Samples for the Determination of Total 25-Hydroxyvitamin D Levels Were Stable at Ambient Temperature

Vitamin D holds important roles in human physiology, and deficiency of this vitamin is of worldwide clinical significance. Serum levels of 25-hydroxyvitamin D (25-OHD) are currently the most appropriate indicator of vitamin D status in the majority of individuals, and thus its preanalytical stability and accurate measurement in clinical laboratories are of significant interest.

We analyzed previously published data from the Vitamin D Standardization Programme commutability study and 2 further datasets: one retrospective observational study and one prospective experiment exploring additional aspects of 25-OHD stability. We analyzed each dataset through the use of robust Bayesian multilevel linear models using the {brms} package in the R statistical computing environment.

Our analyses provided evidence that 25-OHD was stable when shipped at ambient temperature, as the average difference between shipping at frozen vs ambient temperature on total 25-OHD measurements across multiple methods, laboratories, and samples [with assigned values ranging from 12.1 to 41.4 ng/mL (30.2-103.6 nmol/L)] was -0.3 ng/mL [-0.8 nmol/L; 95% credible interval (CI) -3.5-1.7 nmol/L]. Our analyses of orthogonal datasets corroborated these findings, with the estimated difference of storage for 7 days at ambient vs frozen temperature prior to shipping being 0.4 ng/mL (0.9 nmol/L; 95% CI -0.1-1.9 nmol/L).

The data and analyses we present here provide substantial evidence to support 25-OHD stability when shipped at ambient temperature and thus support the continuation of this practice as part of the Vitamin D External Quality Assurance Scheme (DEQAS) quality program.