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Interference of creatinine measurement in CAPD fluid is dependent on glucose and creatinine concentrations
T W Mak1, C K Cheung, C M Cheung
1Department of Chemical Pathology, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong.
Background:
High glucose concentration in CAPD fluid is known to interfere with creatinine measurement, which is required for assessment of peritoneal membrane permeability and adequacy of dialysis. Correction formulae have been proposed but they may be method/analyser-dependent. We studied such interference in detail.
Methods:
CAPD fluid was diluted to prepare six specimens with glucose concentrations ranging from 9.1 to 154.4 mmol/l. To each specimen, creatinine standard was added to give five different concentrations from 50 to 800 mumol/l. The 30 specimens were assayed for creatinine with six routine clinical chemistry analysers (Hitachi 911 and 747, Technicon RAXT and SMAC3, Beckman CX7, and Kodak Ektachem 700). Creatinine interference was calculated by subtracting the apparent creatinine concentration with corresponding baseline creatinine concentration (measured at glucose = 9.1 mmol/l) in the same series.
Results:
At constant creatinine concentration, interference increased with increasing glucose concentration to varying extents (up to 200%) amongst the six analysers. At constant glucose concentration, interference decreased with increasing creatinine concentration in analysers using the alkaline picrate reaction but increased in the Kodak analyser using enzymatic assay.
Conclusion:
Interference of creatinine measurement in CAPD fluid is dependent on glucose and creatinine concentrations, and each centre should derive specific correction formulae for its analytical system.