When determining the carboplatin dosage from the Calvert formula, there are a lack of data when evaluating patients with cachexia or body mass index (BMI)>or=27. If the Cockcroft and Gault (C-G) creatinine clearance (CrCl) equation is utilized and substituted for glomerular filtration rate in the Calvert formula, the chance for inaccurate dosing occurs especially in these populations. Therefore, the purpose of this study is to evaluate and compare the target carboplatin area under the concentration (AUC) with the actual AUC achieved in cachectic or BMI>or=27 patients. In a prospective manner, we evaluated 19 patients with a BMI>or=27 and nine cachectic patients. In the C-G equation to determine creatinine clearance, the adjusted body weight was used for BMI>or=27 patients and serum creatinine value of 70.7 microM (0.8 mg/dl) for the cachectic patients. The carboplatin dose was calculated, administered to the patients, and subsequent carboplatin blood samples were obtained for pharmacokinetic determination. Once the AUC was determined, the results were compared with the expected outcomes from the modified C-G CrCl equation for the Calvert formula, Chatelut and Bénézet equations. The results demonstrated that the modified C-G CrCl equation for the Calvert formula had less bias and more precision than using actual weight in the C-G CrCl equation or using the Chatelut and Bénézet equations. Using the actual weight in overweight and especially obese patients and using a serum creatinine<70.7 microM in cachectic patients will lead to overestimation of the carboplatin clearance and thus AUC.