Interlaboratory method-dependent variability may impact reimbursement policies based on a priori determined...

Vitamin D supplementation is widely used in clinical practice and its administration is based on circulating 25-hydroxyvitamin D [25(OH)D] concentrations. However, 25(OH)D is an analytically challenging biomarker, and the influence of interlaboratory method-dependent variability on the routine applicability of fixed biochemical cut-offs as the only criterion for reimbursements of vitamin D supplementation remains insufficiently defined. We aimed to quantify interlaboratory analytical variability in 25(OH)D measurements using External-Quality-Assessment (EQA) data from the Lombardy region healthcare system.

Retrospective analysis was performed on External Quality Controls data collected through EQA program run by Lombardy Regional Coordination Center for Laboratory Medicine during 2024-2025. Cross-laboratory control assessment utilized statistical measures including mean, coefficients of variation (CVs%), and platform-to-platform analytical comparisons.

Over a two-year period, a total of 2,747 individual 25(OH)D results from approximately 114 laboratories per EQA-round were analyzed. Mean reported 25(OH)D concentration was 28.68 (SD:11.28) ng/mL, with values ranging from 16.94 to 72.57 ng/mL. Across all analytical platforms evaluated, substantial dispersion of results was observed (p < 0.001). Interlaboratory variability was substantial, with CVs ranging from 18.10 to 45.35% and a mean of 29.98% (5.96). Notably, analytical variability remained consistently high across clinically used cut-offs (< 30 and ≥ 30 ng/mL), with similar mean CVs (29.98% (5.85) vs. 29.97% (6.58), respectively; p = 0.996).

In real-world healthcare settings, interlaboratory analytical variability of 25(OH)D EQA measurements is high and independent of clinically adopted thresholds. These findings suggest that approaches to vitamin D administration and reimbursement based on the general application of a priori determined rigid 25(OH)D threshold levels might result methodologically fragile, supporting a greater role for clinical judgment in decisions regarding vitamin D supplementation.