Measurement of Vitamin D metabolites: an international perspective on methodology and clinical interpretation.

The International Quality Assessment Scheme for Vitamin D metabolites (DEQAS) was introduced in 1989. Initially, the aim was to improve the reliability of 25-hydroxyvitamin D (25-OHD) assays but the scheme was extended in 1997 to include 1,25-dihydroxyvitamin D (1,25(OH)(2)D). DEQAS has 95 members in 18 countries (January 2003). Five serum samples are distributed quarterly and participants are given up to 6 weeks to return their results for statistical analysis. The majority of participants use commercial kits for both analytes. A performance target was set by an advisory panel in 1997 and, at present, requires participants to get 80% or more of their results within +/-30% of the All-Laboratory Trimmed Mean (ALTM). The performance targets are under continual review. In 2003, 59% of participants met the target (cf. 52% in 2000). A questionnaire, distributed in January 2003, requested information on methods and the interpretation of results. Reference ranges varied but there was reasonable agreement on the 25-OHD concentrations below which Vitamin D supplementation was advised. A minority (22%) of respondents was unsure whether Vitamin D(3) or Vitamin D(2) was used to treat patients in their locality. The majority (52%) of assays for 1,25(OH)(2)D were done 'on demand' and others for apparently spurious reasons. Most respondents thought participation in DEQAS extremely important and the planned introduction of on-line reporting should enhance its value.