How do I calculate creatinine clearance?

How to Calculate Creatinine Clearance

Primary Recommendation: Use the Cockcroft-Gault Formula

For clinical practice, particularly medication dosing, use the Cockcroft-Gault formula: CrCl (mL/min) = [(140 - age) × weight (kg)] / [72 × serum creatinine (mg/dL)] × 0.85 if female. 1, 2, 3

This is the preferred method endorsed by the American College of Cardiology, American College of Clinical Pharmacy, and American Society of Clinical Oncology specifically for drug dosing purposes, especially for renally-cleared medications with narrow therapeutic windows. 2, 3

Step-by-Step Calculation Process

Required Variables

• Age in years 1
• Weight in kilograms 1
• Serum creatinine in mg/dL 1
• Sex (multiply final result by 0.85 for females) 1, 4

The Formula Breakdown

1. Subtract the patient's age from 140 1
2. Multiply this result by body weight in kg 1
3. Divide by 72 multiplied by serum creatinine in mg/dL 1
4. If female, multiply the entire result by 0.85 1, 4

Unit Conversions

• If serum creatinine is reported in μmol/L, divide by 88.4 to convert to mg/dL 1, 3

Special Population Adjustments

Obese Patients

• Use the mean value between actual body weight and ideal body weight rather than actual weight alone in the Cockcroft-Gault formula. 1, 2, 3
• This prevents overestimation of creatinine clearance that occurs when using actual weight in obese individuals. 5

Elderly Patients

• The Cockcroft-Gault formula consistently underestimates GFR in elderly patients, with the discrepancy most pronounced in the oldest patients. 1, 3
• Despite this limitation, it remains the recommended formula for medication dosing in this population. 6
• Be particularly cautious as "near normal" serum creatinine levels may represent significant renal impairment in elderly patients with low body weight. 1

African American Patients

• The Cockcroft-Gault formula does not include a race-specific adjustment factor. 3
• African Americans have higher baseline serum creatinine due to greater muscle mass (32.5% vs 28.7% of body weight), but this is not accounted for in Cockcroft-Gault. 1, 3

Alternative Formula: MDRD (When Not to Use Cockcroft-Gault)

MDRD Formula

Estimated GFR (mL/min/1.73 m²) = 186 × [serum creatinine (mg/dL)]^-1.154 × [age (years)]^-0.203 × [0.742 if female] × [1.21 if African American] 1, 2

When to Use MDRD Instead

• For assessing chronic kidney disease stages when GFR is less than 90 mL/min/1.73 m². 2, 3
• MDRD is more accurate than Cockcroft-Gault in patients with significantly impaired renal function (stages 3-5 chronic kidney disease). 2, 3
• MDRD provides GFR indexed to body surface area, which differs from the non-indexed Cockcroft-Gault result. 1, 3

When NOT to Use MDRD

• Normal persons or GFR >60 mL/min/1.73 m² 2
• Extremes of body size 2
• Persons older than 70 years of age 2
• Rapidly changing kidney function 2
• Vegetarian diet 2
• Diseases of skeletal muscle, paraplegia, or quadriplegia 2

Critical Laboratory Considerations

Serum Creatinine Measurement Methods

• The Jaffe method overestimates serum creatinine by 5-15% compared to enzymatic methods. 1, 2, 3
• When your laboratory uses enzymatic methods, add 0.2 mg/dL to the serum creatinine value before using the Cockcroft-Gault equation to avoid underdosing medications. 2, 3

Important Clinical Caveats

Inherent Formula Limitations

• Creatinine clearance overestimates true GFR because creatinine is both filtered and secreted by the kidneys. 1, 2, 3
• The Cockcroft-Gault formula underestimates GFR in normal and moderately reduced renal function but overestimates GFR when renal function is significantly impaired. 1, 2
• Never use serum creatinine concentration alone to assess kidney function—minor elevations may represent substantial reductions in GFR, particularly in elderly patients, women, and those with lower muscle mass. 2, 3

When Direct Measurement is Required

• Extremes of age and body size 2, 3
• Severe malnutrition or obesity 2, 3
• Diseases of skeletal muscle 2, 3
• Paraplegia or quadriplegia 2, 3
• Vegetarian diet 2, 3
• Rapidly changing kidney function 2, 3
• Calculating doses for potentially toxic drugs that are renally excreted 2

Critically Ill Patients

• Do not use estimation formulas in critically ill patients. 3
• Measured urinary creatinine clearance is recommended instead, calculated as (Urinary creatinine concentration × Urinary volume) / Serum creatinine, with urine collection over at least 1 hour. 3
• Estimation formulas were developed for stable patients with chronic renal insufficiency and do not accurately reflect renal function in critically ill populations. 3

Clinical Context for Use

Medication Dosing

• The Cockcroft-Gault equation is specifically recommended because most medication dosing studies in renal failure have traditionally used this formula. 6
• Medications should be dosed according to their package inserts regarding renal function, which typically reference Cockcroft-Gault-derived creatinine clearance values. 6

Nephrotoxic Drug Monitoring

• Calculate creatinine clearance before initiating drugs with nephrotoxic potential such as tenofovir or indinavir. 6
• Baseline assessment is particularly important in black HIV-infected patients and those with advanced disease or comorbid conditions due to increased risk of nephropathy. 6