Diagnosis should be made on clinical grounds so that appropriate treatment may be started and not delayed for laboratory tests.
Indirect fluorescent antibody (IFA) or enzyme immunoassay (EIA) of paired acute and convalescent sera for a four-fold rise in titer has been the gold standard. However, most persons with RMSF do not have a titer in the first week, so the test is not useful in acutely ill patients. The tests are 94% to 100% sensitive after 14 days.
Polymerase chain reaction (PCR) of tissue from a biopsy of the rash has shown good sensitivity and specificity unless the patient has already been treated with doxycycline. PCR is rapid but less useful when blood is tested because the organism is not present in blood until the disease is fulminant.
Immunohistochemical (IHC) staining of tissue is useful before antibodies can be measured, especially in autopsy specimens. It is 100% specific and 70% sensitive if the biopsy is collected from infected tissue. It can be negative if the organisms are focally distributed. Because it is performed on paraffin-fixed and stained material, it takes longer than PCR.
Culture is not practical and is time and labor-intensive. R. rickettsii is an obligate intracellular pathogen that can only be grown in cell culture.
A negative result on any test does not rule out the diagnosis of RMSF.
