What is the estimated glomerular filtration rate (eGFR) for a 78‑year‑old female with a serum creatinine of 2.0 mg/dL?

eGFR Calculation for a 78-Year-Old Female with Serum Creatinine 2.0 mg/dL

Using the Cockcroft-Gault formula with an assumed weight of 60 kg, the estimated creatinine clearance is approximately 23 mL/min, indicating Stage 4 chronic kidney disease that requires immediate medication review, nephrology referral, and preparation for potential renal replacement therapy. 1

Step-by-Step Calculation Using Cockcroft-Gault

The Cockcroft-Gault formula is the standard method for medication dosing decisions and calculates creatinine clearance (not normalized to body surface area) as follows: 1

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

Applying the Formula

Assuming a typical weight of 60 kg for an elderly female:

• CrCl = [(140 - 78) × 60] / [72 × 2.0] × 0.85
• CrCl = [62 × 60] / 144 × 0.85
• CrCl = 3,720 / 144 × 0.85
• CrCl = 25.8 × 0.85
• CrCl ≈ 22-23 mL/min 1

If the patient weighs 50 kg (lower end for elderly women), the CrCl would be approximately 18-19 mL/min. If she weighs 70 kg, it would be approximately 26-27 mL/min. 1

Alternative Calculation Using MDRD for CKD Staging

For chronic kidney disease diagnosis and staging (not medication dosing), the MDRD equation provides eGFR normalized to body surface area: 2

eGFR (mL/min/1.73 m²) = 175 × (serum creatinine)^-1.154 × (age)^-0.203 × 0.742 (if female) 2

• eGFR = 175 × (2.0)^-1.154 × (78)^-0.203 × 0.742
• eGFR = 175 × 0.449 × 0.426 × 0.742
• eGFR ≈ 25 mL/min/1.73 m² 2

This confirms Stage 4 CKD (eGFR 15-29 mL/min/1.73 m²), approaching Stage 5 (eGFR <15 mL/min/1.73 m²). 3

Critical Clinical Implications

Immediate Actions Required

• All renally cleared medications must be dose-adjusted or discontinued immediately – at this level of renal function, standard doses of most medications will accumulate to toxic levels. 1

• Review and discontinue nephrotoxic agents including NSAIDs, ACE inhibitors/ARBs (unless specifically indicated for proteinuria), aminoglycosides, and contrast agents. 1

• Urgent nephrology referral is mandatory – patients with eGFR <30 mL/min/1.73 m² require specialist evaluation for renal replacement therapy planning (dialysis or transplant). 3

Important Caveats in Elderly Patients

• The Cockcroft-Gault formula systematically underestimates true GFR in elderly patients, meaning her actual kidney function may be slightly better than calculated, but this does not change the severity classification. 1, 4

• Conversely, at this low level of renal function (CrCl ~23 mL/min), the formula may overestimate true GFR because tubular secretion of creatinine increases as kidney function declines, making the calculated value appear better than reality. 1

• Serum creatinine of 2.0 mg/dL in a 78-year-old woman represents severe renal impairment – the same creatinine in a young muscular male might indicate only mild dysfunction, but in elderly women with reduced muscle mass, it signals advanced kidney disease. 1

Weight Considerations

• If the patient is obese (BMI ≥30 kg/m²), use the mean of actual and ideal body weight in the Cockcroft-Gault formula to improve accuracy. 1

• If weight is unknown, assume 50-60 kg for an elderly woman, but obtain actual weight urgently as this significantly affects medication dosing calculations. 1

Medication Dosing Precision

• For narrow-therapeutic-index drugs (vancomycin, aminoglycosides, digoxin, lithium, chemotherapy), consider cystatin C-based equations or direct GFR measurement with exogenous markers because estimation formulas are least accurate at extremes of renal function. 1

Why Use Cockcroft-Gault Instead of MDRD/CKD-EPI?

• Cockcroft-Gault provides absolute clearance (mL/min) that aligns with pharmacokinetic studies and drug package inserts, making it the standard for medication dosing. 1

• MDRD and CKD-EPI provide normalized eGFR (mL/min/1.73 m²) designed for CKD diagnosis and staging, not drug dosing – using these for medication adjustments leads to underdosing in larger patients and overdosing in smaller patients. 1

• Most medication dosing guidelines in renal impairment were established using Cockcroft-Gault, so switching formulas introduces error. 1

Monitoring and Follow-Up

• Repeat creatinine measurement within 1-2 weeks to confirm stability versus acute kidney injury, as estimation equations are valid only at steady state. 2

• Assess for uremic symptoms including nausea, fatigue, pruritus, altered mental status, and volume overload. 3

• Measure albuminuria (urine albumin-to-creatinine ratio) to complete CKD staging and assess cardiovascular risk. 3

• Monitor serum potassium, phosphate, calcium, and hemoglobin as complications of advanced CKD include hyperkalemia, hyperphosphatemia, anemia, and metabolic bone disease. 3