≥90
60–89
30–59
15–29
<15
| Drug | Normal Dose | Recommendation |
|---|
Cockcroft-Gault Formula — Complete Guide
The Cockcroft-Gault (CG) equation, published in 1976, remains the gold standard for estimating creatinine clearance for drug dosing purposes. It was derived from 249 male patients and validated extensively across diverse populations. Despite its age, it remains the formula of choice because virtually all drug pharmacokinetic studies and prescribing information cite CG-based CrCl for dose adjustment thresholds.
× 0.85 if female
SCr in µmol/L: divide by 88.4 to convert to mg/dL first
Why Age Matters So Much
The (140 − Age) term captures the well-documented decline in GFR with ageing. Renal function peaks in early adulthood and declines at approximately 1 mL/min per year after age 40. Critically, serum creatinine may remain normal in elderly patients even with severely reduced CrCl because creatinine production falls proportionally with muscle mass. An 85-year-old woman weighing 45 kg with creatinine 0.7 mg/dL has a CrCl of only 28 mL/min — severe impairment despite a "normal" creatinine.
The Female Correction Factor (×0.85)
Women have approximately 15% less muscle mass than men of the same weight and age, producing proportionally less creatinine. The 0.85 correction factor accounts for this difference. Without it, CrCl would be systematically overestimated in women.
Choosing the Right Weight
Weight is the most contentious variable in the CG formula:
- Not obese (ABW ≤ 120% IBW): Use actual body weight
- Obese (ABW > 120% IBW): Use IBW — adipose tissue produces minimal creatinine, so ABW overestimates CrCl
- Underweight (ABW < IBW): Use actual body weight
- Oedematous/ascitic patients: Use estimated dry weight or IBW
CrCl vs eGFR — When to Use Each
These are frequently confused. CrCl (Cockcroft-Gault) gives an absolute value in mL/min — ideal for drug dosing. eGFR (CKD-EPI, MDRD) is normalised to 1.73 m² body surface area — ideal for CKD staging and population-level cardiovascular risk. For a 50 kg woman, CrCl will be lower than eGFR; for a 100 kg man, CrCl will be higher. Always use CG-CrCl when checking drug prescribing information.
Drug Dosing in Renal Impairment — Comprehensive Guide
Antibiotics
Aminoglycosides (Gentamicin, Amikacin): Highly nephrotoxic, renally cleared. Use extended interval dosing (5–7 mg/kg once daily) with therapeutic drug monitoring. Reduce dose or extend interval when CrCl <60 mL/min. Avoid if CrCl <20 mL/min unless no alternative. Check levels — trough <1 mg/L for gentamicin.
Vancomycin: Uses AUC-guided dosing (target AUC/MIC 400–600). Loading dose based on actual weight; maintenance adjusted for CrCl. At CrCl <30 mL/min, extend dosing interval to 24–48 hourly. Monitor trough levels if AUC monitoring unavailable.
Piperacillin-Tazobactam: Reduce dose at CrCl <40 mL/min. Extended infusion (4-hour) strategy improves pharmacodynamics especially in renal impairment.
Cardiovascular Drugs
Digoxin: Narrow therapeutic index, primarily renally cleared. Use IBW for dosing calculations. Loading dose same regardless of renal function; maintenance halved if CrCl <60 mL/min. Target levels 0.5–0.9 ng/mL in heart failure.
Atenolol and Sotalol: Both renally cleared — reduce dose at CrCl <35 mL/min. Sotalol requires careful QTc monitoring in renal impairment due to accumulation risk.
Anticoagulants
Enoxaparin (therapeutic): Use 1 mg/kg BD for most patients. If CrCl <30 mL/min, reduce to 1 mg/kg once daily or use unfractionated heparin with anti-Xa monitoring. Avoid LMWH if CrCl <15 mL/min.
Direct oral anticoagulants (DOACs): Apixaban is least renally cleared (27%) — can be used down to CrCl 15 mL/min with dose reduction. Rivaroxaban (33% renal) — avoid if CrCl <15. Dabigatran (80% renal) — avoid if CrCl <30.
Diabetes Medications
Metformin: Risk of lactic acidosis from accumulation. Continue at full dose if CrCl ≥60. Reduce by 50% if CrCl 45–59. Stop if CrCl <45 (NICE) or <30 (some guidelines). Always withhold before contrast procedures if CrCl <60.
SGLT2 inhibitors: Reduced glucose-lowering efficacy at low GFR. Empagliflozin and dapagliflozin approved for heart failure/CKD protection down to eGFR 20+, even without glucose-lowering effect.
NSAIDs and Analgesics
NSAIDs should be avoided in CrCl <60 mL/min — they inhibit prostaglandin-mediated afferent arteriolar vasodilation, reducing GFR acutely. In CKD, they can precipitate acute-on-chronic kidney injury. Use paracetamol as first-line analgesia in renal impairment.
Codeine: Active metabolite (morphine-6-glucuronide) accumulates in renal failure — avoid in CrCl <30. Use low-dose fentanyl or buprenorphine patches as safer alternatives in severe CKD.