COMPARISON OF MEASURED AND PREDICTED CREATININE EXCRETION IS AN UNRELIABLE INDEX OF COMPLIANCE IN PD PATIENTS

Objective: To evaluate the use of the ratio of measured to predicted c reatinine excretion as an index of compliance in peritoneal dialysis ( PD) patients. Design: A prospective analysis. Setting: Academic teachi ng hospital dialysis unit. Patients: Forty-three patients on PD. Measu rements: Creatinine excretion in daily dialysate and urine collections was measured on one occasion in 10 patients and on two occasions in 3 3 patients, and, after adding an estimate for extrarenal creatinine de gradation, was divided by predicted creatinine excretion to give a cre atinine excretion ratio, which has been proposed as an index of compli ance with exchanges in PD patients. Values above 1.24 have been sugges ted to indicate noncompliance. Lean body mass was also estimated from creatinine excretion. Results: The mean creatinine excretion ratio was 1.12, and 30% of patients had a value above 1.3. Only one patient adm itted noncompliance. Studies on four consecutive days of guaranteed co mpliance in 7 patients with high ratios showed that creatinine excreti on remained constant, suggesting that the patients were high creatinin e producers rather than noncompliant. Creatinine excretion was stable when measured at intervals of days, but over months it tended to chang e markedly in many patients. Lean body mass estimations using creatini ne excretion were low in most patients. Conclusion: Comparison of meas ured and predicted creatinine excretion is not a reliable indicator of noncompliance because many compliant patients consistently excrete mo re creatinine than predicted. The standard formulas were not validated in dialysis patients and underestimate creatinine excretion significa ntly in many PD patients. Existing estimates in the literature of nonc ompliance, using this methodology, may not be accurate. Better methods of detecting this problem are required.