Background: Deficiency of vitamin D is commonly associated with hip
fracture and treatment with vitamin D reduces hip fracture rates. Consequently, the
demand for assays to measure 25-hydroxyvitamin D (25-OHD) has increased. The Nichols
Advantage chemiluminescence protein-binding assay (CLPBA) for 25-OHD is a
first-generation automated immunoassay with decreased turnaround time, reduced manual
handling and non-radioactive label.
Methods: We compared the CLPBA to the DiaSorin radioimmunoassay (RIA)
and high-performance liquid chromatography (HPLC) for the measurement of 25-OHD using
161 samples from hip fracture patients and samples before and after institution of
ergocalciferol (vitamin D2) therapy.
Results: A negative bias for the CLPBA at concentrations below 30 nmol/L
and a positive bias at 25-OHD values above 30 nmol/L compared with the RIA resulted
in diagnostic discordance for one in three samples when using 30 and 50 nmol/L as
decision limits. HPLC analysis confirmed the presence of a negative bias for the
CLPBA at low values. Both immunoassays under-estimate 25-hydroxyvitamin
D2.
Conclusions: The discordance between 25-OHD values may be due to
differences in standardization of each assay relative to HPLC. Our results emphasize
the need for assay-specific clinical decision limits.
