Matrix effect in exhaled breath condensate interferon-gamma immunoassay

Manuscripts presenting cytokine (and other protein) concentrations in exhaled breath condensate (EBC) are commonly published with insufficient control experiments performed to provide confidence that the assay was accurate. Our laboratory has identified false positives in EBC with several forms of immunoassay, including ELISA and multiplex bead arrays. In this current brief investigation, we studied EBC samples from ten subjects and examined for the effect of alteration of the matrix on the assay results reported by a commercially available ELISA assay for a commonly reported EBC cytokine (interferon-gamma (IFN-γ)). The commercial immunoassay reported measurable levels of the IFN-γ signal in all samples. However, when we added to the EBC samples the proteinaceous matrix of the zero-standard from the commercial kit, we could identify no IFN-γ signal. These simple findings are consistent with our earlier observations that the EBC matrix requires adjustment for there to be any validity to immunoassays.