Poor agreement between commercial ELISAs for plasma fetuin-A: an effect of protein glycosylation?

BACKGROUND Fetuin-A is a circulating inhibitor of ectopic calcification. Low plasma levels have been associated in some studies with increased vascular calcification, aortic stiffness and mortality in patients with Chronic Kidney Disease (CKD). However, there are other studies examining the association of fetuin-A with vascular parameters and mortality, which do not show these associations. These conflicting data may be explained by methodological differences. METHODS We compared plasma fetuin-A measurements made with two widely-used commercial fetuin-A ELISA kits (Biovendor, Modrice, Czech Republic; Epitope Diagnostics Inc., San Diego, US) in samples from patients with and without CKD. We evaluated the effect of differences in fetuin-A glycosylation status on assay specificity. RESULTS Deming regression analysis showed poor agreement between methods (for CKD cohort: y=-0.05+2.52x, S(y|x)=0.099g/L, R(2)=0.694). The Epitope Diagnostics kit demonstrated significant positive bias and greater specificity for deglycosylated fetuin-A relative to the Biovendor assay. CONCLUSION The apparently contradictory nature of reports of the association of fetuin-A with biological variables may reflect differences in the specificity of different ELISA methods for glycosylated plasma fetuin-A.