Correlation among 25-hydroxy-vitamin D assays

Context: Measurement of circulating 25-hydroxy-vitamin D [ 25(OH) D]) is the accepted clinical indicator of vitamin D status. However, between-laboratory differences in measurement of this analyte exist, which may confound clinical care. Objectives: We investigated the current agreement of 25(OH) D measurement in clinical laboratories and explored the possibility that simple calibration would improve between-laboratory agreement. Design and Participants: Serum obtained from healthy volunteers (age 20-60 yr) and one "calibrator," selected to have a 25(OH) D value near 30 ng/ml, were sent for 25(OH) D measurement in four clinical laboratories (laboratories A-D) using HPLC, liquid chromatography tandem mass spectroscopy, and RIA methodologies. Main Outcome Measures: Serum 25(OH) D. Based upon self-report, the laboratory with the lowest interassay percent coefficient of variation was assigned as the reference to which the others were compared using linear regression and Bland-Altman analyses (Analyse-it; Analyse-it Software, Ltd., Leeds, UK). Results: Good correlation was observed for 25(OH) D measurement between laboratory A and laboratories B-D(R-2 = 0.99, 0.81, and 0.95, respectively). Modest between-laboratory variation was noted; the mean bias ranged from 2.9-5.2 ng/ml. Consistent with a systematic offset, each value in laboratory B was higher than in laboratory A, and 89% of values from laboratories B-D were higher than laboratory A. The use of a single calibrator and correction factor reduced mean between-laboratory bias for laboratories B and D. Conclusions: Measurement of 25(OH) D by clinical laboratories yields similar results. The use of even a single calibrator will improve, but not resolve, between-laboratory variability. Based upon these data, in combination with reported within-individual variability, we recommend that clinicians aim for values greater than 30 ng/ml in their patients.