Fecal electrolyte testing for evaluation of unexplained diarrhea: Validation of body fluid test accuracy in the absence of a reference method

BackgroundValidation of tests performed on body fluids other than blood or urine can be challenging due to the lack of a reference method to confirm accuracy. The aim of this study was to evaluate alternate assessments of accuracy that laboratories can rely on to validate body fluid tests in the absence of a reference method using the example of sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) testing in stool fluid. MethodsValidations of fecal Na⁺, K⁺, and Mg²⁺ were performed on the Roche cobas 6000 c501 (Roche Diagnostics) using residual stool specimens submitted for clinical testing. Spiked recovery, mixing studies, and serial dilutions were performed and % recovery of each analyte was calculated to assess accuracy. Results were confirmed by comparison to a reference method (ICP-OES, PerkinElmer). ResultsMean recoveries for fecal electrolytes were Na⁺ upon spiking=92%, mixing=104%, and dilution=105%; K⁺ upon spiking=94%, mixing=96%, and dilution=100%; and Mg²⁺ upon spiking=93%, mixing=98%, and dilution=100%. When autoanalyzer results were compared to reference ICP-OES results, Na⁺ had a slope=0.94, intercept=4.1, and R²=0.99; K⁺ had a slope=0.99, intercept=0.7, and R²=0.99; and Mg²⁺ had a slope=0.91, intercept=-4.6, and R²=0.91. Calculated osmotic gap using both methods were highly correlated with slope=0.95, intercept=4.5, and R²=0.97. Acid pretreatment increased magnesium recovery from a subset of clinical specimens. ConclusionsA combination of mixing, spiking, and dilution recovery experiments are an acceptable surrogate for assessing accuracy in body fluid validations in the absence of a reference method.