Estimation of creatinine clearance in patients with Cushing's syndrome

The predictive value of the Cockcroft-Gault equation in patients with Cushing's syndrome was evaluated in 23 patients. Patients were subdivided based on total body weight into two groups, obese and nonobese. Estimated creatinine clearance (ECl(cr)) values were obtained by the Cockcroft-Gault method using ideal body weight (IBW) and total body weight (TBW). These values were then compared with a 24-hour measured creatinine clearance (MCl(cr)). ECl(cr) values based on TBW consistently overestimated measured values in all patients (p<0.05). In obese patients with Cushing's syndrome IBW predictions were not statistically different. However, linear regression analysis revealed a poor correlation (r=0.32). Daily creatinine production rates (U(cr)) were calculated and contrasted with an appropriate historical control for obese and nonobese subjects. Nonobese patients revealed a marked reduction in total U(cr) compared with normal-weight controls (p<0.05). Obese patients also showed a reduction in U(cr) when compared with a normal obese control population (p<0.05). Difficulty in predicting creatinine clearance in patients with Cushing's syndrome appears to be related to alterations in U(cr). These data suggest that the pathophysiologic changes that accompany Cushing's syndrome are sufficient to alter U(cr) and may limit the usefulness of existing methods to predict creatinine clearance and renal function in these patients.