marker because muscle produces it at a fairly steady rate and the kidneys clear it; as filtration falls, creatinine rises. The trade-off is that every equation is an approximation, and each carries assumptions that break down at the extremes of age, weight, and muscle mass.

The Two Equations That Matter Most

Cockcroft–Gault (1976) estimates creatinine clearance in mL/min from age, weight, sex, and serum creatinine. Because most drug labels were validated against it, it remains the renal drug-dosing standard. CKD-EPI 2021 estimates eGFR in mL/min/1.73m², is race-free, and is the equation KDIGO and the National Kidney Foundation recommend for CKD staging. The units differ because eGFR is indexed to body-surface area while creatinine clearance is not, which is exactly why the two numbers are not interchangeable.

The Supporting Equations and Their Roles

• MDRD — an earlier eGFR equation, still reported by many laboratories; CKD-EPI is more accurate at higher GFR values.
• Bedside Schwartz — the standard for pediatric eGFR, using height and creatinine.
• Cystatin C–based equations — useful when creatinine is unreliable, such as at very low or very high muscle mass, and as a confirmatory test.
• Salazar–Corcoran — a creatinine-clearance estimate built for obesity, where weight-based Cockcroft–Gault can overestimate.
• Jelliffe — estimates creatinine clearance without a timed urine collection.

How to Choose

Start from the question. If you are dosing a renally-cleared drug, use Cockcroft–Gault creatinine clearance unless the label specifies eGFR. If you are staging chronic kidney disease or tracking it over time, use CKD-EPI 2021 eGFR. If creatinine itself is suspect — a frail patient, an amputee, a bodybuilder — consider a cystatin C–based estimate. The comparison pages below walk through these decisions side by side.

Why the Equations Disagree

Two equations applied to the same patient often return different numbers, and the reason is usually structural rather than an error. Cockcroft–Gault reports raw mL/min scaled to actual body size, while eGFR equations report mL/min/1.73m² normalised to a standard body-surface area, so a large person can show a higher Cockcroft–Gault value and a smaller indexed eGFR for identical kidneys. The equations were also derived from different study populations and calibrated to different creatinine assays. On top of that, creatinine clearance slightly overstates true filtration because the kidney tubules secrete a small amount of creatinine — the reason a measured clearance reads a little above measured GFR. Treat the gap between two estimates as expected, and choose the equation that matches your question rather than averaging them.

Method Explainers

• Cockcroft–Gault equation — creatinine clearance (mL/min) — the drug-dosing standard
• CKD-EPI 2021 equation — race-free eGFR (mL/min/1.73m²) — CKD staging
• MDRD equation — earlier eGFR equation, still widely reported
• Schwartz equation — pediatric eGFR from height and creatinine
• Jelliffe equation — creatinine clearance without a urine collection
• Salazar–Corcoran equation — creatinine clearance in obesity
• Why CrCl overestimates GFR — tubular secretion of creatinine

Equation Comparisons

• CrCl vs eGFR — which to use and when (start here)
• Cockcroft–Gault vs CKD-EPI — dosing vs staging
• MDRD vs CKD-EPI — which eGFR equation
• Creatinine clearance vs GFR — estimate vs gold standard