Overview
Why the Estimate Matters
A laboratory reported eGFR of 40 mL/min/1.73 m² is not automatically the clearance value to use for a drug.
A laboratory-reported eGFR of 40 mL/min/1.73 m² is not automatically the clearance value to use for a drug. If the patient’s body surface area is 1.5 m², the absolute estimate is approximately 40 × 1.5 ÷ 1.73 = 35 mL/min. Indexing allows kidney function to be compared across body sizes; drug clearance depends more directly on the patient’s absolute filtration. That calculation still does not settle every dosing decision. Many older product monographs were developed with Cockcroft–Gault creatinine clearance, whereas current guidance increasingly supports validated, race-free CKD-EPI eGFR for medication decisions. The NP must identify which estimate the specific drug reference requires rather than substituting one value for another without checking. The other decisive question is whether renal function is stable. eGFR equations assume a steady creatinine concentration. In sepsis, dehydration, hypotension, or evolving AKI, a calculated eGFR can look precise while lagging behind the patient’s actual clearance. Trend serum creatinine, urine output, haemodynamics, clinical status, and drug concentrations when available.