Conventional blood tests can produce results that hint at RMSF. Among the typical findings are hypoanotremia (low sodium), thrombocytopenia, white blood cell abnormalities and/or elevated liver enzymes.
Serological assays are used mostly to confirm the diagnosis after treatment has been initiated. Indirect immunofluorescence assays (IFA) of both IgM and IgG antibodies are most commonly employed, but enzyme linked immunosorbent assays (ELISA) and dot immunoassays are also available. Complement fixation is less sensitive, and less frequently used. Immunostaining of biopsied skin rashes can also be performed and is very rapid; results are available in a few hours. However, the test is only 70% sensitive, so a negative result does not exclude the diagnosis.
Polymerase chain reaction (PCR) assays for R. rickettsii DNA are considered perhaps the most timely and specific test for RMSF overall, but are still not widely available.