125I-RIA kit cannot distinguish vitamin D deficiency as well as a more specific assay for 25-hydroxyvitamin D.
OBJECTIVE
To compare a new radioimmunoassay (RIA), measuring vitamin D metabolites, against an established, competitive-protein binding assay (CPBA) that uses rat vitamin D binding protein.


METHODS
For the CPBA, we first isolated 25-hydroxyvitamin D (25(OH)D) by eluting sample through silica cartridges. For the RIA, serum was added to acetonitrile and centrifuged, followed by RIA using 125I-25(OH)D3.


RESULTS
The within-run CV was 1.5% to 4.1% by RIA, compared to 8.1% to 15.0% by CPBA. For both methods, analytical recovery was not significantly different from 100%, and both methods were appropriately linear with sample dilution. We compared RIA values (y axis) from 90 subjects with 25(OH)D ranging from 11 to 232 nmol/L by CPBA (x axis). The slope was 0.9992 (not significantly different from 1). However, the RIA exhibited a bias of 11.15 nmol/L (p < 0.05 vs 0), and a correlation coefficient of r = 0.923. When the comparison was confined to the 37 samples in the lower half of our reference range, the RIA did not compare well: slope, 0.764 (p < 0.05 vs 1.0); intercept 18.7 nmol/L (p < 0.05 vs 0); r = 0.517. For the samples below 36 nmol/L by CPBA, there was no discrimination by RIA between the 11 values below, and the 11 above the decision level (25 nmol/L by CPBA) for vitamin D deficiency.


CONCLUSIONS
We conclude that the RIA is not optimized to detect vitamin D deficiency, and for this purpose it is not a valid substitute for conventional 25(OH)D methods.
Vieth
R
R
Department of Clinical Biochemistry, Mount Sinai Hospital, Toronto, Ontario, Canada.
Chan
A
A
Pollard
A
A
D001667:Binding, Competitive; D002112:Calcifediol / Q000097:blood*; D003362:Cost-Benefit Analysis; D006801:Humans; D007457:Iodine Radioisotopes; D016014:Linear Models; D011485:Protein Binding; D011863:Radioimmunoassay; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014808:Vitamin D Deficiency / Q000175:diagnosis*
D007457:Iodine Radioisotopes; D002112:Calcifediol