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

Area of Science:

• Clinical Chemistry
• Laboratory Medicine
• Endocrinology

Background:

• The Vitamin D External Quality Assessment Scheme (DEQAS) has monitored 25-hydroxyvitamin D (25-OHD) assay performance since 1989.
• DEQAS has experienced rapid growth, serving 670 participants across 35 countries as of July 2009.

Purpose of the Study:

• To assess and improve the performance of 25-OHD assays through external quality assessment.
• To track trends in inter-laboratory imprecision and accuracy for vitamin D measurements.

Main Methods:

• Distribution of five human serum samples quarterly to participating laboratories.
• Analysis of participant results to determine the All-Laboratory Trimmed Mean (ALTM) and standard deviation (SD).
• Provision of preliminary and final reports to participants via the internet and email.

Main Results:

• Significant improvement in mean inter-laboratory imprecision (CV) from 32.0% in 1994 to 15.3% in 2009.
• As of 2009, most major methods achieved results within ±7.4% of the ALTM.
• DEQAS has conducted numerous investigations into the performance of various 25-OHD methods.

Conclusions:

• DEQAS has demonstrably improved the precision of 25-OHD assays over the past 15 years.
• A gas chromatography-mass spectrometry (GC-MS) reference method is under development to further validate assay performance.
• The suitability of the ALTM as the primary target for DEQAS samples will be evaluated using the new reference method.