Table 4

Assessment of vitamin D levels

RadioimmunoassayUses antibodies that recognize both 25(OH)D3 and 25(OH)D2Most commonly used assay; coefficient of variability in assay is 12%–18% in normal range of VTD (85–147.5 nmol/L) and 10%-25% in lower range of VTD (20–62.5 nmol/L)86
Competitive binding protein assayReagent separates VTD from binding proteinsOften yields values about 30% higher (nonspecific) but might not detect 25(OH)D287
  • 25(OH)D3—25-hydroxyvitamin D3, 25(OH)D2—25-hydroxyvitamin D2, VTD—vitamin D.