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) 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 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°C was tested. P 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 concentrations increased rapidly when samples were maintained at RT. This was most relevant for P <10 μM, as concentrations more than doubled within a few hours. Immediate freezing on dry ice and storage at -80°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 was 2.7 μM (interquartile range [IQR] <2.0-4.2) with a median eGFR of 25.1 ml/min per 1.73 m (IQR 20.3-28.1).

Conclusion: We conclude that immediate freezing and maintenance of plasma samples at -80°C facilitates the sample collection process and allows accurate P assessment in large cohorts. The present study may serve as a reference for sample handling to assess P in clinical trials and to determine its role in CKD progression.