Exams and Tests

The diagnosis of RMSF is usually made clinically when a patient's history of their problem includes a fever and a rash (usually on the wrists and ankles first, then to the palms and soles of the feet) that occurs a few days after a tick bite. Treatment should not be delayed while waiting for a definitive diagnosis as the disease may advance rapidly. Pictures of the rash and ticks can be found in the U.S. Centers for Disease Control and Prevention (CDC) Web sites listed below.

A laboratory test that presumes the diagnosis is done in some patients. It is a biopsy of the skin rash stained specifically to show R. rickettsii in the patient's cells. Another test involves a direct immunofluoresence staining of R. rickettsii in skin tissue samples; it can confirm the diagnosis in about 70% of cases and, in some laboratories, can be done the same day the specimen is collected. Other immunological tests are available but are seldom used (indirect hemagglutination, complement fixation or latex agglutination).

In most patients with RMSF, physicians will order other tests to determine how the patient is responding to the infection. Such tests are likely to include platelet counts, CBC, liver function tests, renal function tests, and electrolyte levels. A few patients may need additional tests such as a lumbar puncture, CT scan, or an MRI scan, especially if the patient exhibits neurological changes such as meningismus, confusion, or coma.