What are the interference factors of the vitamin D assay in Vidas (Vitamin D Immunoassay)?

Interference Factors of Vitamin D Assay in VIDAS

The VIDAS vitamin D immunoassay, like other automated immunoassays, is susceptible to interference from heterophilic antibodies, incomplete displacement of 25(OH)D from vitamin D-binding protein, and matrix effects that can cause inaccurate measurements, with classification of samples varying by 4-32% depending on the assay used. 1

Primary Interference Mechanisms

Heterophilic Antibody Interference

• Heterophilic antibodies can bind to the assay components and cause falsely elevated or decreased measurements in immunoassay-based methods like VIDAS 1
• This interference is particularly problematic for competitive protein binding assays and immunoassays compared to LC-MS/MS methods 2

Vitamin D-Binding Protein (DBP) Issues

• Immunoassays face challenges with inconsistent displacement of 25(OH)D from DBP during the measurement process 1
• The assay's ability to disrupt the tight binding between 25(OH)D and DBP (which binds >99% of circulating vitamin D) directly affects measurement accuracy 3
• Matrix effects in certain patient populations, particularly those with elevated urea or retained metabolites (such as hemodialysis patients), can cause incomplete binding disruption between 25(OH)D and DBP, leading to poor assay accuracy 4

Assay-Specific Performance Issues

Standardization Problems

• Immunoassay methods, including VIDAS, show limited improvement in accuracy even with National Institute of Standards and Technology (NIST) standard reference materials, which primarily benefit LC-MS/MS methods 1
• Testing variability between immunoassay methods and between laboratories using the same method ranges from 10-20% 2, 5

Cross-Reactivity and Specificity

• Immunoassays may have variable cross-reactivity with 25(OH)D2 versus 25(OH)D3, potentially underestimating or overestimating total vitamin D status depending on supplementation type 2
• Epimers (such as 3-epi-25OHD3) and isobars (1α-hydroxyvitamin-D3, 7α-hydroxy-4-cholesten-3-one) can interfere with measurements if not chromatographically separated, though this is more relevant for LC-MS/MS methods 6

Clinical Impact of Interference

Misclassification Risk

• The choice of assay can result in 4-32% variation in classifying patients as "deficient" versus "nondeficient" 1, 2
• Automated immunoassays have been shown to falsely assign suboptimal vitamin D status in significant percentages of patients compared to reference methods 4

Population-Specific Considerations

• Total serum 25(OH)D measured by immunoassays may underestimate vitamin D status in African Americans due to differences in DBP levels, though bioavailable 25(OH)D may be similar across races 2
• Special populations (hemodialysis patients, those with altered protein binding) show greater assay deviation and interference 4

Practical Recommendations

Sample Handling

• Use serum as the sample of choice and avoid gel tubes until more data are available on potential interference 2
• Be aware that various confounders influence vitamin D assays and are often overlooked in routine practice 7

Quality Assurance

• Verify that your laboratory participates in the CDC's Vitamin D Standardization Certification Program to maintain quality and comparability of results 1, 5
• Recognize that even eight years after standardization programs began, several commercially available assays still do not comply with VDSP requirements 8

Clinical Correlation

• Always correlate laboratory results substantially with clinical findings, as reports need proper understanding of interfering factors 7
• Consider that measurements of Vit D by different diagnostic laboratories are not uniform due to variation in assay methodology and measurement of different metabolites 7