Comparison of LC-MS/MS and CLIA Methods for Vitamin D Measurement in an Unselected Outpatient Cohort: Statistical Evaluation and Impact on Clinical Classification.

BACKGROUND

This study aimed to compare a chemiluminescence immunoassay (CLIA; Siemens Atellica IM 1600) with an automated liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyser (Thermo Scientific Cascadion SM) for serum 25-hydroxyvitamin D [25(OH)D] measurement in unselected outpatients, and its impact on clinical classification.

METHODS

25(OH)D was measured on 1064 paired serum samples (828 women, 236 men; age 3-111 years); both platforms participated in the Vitamin D Standardisation Program (VDSP). Agreement was assessed by the Wilcoxon signed-rank test, Passing-Bablok regression, Bland-Altman analysis on log-transformed values, Breusch-Pagan testing for heteroscedasticity, consistency-based intraclass correlation coefficient (ICC), Lin's concordance correlation coefficient, and Cohen's kappa with McNemar's test on six- and two-category clinical classifications.

RESULTS

Methods differed significantly (p<2.2×10-16). Passing-Bablok regression showed proportional bias (slope 1.33; 95% confidence interval [CI], 1.28-1.38; LC-MS/MS higher). Consistency ICC was 0.928 (95% CI, 0.919-0.936). Log-Bland-Altman bias was 35.5% (limits of agreement, -18.3% to +124.5%; heteroscedasticity, p=5.18×10-11). Cohen's kappa was 0.32 unweighted and 0.72 weighted on six clinical categories, and 0.56 on a two-class scheme (McNemar p<2.2×10-16). In discordant pairs, CLIA classified deficiency 118-fold more often than LC-MS/MS.

CONCLUSIONS

In our study, despite preserved rank ordering, a non-linear concentration-dependent bias and different limits of agreement caused systematic differences in the classification of vitamin D deficiency. Adoption of automated LC-MS/MS can substantially reclassify outpatients toward sufficiency, with implications for supplementation and laboratory harmonisation.