Serum immunoglobulin G avidity test as a tool for improving diagnosis of cat scratch disease

Cat scratch disease (CSD) is a common cause of subacute infectious regional lymphadenitis, caused by Bartonella henselae. Presently, detection of anti-B. henselae antibodies by immunofluorescence antibody assay or enzyme immunoassay (EIA) is the most widely used diagnostic test for CSD, but both are limited in establishing the timing of infection with B. henselae. In the present work we developed an avidity test for anti-B. henselae immunoglobulin G (IgG) based on EIA to distinguish recent from past CSD. We used 101 serum samples from 79 CSD patients with positive anti-B. henselae IgG as verified by EIA, and systematically developed an avidity assay using various detergent (urea) concentrations and incubation settings to optimize the test conditions to differentiate early CSD (less than 12 weeks) from late CSD (12 weeks or more). After serial experiments, the optimal conditions for performing the avidity test included incubation for 10 min at room temperature with 8 M urea at pH 7.4, and these parameters were used in the study. Our experiments showed that while the avidity indexes (AIs) of the early CSD samples were widely distributed, all the late CSD sera samples had AIs above 43, indicating that an AI <43 can serve as evidence of early CSD. The results of this study indicate that the avidity test can be useful in the serodiagnosis of CSD, particularly when anti-B. henselae IgM antibodies are not detected.