Diagnosis of rickettsial infections, especially scrub typhus and murine typhus which are common causes of fever in Asia, is difficult since it does not present with any distinctive clinical signs compared to other febrile illnesses in this region, except for a necrotic skin lesion (eschar) in some patients. Laboratory tests are therefore crucial to identify of rickettsial infections. Reliable laboratory tests need significant infrastructure and experienced staff, making access to accurate tests very limited in the low-resource endemic regions. Serological tests measure the amount of specific immunoglobulin IgM and IgG antibodies against the pathogen that is causing the rickettsial illness. Serological tests are often used because they are perceived to be cheaper and easier to perform than molecular testing and they use convenient patient samples which are normally serum or plasma. However, serological tests for the diagnosis of rickettsial illnesses is limited and often there is inconsistency in their application and difficulties with interpretation of the final result.

The gold-standard serological test for rickettsial pathogens is the immunofluorescence assay (IFAs which provides a quantitative measure of antibodies in the patient sample. IFAs are not suited to testing large numbers of samples and require specific skills and equipment to read the result (Blacksell et al., 2007). There is a great deal inconsistency in the application of IFA methodologies especially in the area of the types of diagnostic antigens used, the application of diagnostic cut-offs and which antibody isotype should be tested (Blacksell et al., 2007; Dhawan et al., 2020).

Another type of test which is often used is an Enzyme Linked Immunoassays (ELISAs) which provides a semiquantitative result and is often used for screening purposes although can be used for absolute diagnosis however results should be interpreted with caution. There are commercial and in-house ELISAs available for the detection of scrub typhus antibodies (Saraswati et al., 2019; Phanichkrivalkosil et al., 2018; Elders et al., 2020) and only in house ELISAs for the detection of murine antibodies. As is the case with IFA, it is important that the diagnostic cut off the established in each geographic region based on the endemicity and prevailing background antibodies in the community. Generally, there is strong positive relationship between ELISA and IFA antibody levels for both IgM and IgG.

Point of care lateral flow devices are also available for the rapid diagnosis of scrub typhus using Ab based methodologies. These devices tend to lack accuracy and results should be interpreted with caution (Saraswati et al., 2018).

References
Blacksell SD, Bryant NJ, Paris DH, Doust JA, Sakoda Y, Day NP. Scrub typhus serologic testing with the indirect immunofluorescence method as a diagnostic gold standard: a lack of consensus leads to a lot of confusion. Clin Infect Dis. 2007;44(3):391-401.

Dhawan S, Robinson MT, Stenos J, Graves SR, Wangrangsimakul T, Newton PN, et al. Selection of Diagnostic Cutoffs for Murine Typhus IgM and IgG Immunofluorescence Assay: A Systematic Review. Am J Trop Med Hyg. 2020.

Elders PND, Dhawan S, Tanganuchitcharnchai A, Phommasone K, Chansamouth V, Day NPJ, et al. Diagnostic accuracy of an in-house Scrub Typhus Enzyme linked immunoassay for the detection of IgM and IgG antibodies in Laos. PLoS Negl Trop Dis. 2020;14(12):e0008858.

Phanichkrivalkosil M, Tanganuchitcharnchai A, Jintaworn S, Kantipong P, Laongnualpanich A, Chierakul W, et al. Determination of Optimal Diagnostic Cut-Offs for the Naval Medical Research Center Scrub Typhus IgM ELISA in Chiang Rai, Thailand. Am J Trop Med Hyg. 2019;100(5):1134-40.