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

Original Articles

Paul Glendenning,
Jane M Noble,
Mario Taranto,
Alexander A Musk,
Marjory McGuiness,
Peter R Goldswain,
William D Fraser and
Samuel D Vasikaran

Department of Core Clinical Pathology and Biochemistry, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Department of Geriatric Medicine, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Department of Core Clinical Pathology and Biochemistry, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Department of Core Clinical Pathology and Biochemistry, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Critical Care Division, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Department of Geriatric Medicine, Royal Perth Hospital, Perth 6000, Western Australia, Australia;
Department of Clinical Chemistry, University of Liverpool, Liverpool, UK;
Department of Core Clinical Pathology and Biochemistry, Royal Perth Hospital, Perth 6000, Western Australia, Australia

Background: Deficiency of vitamin D is commonly associated withhip fracture and treatment with vitamin D reduces hip fracturerates. Consequently, the demand for assays to measure 25-hydroxyvitaminD (25-OHD) has increased. The Nichols Advantage chemiluminescenceprotein-binding assay (CLPBA) for 25-OHD is a first-generationautomated immunoassay with decreased turnaround time, reducedmanual 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 dueto differences in standardization of each assay relative toHPLC. Our results emphasize the need for assay-specific clinicaldecision limits.

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