Diagnosis of scrub typhus is challenging. The main hurdle is the antigenic and geographical heterogeneity of Orientia tsutsugamushi. Serological tests have used antigen combinations based on locally circulating strains. Surface cell antigen A (ScaA) is an autotransporter protein, and specific antibody responses against its passenger domain in humans were previously reported. This study aimed to develop a new serological test to diagnose scrub typhus. The first objective was to develop a ScaA IgM enzyme-linked immunosorbent assay (ELISA) and estimate the sensitivity and specificity using Bayesian latent class models. The second objective was to narrow down the epitope region and develop a simplified ELISA based on peptide fragments. A ScaA IgM ELISA was developed with a recombinant 101-amino acid fragment of the ScaA autotransporter domain. This amino acid sequence had a high identity across Orientia strains. The new test was evaluated using two cohorts previously collected in Vietnam: a rickettsiosis cohort and a fever cohort. The estimated sensitivity and specificity of a ScaA IgM ELISA were 73.9% and 96.9% in the rickettsiosis cohort and 68.6% and 94.8% in the fever cohort. The antibody response against the recombinant ScaA protein was slower to develop than that against 56kDa type-specific antigen. The estimated sensitivity and specificity were interpreted in a clinical scenario of the scrub typhus prevalence of 3-30% in patients with acute febrile illnesses. To develop a ScaA peptide IgM ELISA, overlapping peptides of the 101-amino acid sequence were investigated. Narrowing down the epitope region reduced the sensitivity but increased the specificity. The sensitivities of the ScaA serological tests were not sufficient. This lack of sensitivity may have been due to the low antigenicity of ScaA and the quantity, timing, and location of its expression in the course of infection. The test sensitivity may improve by combining it with other conserved antigens.