Proficiency testing of 25-Hydroxyvitamin D (25-OHD) assays

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Abstract

The Vitamin D External Quality Assessment Scheme (DEQAS) has been monitoring 25-OHD assay performance since 1989. The scheme has expanded rapidly in recent years and has 670 participants in 35 countries (July 2009). Five samples of human serum are distributed quarterly and the results analyzed to give an All-Laboratory Trimmed Mean (ALTM) and SD. Each participant has internet access to a preliminary report after submission of results and, following the results deadline, a final report is e-mailed to designated staff in each laboratory. The last 15 years has seen an improvement in mean inter-laboratory imprecision (CV), from 32.0% (1994) to 15.3% (2009) and most major methods are now giving results within plus or minus 7.4% of the ALTM (2009). DEQAS has regularly conducted and reported on a number of investigations into the performance of 25-OHD methods. A gas chromatography–mass spectrometry (GC–MS) reference method for 25-OHD is under development and will be used to assess whether the ALTM remains the most appropriate target for DEQAS samples.

Introduction

The International Vitamin D External Quality Assessment Scheme (DEQAS) was established in 1989 after several studies highlighted the poor performance of 25-OHD assays [1], [2], [3]. The scheme has expanded rapidly in recent years (Fig. 1) and has 670 participants in 35 countries (July 2009). The relatively large number of samples and frequency of distribution (5 samples quarterly) fulfills requirements in the US of the Clinical Laboratories Improvement Act (CLIA) for endocrinology tests. Although proficiency testing of 25-OHD assays is not compulsory, over 250 US laboratories have voluntarily registered with DEQAS (July 2009) and its importance in monitoring the performance of 25-OHD assays has recently been acknowledged [4].

The primary aim of DEQAS is to monitor the performance of individual laboratories but the data can also be used to assess the performance of the methods used. Help is available to participants and manufacturers in the evaluation and trouble-shooting of new and existing methods. A DEQAS Advisory Panel was established in 1997 and comprises scientists with acknowledged expertise in the vitamin D field and/or proficiency testing schemes. The Panel sets a performance target and participants achieving acceptable performance over a distribution cycle (1 year) receive a certificate. The target, which is reviewed annually, currently requires 80% or more results to be within 30% of the All-laboratory Trimmed Mean [5].

Section snippets

Sample preparation/transport

Serum is harvested from blood donated with informed consent of patients with haemochromatosis and polycythaemia attending the Oncology outpatient clinic at Charing Cross Hospital (Imperial College Healthcare NHS Trust, London, UK). Permission for DEQAS to use the blood was obtained from the local Ethics Committee and is in accordance with the UK Department of Health guidelines.

Blood is collected into plain polythene bags and left to clot overnight at 4 °C. The serum is transferred to sterile

Results

Fig. 1 shows the increase in DEQAS participant numbers over the last 20 distributions, from 141 in October 2004 to 670 in July 2009. Numbers have approximately doubled since January 2008 when the number of registered participants was 331. The proportion of participants submitting results has remained fairly constant, with a mean return rate of 83.5%.

There has been a reduction in inter-laboratory imprecision (Fig. 2) over the last 15 years, from 32% in 1994 to 15.3% in 2009 (April and July). The

Discussion

DEQAS is the only specialist international Proficiency Testing Scheme for 25-OHD. Its rapid expansion over the last few years has probably resulted from an explosion of interest in vitamin D and the addition of 25-OHD assays to the repertoire of many more routine clinical laboratories.

Whilst the overall performance of 25-OHD assays has improved (better agreement between laboratories and methods), 25-OHD remains a difficult analyte to measure.

Automated methods, which have necessarily abandoned

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