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<h4>Objectives</h4>This study examined individuals with Rickettsia typhi infection in the Lao People's Democratic Republic (Lao PDR) to (a) investigate humoral immune dynamics; (b) determine the differences in reference diagnostic results and recommend appropriate cut-offs; (c) determine differences in immune response after different antibiotic treatments; and (d) determine appropriate diagnostic cut-off parameters for indirect immunofluorescence assay (IFA).<h4>Methods</h4>Sequential serum samples from 90 non-pregnant, adults were collected at seven time-points (days 0, 7, 14, 28, 90, 180 and 365) as part of a clinical antibiotic treatment trial. Samples were tested using IFA to determine IgM and IgG antibody reciprocal end-point titres against R. typhi and PCR.<h4>Results</h4>For all 90 individuals, reciprocal R. typhi IgM and IgG antibody titres ranged from <400 to ≥3200. The median half-life of R. typhi IgM was 126 days (interquartile range 36-204 days) and IgG was 177 days (interquartile range 134-355 days). Overall median patient titres for R. typhi IgM and IgG were significantly different (p < 0.0001) and at each temporal sample collection point (range p < 0.0001 to p 0.0411). Using Bayesian latent class model analysis, the optimal diagnostic cut-off reciprocal IFA titer on patient admission for IgM was 800 (78.6%, 95% CI 71.6%-85.2% sensitivity; 89.9%, 95% CI 62.5%-100% specificity), and for IFA IgG 1600 (77.3%; 95% CI 68.2%-87.6% sensitivity; 99%, 95% CI 95%-100% specificity).<h4>Conclusions</h4>This study suggests suitable diagnostic cut-offs for local diagnostic laboratories and other endemic settings and highlights antibody persistence following acute infection. Further studies are required to validate and define cut-offs in other geographically diverse locations.

Original publication




Journal article


Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases

Publication Date





781.e9 - 781.e16


Lao-Oxford-Mahosot Hospital-Oxford Tropical Medicine Research Unit, Microbiology Laboratory, Mahosot Hospital, Vientiane, Lao People's Democratic Republic; Mahidol-Oxford Tropical Medicine Research Unit, Mahidol University, Bangkok, Thailand.


Humans, Rickettsia typhi, Typhus, Endemic Flea-Borne, Immunoglobulin G, Immunoglobulin M, Antibodies, Bacterial, Anti-Bacterial Agents, Fluorescent Antibody Technique, Indirect, Bayes Theorem, Sensitivity and Specificity, Longitudinal Studies, Adult, Laos, Immunity, Humoral