How is kidney function assessed for drug dosing determination?
The best way is to estimate the GFR. The gold standard is measurement of the clearance of inulin; however, this is cumbersome and impractical for clinical use. Measurement of the 24-hour creatinine clearance (CrCl) is no longer recommended for similar reasoning. This has led to the development of equations to estimate GFR such as the Cockcroft-Gault (CG) and Modification of Diet in Renal Disease (MDRD) study.
These equations use serum creatinine as one of the variables and both generally provide similar dosing recommendations. Adverse events such as drug accumulations are relatively uncommon when the GFR remains >50 mL/min.
It is still important to consider potential analytic interferences in these calculations based on the concurrent drug therapy. Some drugs may artifactually increase or decrease the measured serum creatinine concentration without directly influencing GFR. Drugs that inhibit the tubular secretion of creatinine will raise the serum level (e.g., trimethoprim, cimetidine, and probenecid).
