Issues of methodology, standardization and metabolite recognition for 25-hydroxyvitamin D when comparing the DiaSorin radioimmunoassay and the Nichols Advantage automated chemiluminescence protein-binding assay in hip fracture cases.
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 D(2)) 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 D(2).


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.
D001331:Automation; D001667:Binding, Competitive; D006620:Hip Fractures / Q000097:blood*; D006801:Humans; D008137:Longitudinal Studies; D008163:Luminescent Measurements; D011485:Protein Binding; D011786:Quality Control; D011863:Radioimmunoassay / Q000379:methods*; D012015:Reference Standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood* / Q000378:metabolism*
D014807:Vitamin D; C104450:25-hydroxyvitamin D