Medical Calculators for Pharmacists

Use Cockcroft-Gault (CrCl) for virtually all drug dosing decisions — it is the equation used in the pharmacokinetic studies that established drug dosing guidelines. CKD-EPI is preferred for CKD staging and disease monitoring. The two equations can yield meaningfully different values, especially at extremes of weight or age. For obese patients, use adjusted body weight (ABW = IBW + 0.4 × [actual − IBW]) in Cockcroft-Gault rather than actual body weight to avoid overestimating renal function.

Per 2020 ASHP/IDSA/SIDP guidelines, AUC/MIC-guided monitoring (target AUC 400–600 mg·h/L for serious MRSA infections, assuming MIC ≤1 mg/L) is recommended over trough-only monitoring. Two-level Bayesian pharmacokinetic programs provide the most accurate AUC estimation. Obtain levels approximately 1–2 hours post-infusion (peak) and pre-dose (trough) after steady state (~4 doses). If Bayesian tools are unavailable, trough-only monitoring with a target of 15–20 mg/L is acceptable for serious infections. Reassess dosing with any significant change in renal function.

Key thresholds by CrCl: <90 — begin monitoring renally-cleared drugs; <60 — dose-reduce metformin, nitrofurantoin contraindicated, dose-adjust levofloxacin; <50 — reduce pemetrexed, capecitabine, dose-adjust gabapentin; <30 — reduce meropenem, avoid NSAIDs, metformin contraindicated, significant aminoglycoside adjustment; <15 — avoid most renally-cleared drugs or use dialysis-specific dosing. Always reference the most current drug monograph — thresholds vary by agent and new guidance emerges regularly.

BSA-based dosing (mg/m²) normalizes systemic exposure across body sizes for cytotoxic agents. Mosteller formula (BSA = √[height(cm) × weight(kg) ÷ 3600]) is most widely used. For obese patients (BMI >30), many protocols cap BSA at 2.0–2.2 m² to limit toxicity, though evidence for capping is weak and may result in underdosing. Carboplatin uses the Calvert formula (dose = AUC × [GFR + 25]) which directly incorporates renal function rather than BSA. Always cross-reference institutional oncology protocols.

High-risk QT-prolonging drugs include antiarrhythmics (amiodarone, sotalol, dofetilide), antipsychotics (haloperidol, quetiapine), antibiotics (azithromycin, fluoroquinolones, fluconazole), antiemetics (ondansetron, domperidone), and methadone. Baseline QTc should be obtained before initiating these agents. Action thresholds: QTc >500 ms or increase >60 ms from baseline — consider dose reduction or discontinuation. Risk is compounded by hypokalemia, hypomagnesemia, bradycardia, and polypharmacy. Use CredibleMeds QTc risk categories for drug interaction assessment.

Glucocorticoid equivalent anti-inflammatory doses (relative to hydrocortisone 20 mg = prednisone 5 mg = methylprednisolone 4 mg = dexamethasone 0.75 mg = betamethasone 0.75 mg = triamcinolone 4 mg). These are anti-inflammatory equivalences only — mineralocorticoid potency, half-life, and tissue penetration differ significantly. Dexamethasone has ~25× the anti-inflammatory potency of hydrocortisone with negligible mineralocorticoid effect. When converting, consider the clinical indication, route of administration, and the need for HPA axis recovery time when tapering.