A DEQAS review of the current performance of assays for the measurement of 1,25dihydroxyvitamin D.

The Vitamin D External Quality Assessment Scheme (DEQAS) evaluates participants' performance of analytical methods for vitamin D metabolites, including 1,25dihydroxyvitamin D (1,25(OH)2D), by distributing samples globally on a quarterly basis. In this review an assessment of the performance of current 1,25(OH)2D methods is presented. Data submitted by scheme participants for the 2024/25 DEQAS distribution cycle was analysed and trends between assays were compared. The four distributions across the 2024/25 DEQAS cycle covered a wide concentration range, challenging participants to report results across the full reference interval. Analytical performance is improving; however, variation persists between the major method groups. Participation in an accuracy-based scheme is considered the gold standard, but there is currently no reference measurement procedure available for 1,25(OH)2D. Participant performance is therefore assessed against the Method Laboratory Trimmed Mean (MLTM). To evaluate the validity of this approach, measurement uncertainty was calculated for each method and for the All Laboratory Trimmed Mean (ALTM) across all of the distributions of the 2024/25 cycle. Calculated measurement uncertainty fell within the limits of acceptability (less than 30% of standard deviation) for the ALTM, and for the MTLM of DiaSorin Liaison XL group. For method groups with fewer participants, the uncertainty of the MLTM as a target value fell outside the limits of acceptability, confirming that the ALTM offers a more robust target value. This finding supports the use of the ALTM for assessing participants' performance, with acceptable performance being defined as the ALTM ± 30%, in at least 80% of distributed samples. This performance target represents what can be achieved with the current 'state of the art' of methods for 1,25(OH)2D measurement.