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The determination of uric acid in urine--forgotten problems rediscovered in an external quality assessment scheme.

This article mainly describes the effects of boric acid and borates often used as bacteriostatic agents in urine collection for microbiological examination, on the results obtained in the measurement of uric acid in urine using the uricase [EC.1.7.3.3] method. The bias in results is not unidirectional, the spread of results being much larger than in urine samples not containing borate. Borate ions are also known to inhibit other enzymes such as urease [EC 3.5.1.5]. Results are presented from six national external quality assessment (EQA) surveys carried out in 2007 by INSTAND in Düsseldorf, Germany. Whereas the performance--expressed as the success rate--in samples not containing borate was acceptable (between 90 and 95%), this was not the case where borate was present (success rate 68-72%). The results also showed systematic differences for different kits which were partly due to differences in standardisation/calibration and partly due to interference by borate. Some effects of ascorbic acid and sodium azide on the determination of uric acid in urine using uricase have also been presented. The results show that care must be taken in the preanalytical phase, especially in referral laboratories, in order to prevent wrong interpretation of results due to methodological failure in the unknown presence of borate ions.