Rickettsiales: Laboratory diagnosis

As undifferentiated febrile illnesses, infections caused by organisms in the genus Rickettsia, Orientia, Ehrlichia, and Anaplasma are often difficult to diagnose. Clinical suspicion is the key to the initiation of timely therapy and clinical workup. Serology is the mainstay of diagnostic methods, but since antibody production occurs later in illness, diagnosis is usually retrospective. Serologic confirmation requires seroconversion or a fourfold increase in titers from acute- and convalescent-phase sera. Immunohistochemistry offers a method of diagnosis during acute illness, but this technique is not readily available to most physicians. Molecular methods also offer the chance of making a diagnosis during early illness, but the sensitivity of these assays is dependent on the pathogenesis of the infecting organism and the site of DNA acquisition (e.g., blood, buffy coat, tissue).