Abstract
BACKGROUND: Reference measurement procedures (RMP) have rigorous accuracy specifications. For total 25-hydroxyvitamin D, 25(OH)D, bias ≤1.7% and CV ≤5% are recommended. These quality specifications are impractical for minor analytes, such as 25(OH)D(2). Furthermore, documentation on RMP quality performance specifications for the individual 25(OH)D metabolites and their daily application are missing. METHODS: To assess accuracy, we used zeta-scores. Daily, 5-10 specimens (duplicate) and 3 reference materials (singleton or duplicate) were measured for 25(OH)D(3) and 25(OH)D(2) using JCTLM-accepted LC-MS/MS RMPs. Protocols were repeated on 3-4 occasions to generate campaign results. We used separate zeta-score acceptability criteria for daily (≤|2|) and campaign (≤|1|) evaluations. Allowable imprecision was determined experimentally. RESULTS: Across 7 campaigns, unacceptable daily zeta-scores required repeating 2 runs for 25(OH)D(3) and 5 runs for 25(OH)D(2). Hence, the zeta-scores of acceptable reference material results indicated high accuracy. The allowable imprecision for the RMPs was ≤5% (daily) and ≤ 3% (campaign) for 25(OH)D(3) and ≤ 7% (daily) and ≤ 4% (campaign) for 25(OH)D(2), respectively. CONCLUSIONS: Using zeta-scores and experimentally derived imprecision, we developed a straightforward approach to assess the acceptability of individual 25(OH)D reference measurements, providing also much-needed practical accuracy specifications for 25(OH)D(2).