MEASUREMENT OF CREATININE IN PERITONEAL-DIALYSIS FLUID

The peritoneal equilibration test (PET) is a standardized procedure to assess peritoneal membrane performance in patients on continuous ambulatory peritoneal dialysis (CAPD). The results of this test allow clinicians to prescribe the most suitable of the CAPD regimes now available, and are also helpful in establishing the cause of dialysis failure. The PET lasts for 4 h, during which dialysate samples are taken at 0 h, 2 h and 4 h for glucose and creatinine measurement, and a blood sample at 2 h for creatinine measurement. The dialysates have very high glucose concentrations (up to 125 mmol/L) which cause interference in the Jaffe reaction used to estimate creatinine. Awareness of this effect led Twardowski to quote a correction equation of the form: Corrected creatinine = Apparent creatinine - F x glucose. A value for the factor F was provided by the laboratory analysing his PET samples. A similar factor (0.853, when creatinine concentrations are in μmol/L and glucose in mmol/L) is also given in worksheets provided by Baxter Healthcare for the recording of PET results, in which it is suggested that the quoted factor may be used if none is provided by the laboratory analysing the samples. In our experience this resulted in negative creatinine concentrations in some cases and we questioned the validity of the indiscriminate use of this factor. To study variations in the extent of glucose interference between different analysers and methods, identical samples from one patient's PET were analysed in five laboratories by a variety of creatinine methods.