If the patient presented late in the course of her illness, both NAAT and culture may be negative. In this case, serologic tests that measure antibodies may be useful. Infection with pertussis produces IgA (except in infants), IgM, and IgG antibodies.
The problem is that the presence of antibodies could indicate a current/past exposure to B. pertussis or immunization. Vaccination can produce both IgA and IgM antibodies in addition to IgG, so serologic antibody testing cannot differentiate vaccine response from infection.
A common testing protocol is to first use an enzyme-linked immunosorbent assay (ELISA). Positive results may then be confirmed using an immunoblot assay. Some testing laboratories may also recommend paired sera when testing for IgG antibodies, with a convalescent specimen following the acute specimen within 30 days.