PLASMA VITAMIN-C ASSAYS - A EUROPEAN EXPERIENCE

Assay procedures for plasma concentrations of vitamin C, and hence for vitamin C status, currently in use in European population surveillanc e laboratories and elsewhere, are based on a wide range of disparate t echniques and reactions. The problem of achieving harmonisation betwee n these techniques, and between laboratories, is further complicated b y the instability of the vitamin, and the existence of several chemica l forms. In the course of a European Community FLAIR programme, of int er-laboratory assay method comparisons for micronutrients, nine Europe an laboratories performed either one or two coordinated ring tests, an d some coordinated checks on in-house plasma samples, spiked with a lo w level of ascorbic acid. The principal conclusion was that good agree ment was difficult to obtain, between laboratories and between methods , at plasma vitamin C concentrations close to the cut-off for biochemi cal deficiency, i.e. ca. II mu M vitamin C in untreated plasma. By the second ring test, and after the correction of some aberrant results d ue to malfunctioning apparatus, interlaboratory coefficients of variat ion of 13-20% were achieved, within the biochemically ''normal'' range of 36-94 mu M. There remains, however; a need for critical within and between-laboratory checks and comparisons, and for new quality contro l materials, with assigned values.