Diagnostic Discordance Between IgM Serology and TSA-56 Real-Time Polymerase Chain Reaction (PCR) for Detecting Scrub Typhus Among Patients With Acute Undifferentiated Febrile Illness in Western Odisha

Background: Scrub typhus (ST), caused by Orientia tsutsugamushi, is an important cause of acute undifferentiated febrile illness (AUFI) in India.Diagnosis remains challenging because clinical manifestations are often nonspecific and the performance of diagnostic assays varies according to the stage of illness.Serological assays are widely used, whereas molecular assays may detect infection during the period of rickettsemia.The present study evaluated the occurrence of ST among patients with acute undifferentiated fever in Western Odisha and examined the discordance between IgM-based serology and TSA-56 real-time polymerase chain reaction (qPCR).Methods: An observational study was conducted at Bhima Bhoi Medical College and Hospital (BBMCH), Balangir, Odisha.Patients presenting with AUFI to the Medicine and Pediatrics outpatient departments were enrolled after written informed consent.A total of 397 febrile patients were included.Serum was tested using an IgM immunochromatographic test (ICT) and IgM enzyme-linked immunosorbent assay (ELISA), while EDTA blood was used for molecular testing.DNA was extracted by a column-based method, and TSA-56 qPCR was performed using a commercial qPCR assay.Complete blood counts were also evaluated.Results: The study population comprised 244 males (61%) and 153 females (39%), with an age range of 3-85 years and a mean age of 37.11 ± 18.41 years.ST positivity was 16.8% (67/397) by IgM ICT and 19.64% (78/397) by IgM ELISA.TSA-56 qPCR was positive in 37 of 178 tested samples (20.78%).Among the samples evaluated by both methods, 12/257 (4.6%) ELISA-negative samples were qPCR-positive, whereas 6/78 (7.7%) ELISA-positive samples were qPCR-negative.Compared with the ST-negative group, patients classified as positive had a higher mean white blood cell count (9.94 ± 3.93 vs. 7.77 ± 3.57 × 10³/µL; p = 0.002) and lower mean platelet count (160.30± 87.64 vs. 220.31± 107.88 × 10³/µL; p = 0.001).Conclusion: The findings demonstrate clinically relevant discordance between serological and molecular detection of ST in patients with acute undifferentiated fever in Western Odisha.The presence of qPCRpositive cases among ELISA-negative patients suggests that reliance on serology alone may miss some infections, particularly during early illness.Conversely, qPCR negativity among ELISA-positive patients may reflect differences in the timing of antibody and bacterial DNA detection.A combined diagnostic approach may therefore provide more comprehensive case detection in endemic clinical settings.