Determination of the avidity of immunoglobulin G (IgG) directed against a specific marker has become an established diagnostic tool for identifying or excluding acute infections with pathogens. A novel assay format termed AVIcomp (avidity competition based on mass action) circumventing the conventional chaotropic format has been developed for determination of the avidity of marker-specific IgG in patient specimens. Its applications for cytomegalovirus (CMV) and Toxoplasma gondii are presented. Specific high-avidity IgG from the patient specimen is selectively blocked using a soluble antigen in a sample pretreatment reagent, and the amount of remaining specific low-avidity IgG is determined relative to that in an untreated control. The comparison of the conventional chaotropic format, represented by the Radim CMV IgG Avidity assay, and the newly developed AVIcomp method, as exemplified by the Architect CMV IgG Avidity assay, on blood drawn within 4 months after seroconversion revealed a sensitivity of 100% (97.3% by an alternative calculation) for the AVIcomp format versus 87.5% (75.7% by an alternative calculation) for the chaotropic avidity assay. The specificity on 312 CMV IgG reactive and CMV IgM nonreactive specimens from pregnant women was 100% for the AVIcomp assay and 99.7% for the conventional avidity assay. The Architect Toxo IgG Avidity assay showed an agreement of 97.2% with the bioMérieux Vidas Toxo IgG Avidity Assay employing chaotropic reagents. These performance data suggest that the AVIcomp format shows superior sensitivity and equivalent specificity for the determination of IgG avidity to assays based on the chaotropic method and that the AVIcomp format may also be applicable to other disease states.