Renal Dosing: GFR-Based Adjustments

Renal impairment alters drug pharmacokinetics across all four ADME phases but most significantly affects elimination. Drugs and their metabolites that are primarily renally excreted accumulate in kidney disease, increasing the risk of dose-dependent toxicity. The GFR (estimated using CKD-EPI equation incorporating creatinine, age, sex, and race) serves as the primary guide for dose adjustment. For renally eliminated drugs, two strategies are used: dose reduction (maintaining dosing interval but reducing each dose) or interval extension (maintaining dose but lengthening the time between doses). The choice depends on whether the drug's efficacy is concentration-dependent (peak-dependent — aminoglycosides: extend interval to maintain high peaks) or time-dependent (maintain levels above MIC for longer — beta-lactams: reduce dose or increase frequency). Beyond elimination, renal failure affects protein binding (uremic toxins displace drugs from albumin, increasing free drug fraction — important for phenytoin, warfarin), volume of distribution (fluid overload increases Vd for hydrophilic drugs), and drug metabolism (CYP3A4 and intestinal P-glycoprotein activity are reduced in CKD). The Cockcroft-Gault equation is still used by many drug manufacturers for dosing recommendations because FDA labeling studies used it, even...