Variation in clinical vitamin D status by DiaSorin Liaison and LC-MS/MS in the presence of elevated 25-OH vitamin D₂

Abstract

Background

We compared total 25-OH vitamin D status measured by DiaSorin Liaison and tandem mass spectrometry (LC-MS/MS) among patients with high and low 25-OH vitamin D₂.

Methods

Total 25-OH vitamin D was measured in plasma containing high (> 25 nmol/l or > 50%, n = 26) and low (< 2.5 nmol/l, n = 29) 25-OH vitamin D₂ using DiaSorin Liaison and an LC-MS/MS method using NIST 972-verified calibrators. Samples were classified as vitamin D adequate (total 25-OH vitamin D ≥ 50 nmol/l), and inadequate or deficient (< 50 nmol/l) by each method. Deming and multiple linear regression were used to compare methods.

Results

Samples were significantly more likely to be classified as inadequate or deficient by DiaSorin Liaison (36%) vs LC-MS/MS (9%). This increased in the presence of high 25-OH vitamin D2 (42% vs 0%). Total 25-OH vitamin D by DiaSorin Liaison was 26.0 nmol/l lower than LC-MS/MS, which increased to 34.1 nmol/l among samples with high 25-OH vitamin D₂. This was attributed to lower recovery of 25-OH vitamin D₂ (proportional bias = 0.64 nmol/l) by DiaSorin Liaison, independent of D₃ (proportional bias = 0.86 nmol/l).

Conclusions

Patients were more likely to be classified as vitamin D inadequate or deficient by DiaSorin Liaison compared to an LC-MS/MS method, which was in part due to the presence of 25-OH vitamin D₂.

Introduction

Vitamin D₂ is the only FDA-approved form of vitamin D for pharmacologic treatment in the United States, despite it being less effective than D₃ in raising total blood 25-OH vitamin D [1]. High doses (e.g. 50 000 IU/week) are used in hospitals together with plasma 25-OH vitamin D measurement to treat frank vitamin D deficiency, which is defined by the Institute of Medicine (IOM) as a plasma 25-OH vitamin D concentration below 30 nmol/l [2]. Inadequacy is thought to occur between 30–50 nmol/l, whereas adequacy is defined as a plasma 25-OH vitamin D concentration between 50 and 125 nmol/l [2]. Adverse effects may occur > 125 nmol/l [2].

In 2004, the U.S. Food and Drug Administration (FDA) approved the DiaSorin Liaison chemiluminescence assay for clinical measurement of total 25-OH vitamin D [2]. Since then, DiaSorin Liaison has become the most popular assay in hospitals and commercial clinical laboratories. For example, in the January 2011 survey of the Vitamin D External Quality Assessment Scheme (DEQAS), DiaSorin Liaison was used by approximately 38% of all laboratories [3]. DiaSorin claims in its package insert that Liaison has 104% cross-reactivity with 25-OH vitamin D₂ compared to D₃ among samples spiked with up to 250 nmol/l of 25-OH vitamin D₂. However in DEQAS surveys, challenge samples that have lower total 25-OH vitamin D by DiaSorin Liaison compared to HPLC or liquid chromatography–tandem mass spectrometry (LC-MS/MS) often contain high 25-OH vitamin D₂ [3]. For example, in the January 2011 survey, mean 25OH vitamin D in sample 390, which contained 50% endogenous 25-OH vitamin D₂ according to DEQAS organizers, was only 50.7 nmol/l (sd = 6.3) by DiaSorin Liaison but was 71.4 nmol/l (sd = 9.0) by LC-MS/MS. This underestimation by DiaSorin Liaison has been previously reported [4], and documented in several other immunoassays including the Immunodiagnostic Systems (IDS) RIA [4], [5], DiaSorin RIA [5], [6], the withdrawn Nichols Advantage assay [4], [5], and more recently the Roche EMODULAR Analytics E170 assay [7].

Several reports highlight the impact of assay differences in the measurement of 25-OH vitamin D on clinical decision-making [8], [9]. As such, we sought to compare the prevalence of vitamin D adequacy, inadequacy and deficiency using DiaSorin Liaison and LC-MS/MS among patients who were not taking vitamin D₂, and those who were receiving vitamin D₂ supplements.