Rapid Point-of-Care Tests Using Staphylococcal Protein A Can Detect Early IgM Responses in HIV-1 and Treponema pallidum Infections.

Serological assays detecting IgM antibodies in addition to IgG antibodies have a diagnostic advantage in finding early infections. Staphylococcal protein A (SpA), widely used as an antibody-detecting reagent in various immunoassays, is considered to have a high binding affinity mainly to IgG, although its interaction with other classes of immunoglobulins has also been documented. Using 28 samples from 22 HIV-1 seroconversion panels, the present study demonstrated detection of early IgM antibodies by SpA-based rapid point-of-care tests, including DPP HIV 1/2, DPP HIV-Syphilis, STAT-PAK HIV 1/2, and Sure Check HIV 1/2. Samples with predominant IgM antibodies were identified by in-house IgM assays and confirmed by pretreatment with 0.1 M 2-mercaptoethanol. Likewise, the detection of treponemal IgM antibodies was shown by DPP HIV-Syphilis assay in eight samples collected at early syphilis infection. Direct interaction between IgM and SpA immobilized in solid phase or in solution was demonstrated with purified human polyclonal IgM. A strong correlation was found between the antibody levels detected by SpA and anti-IgM reagent in the early seroconversion samples, thus supporting the evidence for IgM binding by SpA. These assays demonstrated the ability to detect IgM antibodies, which may increase test sensitivity in early infections due to a reduced serodiagnostic window.