Assessment of serum total 25-hydroxyvitamin D assay commutability of Standard Reference Materials and College of American Pathologists Accuracy-Based Vitamin D (ABVD) Scheme and Vitamin D External Quality Assessment Scheme (DEQAS) materials: Vitamin D Standardization Program (VDSP) Commutability Study 2

An interlaboratory study was conducted through the Vitamin D Standardization Program (VDSP) to assess commutability of Standard ReferenceMaterials (R) (SRMs) and proficiency testing/external quality assessment (PT/EQA) samples for determination of serum total 25-hydroxyvitamin D [25(OH)D] using ligand binding assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). A set of 50 single-donor serum samples were assigned target values for 25-hydroxyvitamin D-2 [25(OH)D-2] and 25-hydroxyvitamin D-3 [25(OH)D-3] using referencemeasurement procedures (RMPs). SRMand PT/EQA samples evaluated included SRM972a (four levels), SRM2973, six College of American Pathologists (CAP) Accuracy-Based Vitamin D (ABVD) samples, and nine Vitamin D External Quality Assessment Scheme (DEQAS) samples. Results were received from 28 different laboratories using 20 ligand binding assays and 14 LC-MS/MS methods. Using the test assay results for total serum 25(OH)D (i.e., the sum of 25(OH)D-2 and 25(OH)D-3) determined for the single-donor samples and the RMP target values, the linear regression and 95% prediction intervals (PIs) were calculated. Using a subset of 42 samples that had concentrations of 25(OH)D-2 below 30 nmol/L, one or more of the SRM and PT/EQA samples with high concentrations of 25(OH)D-2 were deemed non-commutable using 5 of 11 unique ligand binding assays. SRM 972a (level 4), which has high exogenous concentration of 3-epi-25(OH)D-3, was deemed non-commutable for 50% of the LC-MS/MS assays.

Automated summary

The study conducted by VDSP evaluated the commutability of SRMs and PT/EQA samples for the determination of serum total 25(OH)D. Results showed that some SRM and PT/EQA samples were deemed non-commutable with specific assays within certain concentration ranges.