Multi-institutional validation study of carboplatin dosing formula using adjusted serum creatinine level

Creatinine clearance (Ccr) is widely used as a practical substitute for glomerular filtration rate (GFR) in the Calvert formula: Carboplatin dose (mg) = target area under the concentration versus time curve (AUC, mg ml^-1min) × [GFR (ml min^-1) + 25]. However, it causes systematic overdosing when the creatinine levels are measured by an enzymatic peroxidase-antiperoxidase method (PAP-Cr). We previously suggested an amended dosing formula to adjust this overdosing: Carboplatin dose (mg) = AUC (mg ml^-1min) × [adjusted Ccr (ml min^-1) + 25], where the Ccr was adjusted by adding 0.2 (mg dl^-1) to serum PAP-Cr. In this study, we prospectively validated this formula in 55 patients from six institutions. Target AUC ranged from 3 to 7 mg ml^-1min, and Ccr was measured by 24-h urine collection. Estimation of carboplatin clearance with the amended formula was unbiased [mean prediction error (MPE) ± SE = 2.9 ± 3.4%] and acceptably precise [root mean squared error (RMSE) = 24.7%], whereas the Calvert formula using non-adjusted Ccr overpredicted carboplatin clearance systematically (MPE ± SE = 24.9 ± 4.9% and RMSE = 36.1%). The improvement in the bias and precision of the estimation was seen in all of the participating institutions as shown by decrease in the absolute value of MPE and RMSE for each institution. The Chatelut formula also highly overestimated carboplatin clearance when PAP-Cr was used, but the adjustment of PAP-Cr yielded a decrease in MPE by 30.4% and in RMSE by 21.3%. These results confirmed the necessity of adjusting the serum PAP-Cr in carboplatin dosing formulas.