The measurement of plasma oxalate and when this is useful

Methods for measurement of plasma oxalate have been unsatisfactory for a number of reasons. First, the plasma oxalate value is normally very low, in vivo isotopic methods giving values of about 1 µ mol/1 (1, 2). Second, there have been serious indications that the plasma oxalate level rises when blood plasma samples are allowed to stand in vitro and that the higher plasma oxalate values reported by chemical methods are erroneous (3, 4). Recently we reported (5) a continuous flow ”AutoAnalyser” method for urinary oxalate with sufficient sensitivity to measure oxalate on urine samples diluted 50 fold. In the course of this work we rediscovered the observation made 50 years earlier (6) that ascorbate spontaneously converts to oxalate at alkaline pH.