How do I calculate estimated glomerular filtration rate (eGFR) from serum creatinine, age, sex, race, and weight?

Calculating eGFR from Serum Creatinine

Use the 2021 CKD-EPI equation without race for diagnosing and staging chronic kidney disease, and use the Cockcroft-Gault formula specifically for medication dosing decisions. 1, 2

Primary Calculation Methods

For CKD Diagnosis and Staging: CKD-EPI Equation (2021, Race-Free)

The 2021 CKD-EPI creatinine equation without race adjustment is the current standard for estimating GFR in adults, offering superior accuracy over older formulas and eliminating race-based bias that contributes to health disparities. 1, 2, 3

The formula structure uses:

• Serum creatinine (mg/dL)
• Age (years)
• Sex (with a 1.018 multiplier for females)
• No race coefficient 1

Clinical laboratories should automatically calculate and report eGFR (rounded to the nearest whole number as mL/min/1.73 m²) whenever serum creatinine is measured, with values <60 mL/min/1.73 m² flagged as low. 2, 3

For Medication Dosing: Cockcroft-Gault Formula

The Cockcroft-Gault equation remains the standard for medication dosing decisions because drug manufacturers and pharmacokinetic studies have historically used this formula to establish renal dosing guidelines. 2

CrCl (mL/min) = [(140 - age) × weight (kg)] / [72 × serum creatinine (mg/dL)] × (0.85 if female) 1, 2

This formula yields an absolute clearance (mL/min) rather than a body-surface-area-indexed value, which aligns with how most drug package inserts specify dosing adjustments. 2

Critical Conversion and Adjustment Factors

Laboratory Conversions

When serum creatinine is reported in µmol/L, divide by 88.4 to convert to mg/dL before applying the Cockcroft-Gault equation. 2, 3

Serum creatinine assays must be calibrated to isotope-dilution mass spectrometry (IDMS) reference methodology, achieving a coefficient of variation <2.3% and bias <3.7%. 2, 3

Enzymatic methods are strongly preferred for creatinine measurement because the Jaffe method overestimates concentrations by roughly 5-15% compared with enzymatic assays. 2, 3

Body Weight Adjustments

For obese patients (BMI ≥30 kg/m²), use the mean of actual body weight and ideal body weight in the Cockcroft-Gault formula to improve accuracy. 2

When to Add Cystatin C for Confirmation

In adults with an eGFRcr of 45-59 mL/min/1.73 m² who lack other markers of kidney damage (e.g., albuminuria), measure cystatin C to confirm CKD. 1, 2, 3

The combined creatinine-cystatin C equation (eGFRcr-cys) improves GFR estimation accuracy and correctly reclassifies approximately 17% of patients with borderline values. 1, 2

Cystatin C testing should also be considered in patients with:

• Extreme muscle mass (very high or very low)
• Severe malnutrition or obesity
• Advanced liver disease
• High-turnover malignancies
• Vegetarian diets
• Paralysis or neuromuscular diseases 2, 3

When Direct GFR Measurement Is Required

Direct measurement of GFR using exogenous filtration markers (inulin, iohexol, ¹²⁵I-iothalamate) is recommended for:

• Dosing drugs with narrow therapeutic indices (vancomycin, aminoglycosides, lithium, digoxin, chemotherapy agents)
• Extremes of age or body size
• Severe cachexia or amputation
• Glomerular diseases requiring immunosuppression decisions
• Kidney transplant donor evaluation
• Rapidly changing renal function or acute kidney injury 2, 3

Common Pitfalls and How to Avoid Them

Never Use Serum Creatinine Alone

Serum creatinine alone should never be used to assess kidney function because it markedly underestimates renal impairment, especially in older adults and individuals with low muscle mass. 2, 3

A "normal" serum creatinine of 1.2 mg/dL can represent a creatinine clearance of ~110 mL/min in a young adult but only ~40 mL/min in an elderly patient. 2

When serum creatinine significantly increases, GFR has already decreased by at least 40%, making it a late indicator of renal dysfunction. 2

Understanding Formula Limitations

The Cockcroft-Gault formula systematically underestimates true GFR in elderly patients, with the greatest discrepancy in the oldest age groups. 2

Conversely, at low levels of renal function (CrCl ~30-50 mL/min), the formula may actually overestimate true GFR because tubular secretion of creatinine increases as kidney function declines. 2

All creatinine-based clearance equations overestimate true GFR by roughly 10-40% because creatinine is both filtered and secreted by renal tubules, with greater overestimation as renal function declines. 2

Do Not Confuse Clinical Purposes

Never use MDRD or CKD-EPI equations (which provide mL/min/1.73 m²) for medication dosing, as this leads to underdosing in larger patients and overdosing in smaller patients. 2

Never use Cockcroft-Gault for CKD diagnosis or staging, as it was not designed or validated for this purpose. 2, 3

Clinical Interpretation Thresholds

An eGFR <60 mL/min/1.73 m² represents loss of half or more of normal adult kidney function and is associated with increased risk for CKD complications and cardiovascular events. 2, 3

eGFR ≥60 mL/min/1.73 m² without albuminuria does not meet criteria for chronic kidney disease. 3

For medication dosing, all renally cleared drugs require dose adjustment when creatinine clearance falls in the 30-60 mL/min range. 2