How to Calculate Creatinine Clearance
Use the Cockcroft-Gault Formula for Medication Dosing
For a patient with GFR 93 mL/min/1.73 m² (normal kidney function), calculate creatinine clearance using the Cockcroft-Gault equation: CrCl (mL/min) = [(140 - age) × weight (kg)] / [72 × serum creatinine (mg/dL)] × (0.85 if female). 1, 2
Why This Formula Matters
The Cockcroft-Gault equation is specifically recommended for medication dosing decisions because virtually all pharmacokinetic studies that established renal dosing guidelines used this formula 1, 3
Drug manufacturers and FDA package inserts reference Cockcroft-Gault-derived creatinine clearance values, not eGFR 1
The MDRD and CKD-EPI equations estimate GFR normalized to body surface area (mL/min/1.73 m²), which is designed for diagnosing and staging chronic kidney disease, not for medication dosing 1, 3
Step-by-Step Calculation
Required parameters:
- Patient's age in years 2
- Weight in kilograms 2
- Serum creatinine in mg/dL (if reported in μmol/L, divide by 88.4 to convert) 4, 2
- Sex (multiply final result by 0.85 for females) 1, 2
Example calculation for a 70-year-old male, 75 kg, serum creatinine 1.0 mg/dL:
- CrCl = [(140 - 70) × 75] / [72 × 1.0] = 5,250 / 72 = 72.9 mL/min 2
Critical Adjustments and Pitfalls
For obese patients: Use the mean value between actual and ideal body weight, not actual weight alone, as this provides more accurate estimation 4, 1, 2
Laboratory method matters: If your laboratory uses enzymatic (PAP) methods instead of the older Jaffe method, add 0.2 mg/dL to the serum creatinine value before calculating to avoid underdosing medications 4, 2
Never use serum creatinine alone to assess kidney function—this significantly underestimates renal insufficiency, especially in elderly patients with reduced muscle mass 4, 1
Understanding Your Patient's GFR of 93
A GFR of 93 mL/min/1.73 m² represents normal kidney function (Stage 1 if kidney damage present, or normal if no damage) 1
GFR ≥90 mL/min/1.73 m² is considered normal, though it declines by approximately 1% per year after age 40 1
For patients with GFR ≥60 mL/min, most medications require no dose adjustment 1
When Cockcroft-Gault May Be Inaccurate
The formula has limitations in:
- Elderly patients (systematically underestimates GFR in the oldest patients) 4, 1, 2
- Patients with significantly impaired renal function (may overestimate GFR due to increased tubular secretion) 1, 2
- Extremes of body composition (severe obesity, cachexia, amputation) 1, 3
For drugs with narrow therapeutic indices (vancomycin, aminoglycosides, chemotherapy), consider cystatin C-based equations or direct GFR measurement using exogenous markers when estimates seem unreliable 1, 3
Key Distinction: CrCl vs eGFR
Creatinine clearance (Cockcroft-Gault) = absolute clearance in mL/min, used for drug dosing 3
eGFR (MDRD/CKD-EPI) = normalized to body surface area in mL/min/1.73 m², used for CKD diagnosis and staging 3
Using eGFR for drug dosing leads to underdosing in larger patients and overdosing in smaller patients because body surface area normalization doesn't reflect actual drug clearance 1, 3