Diagnosis of Rickettsial and Related Infections 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) Differentiating rickettsial from other infections Rickettsial and related diseases must be differentiated from other acute infections, primarily meningococcemia, rubeola, and rubella.