Assessment of Plasma Oxalate Concentration in Patients With CKD

Introduction: Alterations in oxalate homeostasis are associated with kidney stone disease and progression of chronic kidney disease (CKD). However, accurate measurement of plasma oxalate (P-Ox) concentrations in large patient cohorts is challenging as prompt acidification of samples has been deemed necessary. In the present study, we investigated the effects of variations in sample handling on P-Ox results and examined an alternative strategy to the established preanalytical procedures. Methods: The effect of storage time at room temperature (RT) and maintenance of samples at -80 degrees C was tested. P-Ox was measured in 1826 patients enrolled in the German Chronic Kidney Disease (GCKD) study, an ongoing multicenter, prospective, observational cohort study. Results: We demonstrate that P-Ox concentrations increased rapidly when samples were maintained at RT. This was most relevant for P-Ox <10 mu M, as concentrations more than doubled within a few hours. Immediate freezing on dry ice and storage at -80 degrees C provided stable results and allowed postponement of acidification for >1 year. In the patients of the lowest estimated glomerular filtration rate (eGFR) quartile, median P-Ox was 2.7 mu M (interquartile range [IQR] <2.0-4.2) with a median eGFR of 25.1 ml/min per 1.73 m(2) (IQR 20.3-28.1). Conclusion: We conclude that immediate freezing and maintenance of plasma samples at -80 degrees C facilitates the sample collection process and allows accurate P-Ox assessment in large cohorts. The present study may serve as a reference for sample handling to assess P-Ox in clinical trials and to determine its role in CKD progression.