Diagnosis and identification of rickettsiae is challenging due to the non-specific clinical manifestations of the disease, combined with the shortcomings of currently available diagnostic modalities. Serological tests remain widely used in clinical settings and for epidemiological and sousveillance purposes. Molecular methods are often used for acute phase detection and species-specific identification.

Serology

Serological methods have been the mainstay for rickettsial disease diagnostics since the Weil-Felix test was developed. The Weil-Felix test is rarely used now given its poor sensitivity and specificity, and the advancements in serological assays. The current reference-standard for rickettsial diagnositcs is the indirect immunofluorescence assay (IFA). While the IFA has a high sensitivity and specificity, it is prone to subjectivity. An technique similar to the IFA is the immunoperoxidase assay (IIP), exhibiting comparable levels of sensitivity and specificity. The microimmunofluorescence assay (MIF) is a serological assay that allows for the detection of multiple rickettsial antigens, however it is often hindered by cross-reactivity. An alternative gold standard serological assay is the enzyme-linked immunosorbent assay (ELISA), which is widely used and better suited for low-resource settings.

Molecular

Molecular methods are more commonly used for direct detection of rickettsial species. Nucleic acid amplification tests (NAAT) and immunohistochemistry (IHC) assays allow for identification at the species level. NAAT has often been used to identify Rickettsia species directly from clinical samples. Many different probes and primers are used for the identification of rickettsial DNA, i.e. 16S rRNA gene, gltA (citrate synthase), ompA (outer membrane protein A), sca0, sca4, and sca5 (outer membrane proteins), HSP60, and 17-kDa gene. Conventional, nested, and real-time PCR techniques are all commonly used for detection of these genes. PCR demonstrates good sensitivity and specificity, and is more reliable during acute stages of illness.

Culture

Cell cultures are often performed for the isolation of rickettsiae. Culture methods are also often used as a confirmatory procedure in clinical settings.

References
An Update on the Laboratory Diagnosis of Rickettsia spp. Infection