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Correlation among 25-Hydroxy-Vitamin D Assays

University of Wisconsin Osteoporosis Clinical Center & Research Program, University of Wisconsin, Madison, Wisconsin 53705

Address all correspondence and requests for reprints to: Neil Binkley, M.D., 2870 University Avenue, Suite 100, Madison, Wisconsin 53705. E-mail: nbinkley{at}wisc.edu.

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 (R² = 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.