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

Sies, Christiaan W.1; Florkowski, Christopher M.1; Frampton, Christopher M. A.2; O’Donnell, John L.3; Chapman, Peter T.3; Stamp, Lisa K.2,3

doi: 10.1097/PAT.0000000000000095
Chemical Pathology

Summary: 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.

1Clinical Biochemistry Unit, Canterbury Health Laboratories, Christchurch

2Department of Medicine, University of Otago, Christchurch

3Department of Rheumatology, Immunology and Allergy, Christchurch Hospital, Christchurch, New Zealand

Address for correspondence: Professor L. Stamp, Department of Medicine, University of Otago, Christchurch, PO Box 4345, Christchurch, New Zealand. E-mail: lisa.stamp@cdhb.health.nz

Received 20 August, 2013

Revised 27 November, 2013

Accepted 10 December, 2013

© 2014 Royal College of Pathologists of Australasia
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