Serologic testing can be useful in determining the cause of IE in true culture-negative cases, which are usually caused by organisms that are difficult to culture including Coxiella burnetti, Bartonella spp., Chlamydia spp., and Legionella species. The immune response to C. burnetti involves development of antibodies against phase 1 and phase 2 antigens. In acute infection, IgM and IgG antibodies develop against phase 2, and only IgM antibodies develop against phase 1. Endocarditis is a manifestation of chronic Q fever, which is characterized by high anti-phase 1 IgG titers. Positive Q fever serology, defined as a phase 1 IgG titer of >1:800, is listed as one of the major modified Duke criteria. A Bartonella antibody titer of 1:1,600 has been reported to have a positive predictive value of 88% for Bartonella IE . However, titers may not be reproducible given lot-to-lot variability of antigen preparations used for testing. Patients with Bartonella infection also frequently develop cross-reacting antibodies that result in false-positive Chlamydia spp. serology. Additional assays to be considered in culture-negative IE cases include serologic studies for Brucella species and Legionella serology or urinary antigen testing.
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