Evaluation of a 1,25-dihydroxyvitamin D enzyme immunoassay.
BACKGROUND
Radioactive reagents are used in most assays for measurement of 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. We evaluated a 1,25(OH)(2)D enzyme immunoassay (EIA) from IDS Ltd. that uses solid-phase immunoextraction and colorimetric detection and compared results to those of the thymus radioreceptor assay (RRA) for 1,25(OH)(2)D.


METHODS
We collected serum samples (n = 145) representing an even distribution (0-200 pmol/L) of 1,25(OH)(2)D concentrations and Vitamin D External Quality Assessment Scheme (DEQAS) proficiency survey samples from 2004 surveys (n = 15) and stored them at -20 degrees C. We analyzed all samples with both EIA and RRA methods. We calculated imprecision using 5 QC samples in quadruplicate in each run (n = 6), including both pooled patient material used for QC with the RRA and QC material included in the EIA reagent set. We evaluated calibration stability by analyzing calibrators from different lots on the same plate and determining if calculated sample values drifted significantly.


RESULTS
Deming linear regression between IDS EIA and RRA methods yielded slope 1.25 (95% CI 1.13-1.37), y-intercept -3 (95% CI -18 to 12), R(2) = 0.74. DEQAS proficiency survey samples for 2004 were all within 30% of the all-methods-trimmed mean. Imprecision CVs were 12%-16% within-run and 15%-20% between-run.


CONCLUSIONS
We find no evidence of inferiority to the classic calf-thymus receptor assay for 1,25(OH)(2)D and no disadvantage in the results generated by the IDS EIA using samples from the major proficiency survey for 1,25(OH)(2)D. According to the product insert, however, the IDS EIA underestimates 1,25(OH)(2)D(2) compared with the D(3) form.
Seiden-Long
Isolde
I
Department of Laboratory Medicine and Pathobiology, University of Toronto Pathology and Laboratory Medicine, Mount Sinai Hospital, Toronto, Canada.
Vieth
Reinhold
R
D000818:Animals; D002417:Cattle; D003124:Colorimetry; D006801:Humans; D007124:Immunoenzyme Techniques / Q000379:methods; D066298:In Vitro Techniques; D016014:Linear Models; D011869:Radioligand Assay; D018167:Receptors, Calcitriol / Q000737:chemistry / Q000378:metabolism; D013950:Thymus Gland / Q000378:metabolism; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood / Q000737:chemistry
D018167:Receptors, Calcitriol; D014807:Vitamin D; C097949:1,25-dihydroxyvitamin D