Diagnosis of RMSF Clinical features Biopsy of rash with fluorescent antibody staining to detect organisms Acute and convalescent serologic testing (serologic testing not useful acutely) Polymerase chain reaction (PCR) Clinicians should suspect Rocky Mountain spotted fever in any seriously ill patient who lives in or near a wooded area anywhere in the Western Hemisphere and has unexplained fever, headache, and prostration, with or without a history of tick contact.