Vitamin D testing continues to have lots of varition between systems – Nov 2012

Evaluation of two automated immunoassays for 25-OH vitamin D: Comparison against LC-MS/MS.

J Steroid Biochem Mol Biol. 2012 Nov 29.
Hsu SA, Soldo J, Gupta M.
Cleveland Clinic, Department of Clinical Pathology, Cleveland, OH 44195, United States.

BACKGROUND/METHODS: A total of 95 human serum specimens, and a 12 specimen precision panel, were measured by 2 automated immunoassays (investigation use only DiaSorin LIAISON(®) 25 OH Vitamin D TOTAL Assay [LSN], and Siemens ADVIA Centaur(®) Vitamin D Total (VitD) assay [Centaur]) and the results compared against LC-MS/MS [LCMS] used as the reference method (Esoterix Inc.). For functional sensitivity and precision, 12 serum specimens [range 1.2-148ng/mL] were run in six replicates [N=30] or four replicates [N=20], respectively, for 5 consecutive days.

RESULTS: Passing-Bablok fit and Difference plot analysis [N=92] showed that although both immunoassays had comparable correlation coefficient [r] values to LCMS (0.936 and 0.933), the LSN assay results were statistically equivalent to those given by LCMS (slope 0.93, intercept -2.5), whereas the results of the Centaur assay showed overall significant assay bias compared to LCMS (slope 1.30, intercept -15.8) and this bias was more significant for doses <30ng/mL by LCMS [bias -30.4%; 95% limits of agreement -72.4% to 11.7%]. For specificity, based on 25-OHD(2) and 25-OHD(3) levels assessed by LCMS, we divided the specimens into 2 groups, one with detectable 25-OHD(2) [Group 1, N=41] and the other with no detectible 25OHD(2) [Group 2, N=51]. The 2 groups showed comparable correlation coefficient [r] values between the methods, but showed significant differences in slope: Centaur [1.48 with group 1 and 1.18 with group 2] compared to LSN [0.91 with Group 1 and 0.96 with Group 2]. LSN demonstrated better precision [total CV range 5.5-10.0%] compared to Centaur [total CV range 11.0-16.3%]. Functional sensitivity was calculated per EP-17A: 2.15ng/mL by LSN and 4.57ng/mL by Centaur.

CONCLUSIONS: Though there was good overall correlation, substantial bias was present in Centaur. Although LSN had a slope and intercept that was not significantly different than LCMS, Centaur had a significantly higher slope in specimens containing measurable 25-OHD(2) levels, a large negative intercept and a significant negative dose bias for doses <30ng/mL by LCMS, suggesting the Centaur assay would report a higher frequency of patients with apparent vitamin D insufficiency/deficiency at the low end and apparent vitamin D toxicity at the high end compared against LCMS. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Copyright © 2012 Elsevier Ltd. All rights reserved.

PMID: 23202147

Note: 100 nmol = 40 ng

Level of quantification generally 10 nmol, but one is much higher, 50 nmol

Day-to-day variation is an acceptable 7nmol for blood level of about 170 nmol

Frequently failed to correctly say blood level was > 40 nmol (false negative)

10X fewer times failed to correctly say blood level was < 40 nmol (false positive)

Ideally all colored bars in a group should be the same height

These are worst-case examples of discrepancies between readings

Sample 4: most measured about 70 nmol (28 ng), but one measured 19 nmol ( 8 ng)

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PDF is attached at the bottom of this page

See also VitaminDWiki

• Low cost vitamin D Blood Tests
• Vitamin D test results still vary by about 9 percent – March 2012
• Different measurements from 6 vitamin D testing systems – Mar 2012
• All items in category Test for D 160 items
• Quick self test of vitamin D deficiency not quantitative, but FREE

See also web

• Vitamin D blood test: Past and present debates, difficulties, and developments Vitamin D Council Jan 2013
  Review of Vitamin D Assays: Past and Present Debates, Difficulties, and Developments. Calcif Tissue Int. 2013 Jan 13. behind a $5/month paywall
  16 types of tests for vitamin D, most use Immunoassay methods