Comparison of urine oxalate measurement from paired samples acidified and non-acidified by the laboratories prior to analysis. Acidified samples were acidified after thawing by the receiving Laboratories (D2, open circles, and E, closed circles) and processed for oxalate measurement by an HPLC-based method. Data for individual sample pairs are connected by a dashed line for Laboratory D2 and continuous line for Laboratory E. Mean and SD is shown by the squares. P < 0.001 acidified versus non-acidified samples, n = 20 per group, with 10 samples per laboratory.