Issues of standardization and assay-specific clinical decision limits for the measurement of 25-hydroxyvitamin D

Department of Core Clinical Pathology and Biochemistry Royal Perth Hospital Perth, Western Australia 6008 Australia E-mail: paul.glendenning{at}health.wa.gov.au

Dear Sir:

The thoughtful editorial by Holick (1) and the recent article by Nesby-O’Dell et al (2) emphasize the importance of vitamin D nutrition and the high prevalence of vitamin D deficiency. However, the prevalence of vitamin D deficiency will continue to vary between studies until issues of 25-hydroxyvitamin D [25(OH)D] assay standardization, assay performance, and critical decision limits have been resolved. Such analytic issues have received relatively little attention in the recent literature.

Different assays for 25(OH)D will give different results, principally because assays are not currently standardized and because there is no accepted definitive method; however, HPLC is commonly offered as a suitable reference method. The only publication that explored the issue of assay comparison concluded that 25(OH)D concentrations measured by competitive protein binding assay (CPBA) were 80% higher than those measured by HPLC; serum 25(OH)D concentrations measured by radioimmunoassay (DiaSorin; INCSTAR Corp, Stillwater, MN) gave intermediate values (3). However, not all CPBAs are the same, as exemplified by Souberbielle et al (4). Souberbielle et al noted that their in-house CPBA gave values that were 30% lower than those measured with the INCSTAR radioimmunoassay, in contrast with Lips et al (3), who documented the opposite trend. To further increase the complexity of interpretation, an automated CPBA was recently developed, as was an enzyme-linked immunosorbent assay (5). In the international comparison study by Lips et al (3), these issues translated to a 38% difference in values between laboratories. Consequently, it is hardly surprising that, because of a lack of standardization of 25(OH)D assays, different clinical decision limits are proposed by different investigators to define vitamin D sufficiency. These issues are not unique to the measurement of 25(OH)D (6). Until these issues are resolved by each assay manufacturer, concern about the appropriate use and interpretation of 25(OH)D assay results will remain. It is clear that in the absence of efforts to standardize 25(OH)D assays, assay-specific clinical decision limits are urgently needed.

REFERENCES

1. Holick MF. Too little vitamin D in premenopausal women: why should we care? Am J Clin Nutr 2002;76:3–4.
2. Nesby-O’Dell S, Scanlon KS, Cogswell ME, et al. Hypovitaminosis D prevalence and determinants among African American and white women of reproductive age: third National Health and Nutrition Examination Survey, 1988–1994. Am J Clin Nutr 2002;76:187–92.
3. Lips P, Chapuy MC, Dawson-Hughes B, Pols HA, Holick MF. An international comparison of serum 25-hydroxyvitamin D measurements. Osteoporos Int 1999;9:394–7.
4. Souberbielle JC, Lawson-Body E, Kindermans C. Authors’ response: vitamin D status and redefining serum PTH reference range in the elderly. J Clin Endocrinol Metab 2002;87:947 (letter).
5. Roth HJ, Zahn I, Alkier R, Schmidt H. Validation of the first automated chemiluminescence protein-binding assay for the detection of 25-hydroxycalciferol. Clin Lab 2001;47:357–65.
6. Stenman UH. Immunoassay standardization: is it possible, who is responsible, who is capable? Clin Chem 2001;47:815–20.