Variability in vitamin D assays impairs clinical assessment of vitamin D status

Authors

  • J. K. C. Lai,

    Corresponding author
    1. National Centre for Epidemiology and Population Health, The Australian National University, Canberra, Australian Capital Territory
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  • R. M. Lucas,

    1. National Centre for Epidemiology and Population Health, The Australian National University, Canberra, Australian Capital Territory
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  • E. Banks,

    1. National Centre for Epidemiology and Population Health, The Australian National University, Canberra, Australian Capital Territory
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  • A.-L. Ponsonby,

    1. Murdoch Childrens Research Institute, Royal Children's Hospital, Melbourne, Victoria, Australia
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  • Ausimmune Investigator Group

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    • The Ausimmune Investigator Group includes: Dr Caron Chapman, Professor Alan Coulthard, A/Professor Keith Dear, Professor Terry Dwyer, Professor Trevor Kilpatrick, A/Professor Robyn Lucas, Professor Tony McMichael, Professor Michael P Pender, Professor Anne-Louise Ponsonby, A/Professor Bruce Taylor, Dr Patricia Valery, Dr Ingrid van der Mei, Dr David Williams.


  • Funding: Funding for the Ausimmune Study was provided by the National Multiple Sclerosis Society of the United States of America, the National Health and Medical Research Council of Australia and Multiple Sclerosis Research Australia. A/Professor Lucas is supported by a Multiple Sclerosis Research Australia Fellowship and the Royal Australasian College of Physicians Cottrell Fellowship. Professor Banks is supported by an Australian National Health and Medical Research Council Senior Research Fellowship.

  • Conflict of interest: None.

Jeffrey K. C. Lai, National Centre for Epidemiology and Population Health, The Australian National University, Canberra, ACT 0200, Australia. Email: jeffrey.lai@anu.edu.au

Abstract

Background:  Measuring serum 25(OH)D concentration is common in clinical practice despite the questionable reliability of assays.

Aims:  The aim of the present study was to examine agreement in 25(OH)D concentrations measured by different assays and laboratories, and consider related clinical implications.

Methods:  Serum samples from 813 participants in the Australian Multicentre Study of Environment and Immune Function (the Ausimmune Study) were assayed for 25(OH)D concentration. Duplicate samples from subsets of subjects were sent to different laboratories, two using DiaSorin Liaison (Laboratory A and B) and one using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS – selected here as the nominal gold standard). Pairwise within-assay (both within-laboratory and between-laboratories) and between-assay agreement was examined using Deming regression and Bland-Altman plots. Common 25(OH)D cut-points for classification of vitamin D deficiency were used to compare the different assays.

Results:  25(OH)D concentrations measured using Liaison were substantially lower at Laboratory A than at Laboratory B (mean bias −11.60 nmol/L, 95% limits of agreement −46.39, 23.18). Both Liaison assays returned much lower 25(OH)D concentrations than LC-MS/MS (mean bias up to −26.05 nmol/L, 95% limits of agreement of −13.21, 65.31). For Laboratory A participants, 46% (355/765) were classified as vitamin D deficient (25(OH)D <50 nmol/L) using Liaison compared with 17% (128/765) using LC-MS/MS. For Laboratory B participants, the respective figures were 36% (76/209) and 20% (41/209). Hence, between 1-in-5 and 1-in-3 participants were misclassified as ‘deficient’.

Conclusion:  Bias and variability in 25(OH)D measurements sufficient to affect significantly clinical decision-making were found both between-laboratories and between-assays. The adoption of common standards to allow assay calibration is required urgently.

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