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Vitamin D test results vary as much as 30 ng between test labs – Jan 2012

25-Hydroxyvitamin D Assay Variations and Impact on Clinical Decision Making.

J Clin Endocrinol Metab. 2012 Jan 11.
Barake M, Daher RT, Salti I, Cortas NK, Al-Shaar L, Habib RH, El-Hajj Fuleihan G.
Division of Endocrinology and Metabolism, Department of Internal Medicine (I.S., G.E.-H.F.); Department of Pathology and Laboratory Medicine (R.T.D., N.K.C.); Outcomes Research Unit (L.A.-S., R.H.H.); and Calcium Metabolism and Osteoporosis Program (G.E.-H.F.), American University of Beirut-Medical Center, Beirut 1107 2020, Lebanon.

Context:Laboratories are increasingly shifting to new automated 25-hydroxyvitamin D (25-OHD) assays, with subsequent variability in results.Objective/Setting:We describe the experience at our center with such a shift and illustrate its clinical implications.Methods:25-OHD levels were measured in 494 patients using Immunodiagnostic Systems RIA (IDS-RIA) and DiaSorin Liaison assays. Sources of variability between the assays were investigated in a subset of 83 samples, retested in the reference laboratory in the United States, and by reviewing the performance reports issued by the International Vitamin D External Quality Assessment Scheme, DEQAS. 25-OHD cut-points for target levels were used to compare the two assays.

Results:25-OHD concentrations were significantly lower when measured with Liaison as compared to IDS-RIA:
mean bias was -5 ng/ml,
range was -38.1 to 18.7 ng/ml, P < 0.001;
the absolute bias was independent of 25-OHD value.
Interassay variability was also detected in values obtained in the reference laboratory and in DEQAS reports.

Using 20 ng/ml as the target 25-OHD level,

  • 52% of patients required treatment when tested by Liaison, as opposed to
  • 36% by IDS-RIA (P < 0.001).

Using 30 ng/ml as the desirable level, the proportions were

  • 79 % and
  • 64 %, respectively (P < 0.001).

The two assays agreed in only 41-68% of subjects, proportions that depended on criteria used to define agreement.

Conclusion:A change in 25-OHD assays has a significant impact on results, patient classification, and treatment recommendations.
Such variability cannot be ignored when deriving and applying vitamin D guidelines.
It also renders universal assay standardization a pressing call.

PMID: 22238386
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So, when comparing your vitamin D test results to those from a different machine/lab:
Place only some trust in the most significant digit.

Example 34.9 ==> 3x.x   Could easily be 2x.x, 4x.x   and even little as 1x.x or as much as 5x.x

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