DEQAS was incorporated in 1989. Its objective is to ensure the analytical reliability of 25 hydroxyvitamin D (25OHD) and 1,25 dihydroxyvitamin D (1,25(OH)2D) assays.

Features include:

CAP accredited laboratories in the USA and Canada can use DEQAS as their primary PT scheme for 25-OHD assays.


Serum is harvested from blood donated by patients undergoing therapeutic venesection for haemochromatosis or polycythaemia.

Permission to use the blood has been obtained from the local Ethics Committee and the UK Department of Health. All patients have given their informed consent.

Liquid serum pools (5) are distributed quarterly (January, April, July, October) at ambient temperature. All pools are screened for hepatitis B, C and HIV before dispatch. Performance assessment of 25OHD assays is restricted to the first 4 samples. The 5th sample is reserved for investigative purposes.

All samples are unadulterated human serum and do not contain preservative. Serum which has been artificially manipulated to give high or low 25-OHD levels, or diluted with animal sera will give spurious results in some assays and so are NEVER used to assess participant performance. We do spike for experimental and educational purposes but not for assessment of a participant's performance.

Laboratories are given approximately 5 weeks to return results. Data are statistically trimmed to produce an All-Laboratory Trimmed Mean (ALTM), SD and CV. Following the submission of results participants have on-line access to an interim report with updated statistics.

After the results deadline, the final report is prepared and participants are notified when this is available to be downloaded.

Details of DEQAS reports and their interpretation are given on this website. Go to the home page and click on 'Reports'.

The Occasional Analyst

Colleagues (eg. vitamin D researchers) whose workload would not justify full participation in DEQAS, but who wish to check the accuracy of their assays, can purchase samples as and when required. A full statistical analysis is provided. Please e-mail the DEQAS administrator for further details (

Advisory Panel

The Advisory Panel for the DEQAS was set up in 1997 and consists of scientists with acknowledged expertise in vitamin D methodology and/or external quality assessment schemes.

The panel will advise the DEQAS organiser on:

Members of the Panel are available to offer advice to participants with queries arising from their DEQAS performance or other matters related to vitamin D.

Initial contact should be made through the DEQAS Organiser (

Proficiency Certificate

A proficiency certificate is issued annually to those laboratories meeting the performance targets set by the DEQAS Advisory Panel for 25OHD* and 1,25(OH)2D** assays.

* 75% or more of the 'useable' results (generally 16) should fall within ± 25% of the Target Value (the NIST assigned value).

** 80% or more of the 'useable' results (generally 20) should fall within ± 30% of the Target Value (the All-Laboratory Trimmed Mean - ALTM).

Performance targets are periodically reviewed.