Can I use the eGFR reported on the lab form instead?

No. eGFR calculated by CKD-EPI or MDRD is used for CKD staging, not for drug dosing. Clinical pharmacokinetic studies used Cockcroft-Gault CrCl — that is the only equation validated for dose adjustment. They can differ by 15–40% in elderly and obese patients.

Should I use actual weight or ideal weight?

Use actual body weight unless the patient is obese (actual weight >30% above IBW). For obese patients, use ideal body weight — actual weight significantly overestimates CrCl, leading to dangerous overdosing of renally cleared drugs like aminoglycosides.

What weight should I use for a very underweight patient?

For underweight patients, use actual body weight (which is less than IBW). Never use IBW if it's higher than actual weight.

Creatinine Clearance (CrCl) — Free Pharmacy Calculator

🎯

Gentamicin order for an 82-year-old, 56 kg, serum creatinine 140 µmol/L. The dose on the chart looks like it's written for a 30-year-old. You need the CrCl before you dispense.

Creatinine Clearance (CrCl)

Renal

Age (years)

Sex

Weight (kg)

Imperial mode auto-converts lb→kg

Serum creatinine unit

Serum creatinine

Normal: 60–110 µmol/L

CrCl = [(140−age) × weight × F] ÷ (0.814 × SCr µmol/L) F = 1.0 for males · F = 0.85 for females
CKD Stages: ≥90 Normal · 60–89 Mild · 30–59 Moderate · 15–29 Severe · <15 Failure

💡 Use actual body weight unless obese, in which case use adjusted body weight. Always use the lower of actual or ideal body weight for elderly patients.

Reference: Cockcroft DW & Gault MH — Nephron 1976 · AMH Renal Dosing

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Estimates creatinine clearance (CrCl) using the Cockcroft-Gault equation. CrCl is the pharmacokinetic parameter used to dose-adjust renally cleared medications — aminoglycosides, vancomycin, direct oral anticoagulants, metformin, digoxin and many others.

2 Formula & professional reasoning

CrCl (mL/min) = [(140 − Age) × Weight(kg)] ÷ [72 × Serum Creatinine(mg/dL)]  ×  0.85 (if female)
AU units: Serum Creatinine in µmol/L ÷ 88.4 to convert to mg/dL

The Cockcroft-Gault equation (1976) remains the standard for renal dose adjustment in pharmacy because clinical pharmacokinetic studies use it as the reference. It uses actual body weight for most patients (ideal body weight for obese patients). The 0.85 female correction accounts for lower muscle mass. eGFR from laboratory reports should NOT be substituted for Cockcroft-Gault CrCl for dose adjustment — they can differ significantly in extreme weights.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Young healthy adult

Given: Age 28, male, 75 kg, SeCr 80 µmol/L (0.90 mg/dL)

Working: [(140−28) × 75] ÷ [72 × 0.90] = 8400 ÷ 64.8

Answer: CrCl 130 mL/min — Normal renal function

💡 No dose adjustment required for most medications.

Standard

Elderly patient — renally impaired

Given: Age 82, female, 56 kg, SeCr 140 µmol/L (1.58 mg/dL)

Working: [(140−82) × 56] ÷ [72 × 1.58] × 0.85 = 3,248 ÷ 113.8 × 0.85

Answer: CrCl 24 mL/min — Severe impairment

💡 Gentamicin: extended interval dosing or avoid. Metformin: contraindicated. Dabigatran: contraindicated. Dose reduction required for most renally cleared drugs.

Advanced

Obese patient — use IBW not actual

Given: Age 65, male, 130 kg (IBW 75 kg), SeCr 110 µmol/L (1.24 mg/dL)

Working: Use IBW 75 kg: [(140−65) × 75] ÷ [72 × 1.24] = 5625 ÷ 89.3

Answer: CrCl 63 mL/min — Mild impairment

💡 Using actual weight 130 kg would overestimate CrCl by 73%. Always use IBW for obese patients in Cockcroft-Gault.

4 Sanity check

CrCl ≥60 mL/min

Normal or mildly reduced — most drugs at full dose

Check individual drug reference for mild impairment thresholds.

CrCl 30–59 mL/min

Moderate renal impairment — dose reduction often needed

Most aminoglycosides, DOACs, antivirals, metformin need review.

CrCl 15–29 mL/min

Severe impairment — many drugs contraindicated or halved

Metformin, DOACs, most NSAIDs: avoid. Aminoglycosides: high risk.

CrCl <15 mL/min

Kidney failure — specialist review essential

Dialysis patients: CrCl-based dosing unreliable. Contact renal pharmacist.

5 Common errors

Error	Cause	Consequence	Fix
Using eGFR instead of CrCl	Lab reports display eGFR prominently	Dose adjustment errors — can differ by 20–40% from Cockcroft-Gault CrCl	eGFR (CKD-EPI) is for staging CKD. Cockcroft-Gault CrCl is for drug dosing. They are NOT interchangeable.
Using actual weight in obese patients	Default to actual body weight	CrCl significantly overestimated in obese patients	Use ideal body weight (IBW) for patients where actual weight exceeds IBW by >30%
Not adjusting for muscle mass in elderly	Serum creatinine appears 'normal'	Renal function overestimated — creatinine low due to low muscle mass not good kidneys	In elderly, frail patients a 'normal' serum creatinine can represent significant renal impairment — CrCl calculation is essential
Using US units when AU units available (or vice versa)	Unit confusion	CrCl off by factor of 88	AU labs report serum creatinine in µmol/L — divide by 88.4 to convert to mg/dL for the original equation, or use the calculator's µmol/L mode directly