Background: Vitamin D (vit-D) deficiency is highly prevalent in the Korean population, highlighting
the need for accurate measurements. In this study, the interferences by endogenous and
cross-reactive substances were compared between routine vit-D immunoassays and mass spectrometry
(MS) methods.
Methods: Two MS methods and 4 immunoassays from different manufacturers (Abbott, Beckman
Coulter, Roche, Siemens) were compared. Residual samples that were icteric, lipemic, hemolyzed,
high in rheumatoid factor, from myeloma patients, or patients undergoing hemodialysis were
collected. Also, 4 levels of National Institute of Standards and Technology (NIST) Standard
Reference Material 972a, and 12 samples serially spiked with 3-epi-25-OH-D3 were prepared.
Results: Significant interferences were observed in hemolytic (Roche), icteric (Beckman and
Siemens) and lipemic samples (all 4 immunoassays). Level 4 NIST material and 3-epi-25-OH-D3-
spiked samples induced significant cross-reactivity, yielding higher total vit-D measurements in
non-epimer-separating MS methods, and both the Beckman and Roche immunoassays.
Conclusion: Most observed interferences were consistent with manufacturers’ claims, but overall
improvement of immunoassay bias limits is required. Awareness of potential interference is
important to increase the accuracy of vit-D measurements. Moreover, care is due when interpreting
vit-D results of newborns, infants and less commonly, pregnant women, who are known to
have physiologically high levels of the highly cross-reactive 3-epi-25-OH-D3.
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