Creatinine Clearance Calculator (Cockcroft-Gault)

Creatinine clearance (CrCl) estimates the glomerular filtration rate (GFR) — the rate at which the kidneys filter blood — using serum creatinine levels, age, weight, and sex. It is the primary clinical measure of kidney function and directly affects drug dosing for renally-cleared medications.

Cockcroft-Gault formula (most widely used clinically): CrCl (mL/min) = [(140 − Age) × Weight (kg)] / [72 × Serum Creatinine (mg/dL)] × 0.85 for females (women have lower muscle mass and thus lower creatinine production)

Kidney function stages by GFR/CrCl:

Alternative: MDRD equation (for research and eGFR reporting): eGFR = 175 × Serum Creatinine^(−1.154) × Age^(−0.203) × 0.742 (if female) × 1.212 (if Black)

Drug dosing implications: Renal clearance affects: antibiotics (vancomycin, aminoglycosides), anticoagulants (enoxaparin), antivirals (acyclovir, tenofovir), and many cardiac drugs. Nephrotoxic drugs require CrCl monitoring every 48–72 hours in critical care.

Worked example: Patient: 68-year-old male, weight 80 kg, serum creatinine 1.4 mg/dL. CrCl = [(140 − 68) × 80] / [72 × 1.4] = [72 × 80] / [100.8] = 5,760 / 100.8 = 57.1 mL/min → Stage G3a (mildly to moderately decreased kidney function)

A vancomycin dose that would be appropriate for normal kidneys would require significant reduction at this CrCl to prevent drug accumulation and nephrotoxicity.

Important: Creatinine-based estimates are less reliable in elderly, malnourished, or obese patients. Cystatin C-based eGFR is preferred in these populations.

Obesity adjustment. Cockcroft-Gault uses weight as a proxy for muscle mass, and overestimates CrCl in obese patients because adipose tissue produces little creatinine. When actual body weight exceeds ideal body weight (IBW) by more than 30%, the convention is to substitute an adjusted body weight in the formula: ABW = IBW + 0.4 × (actual weight − IBW). The IBW itself comes from the Devine formula (men: 50 + 2.3 × inches over 60; women: 45.5 + 2.3 × inches over 60). For underweight patients, use actual body weight rather than IBW — substituting up never helps the estimate.

A common pitfall — “normal” creatinine isn’t always normal kidney function. A frail elderly patient or a malnourished one produces less creatinine because they have less muscle, so the serum value reads in the normal range even when CrCl is genuinely poor. A creatinine of 0.8 mg/dL in a 90-year-old can correspond to a CrCl below 30 mL/min. This is the case where Cystatin C is the better marker.