Specificity of EIA immunoassay for complement factor Bb testing.

During the alternative complement pathway activation, factor B is cleaved in two fragments, Ba and Bb. Concentration of those fragments is about 2 logs lower than of factor B present in the blood, which makes fragment detection challenging because of potential cross-reactivity. Lack of information on Bb assay cross-reactivity stimulated the authors to investigate this issue.We ran 109 healthy donor EDTA plasmas and 80 sera samples with both factor B immunodiffusion (The Binding Site) and Quidel Bb EIA assays.During the study it was shown that physiological concentrations of gently purified factor B demonstrated approximately 0.15% cross-reactivity in the Quidel Bb EIA assay. We also observed that Bb concentration in serum is higher than in plasma due to complement activation during clot formation which let us use sera as samples representing complement activated state.Our study demonstrated that despite the potential 0.15% cross-reactivity between endogenous factor B and cleaved Bb molecule, measuring plasma concentrations of factor Bb is adequate to evaluate the activation of the alternative complement pathway.