Clinical vitamin B6 analysis:: An interlaboratory comparison of pyridoxal 5′-phosphate measurements in serum

Background: Recent investigations into the role vitamin B, plays in reducing risk of stroke and cardiovascular disease have heightened interest in vitamin B, intake and its relationship to clinical status indicators. Because a true reference method and certified reference materials are lacking, little is known about the relative analytical performance of clinical vitamin B, assays. Methods: Ten laboratories experienced in clinical vitamin B, analysis participated in a 3-day analysis of 69 serum and 3 aqueous specimens for pyridoxal 5'-phosphate (PLP). Laboratories used either HPLC-based or enzymatic assays. Results were analyzed for imprecision, recovery, and bias relative to consensus means. Results: Among laboratories, mean within-day CVs (3 specimens x 3 measurements/day) were 0.6%-37% and between-day CVs (20 specimens x 1 measurement/day x 3 days) were 1.4%-26%. Mean recoveries of added PLP were 53%-144%, and mean sample pool mixing recoveries were 75%-119%. Consensus means calculated for 20 serum specimens gave mean relative biases between measurement of -10.0% to 24.3% among participating laboratories over a range of 15.8-319 nmol/L PLP. Measurement imprecision and biases were evaluated against empirically derived performance criteria based on biological variation. Three of 10 laboratories met optimum imprecision requirements and had 90% or more of measurements satisfy optimum criteria for biases among methods. All 10 laboratories met minimum imprecision requirements, but 25%-53% of the results reported by 4 of the 7 suboptimal laboratories failed to satisfy the minimum criteria for bias. Conclusion: Agreement among vitamin B-6 methods is good, but large differences in laboratory proficiency exist, pointing to the need for vitamin B-6 reference materials and external quality assurance programs. (c) 2005 American Association for Clinical Chemistry.