Plasma vitamin C assays: a European experience. EC FLAIR Concerted Action No. 10: Micronutrient Measurement, Absorption and Status.

Assay procedures for plasma concentrations of vitamin C, and hence for vitamin C status, currently in use in European population-surveillance laboratories and elsewhere, are based on a wide range of disparate techniques and reactions. The problem of achieving harmonisation between these techniques, and between laboratories, is further complicated by the instability of the vitamin, and the existence of several chemical forms. In the course of a European Community FLAIR programme, of inter-laboratory assay method comparisons for micronutrients, nine European laboratories performed either one or two coordinated ring tests, and some coordinated checks on in-house plasma samples, spiked with a low level of ascorbic acid. The principal conclusion was that good agreement was difficult to obtain, between laboratories and between methods, at plasma vitamin C concentrations close to the cut-off for biochemical deficiency, i.e. ca. 11 microM vitamin C in untreated plasma. By the second ring test, and after the correction of some aberrant results due to malfunctioning apparatus, interlaboratory coefficients of variation of 13-20% were achieved, within the biochemically "normal" range of 36-94 microM. There remains, however, a need for critical within and between-laboratory checks and comparisons, and for new quality control materials, with assigned values.