Comparison of two plasma urate assays in patients receiving vitamin C supplementation

The aim of this study was to compare plasma urate (PU) concentrations using two different assays in patients receiving vitamin C supplementation. PU was measured using two routinely available enzymatic uricase methods: (1) uric acid plus method (ascorbate oxidase assay), and (2) uric acid method (non-ascorbate oxidase assay). Twenty patients receiving allopurinol were randomised to an increase in allopurinol dose or commence vitamin C 500  mg/d on a 1:1 ratio. Twenty patients not receiving allopurinol were randomised to start allopurinol or vitamin C 500  mg/d on a 1:1 ratio. Trough fasting samples for plasma ascorbate and urate were measured weekly until week 8. There was no significant difference in the mean PU measured by the two assays. In patients not receiving supplemental vitamin C the mean PU concentrations were identical for both assays. For patients receiving supplemental vitamin C the mean PU concentrations for the ascorbate oxidase assay was 0.525  mmol/L (SE 0.034) and for the non-ascorbate oxidase assay 0.510  mmol/L (SE 0.033), p = 0.079.There is a small non-significant difference in measured PU in patients receiving supplemental vitamin C between the two assays. The assay which does not include ascorbate oxidase results in consistently lower PU concentrations compared to the assay which includes ascorbate oxidase.