# Comparative Evaluation of IgM Enzyme-Linked Immunosorbent Assay (ELISA) and Real-Time PCR for the Early Diagnosis of Scrub Typhus in Patients With Febrile Illness

## TL;DR

This study compares ELISA and PCR for early diagnosis of scrub typhus, finding PCR more effective in the first week of fever.

## Contribution

The study demonstrates that RT-PCR outperforms IgM ELISA for early diagnosis of scrub typhus within the first seven days of fever.

## Key findings

- RT-PCR detected 100% of scrub typhus cases in patients with fever duration less than seven days.
- IgM ELISA-positive but RT-PCR-negative cases were mostly from patients with fever duration of seven to 14 days.
- Combining RT-PCR in early stages and IgM ELISA in later stages improves diagnostic accuracy.

## Abstract

Introduction: Scrub typhus, caused by Orientia tsutsugamushi, is a major cause of acute undifferentiated febrile illness (AUFI) in India. Timely diagnosis is challenging due to the non-specific clinical presentation. While IgM enzyme-linked immunosorbent assay (ELISA) is widely used, molecular methods like real-time PCR (RT-PCR) offer the potential for early detection. This study compared the performance of IgM ELISA and RT-PCR in diagnosing scrub typhus in patients with febrile illness.

Methods: A cross-sectional study was conducted from January to April 2024. Blood samples were collected from 156 patients with febrile illness of less than 14 days. All samples were tested for scrub typhus IgM ELISA. ELISA-positive samples and a random selection of ELISA-negative, clinically suspected samples were subjected to RT-PCR for confirmation.

Results: Of the 156 patients, 46 (29.5%) were positive by IgM ELISA. Among these 46 ELISA-positive cases, 15 (32.6%) were positive by RT-PCR. A stark contrast was observed when analyzed by fever duration. Among the 15 RT-PCR positive cases, 100% had a fever duration of less than seven days. In contrast, only 17 (37.0%) of the 46 IgM ELISA-positive cases presented within the first seven days of fever (p<0.001). The 31 IgM ELISA-positive but RT-PCR-negative cases were predominantly from the later febrile phase (seven to 14 days).

Conclusions: RT-PCR is a superior tool for the early diagnosis of scrub typhus within the first week of fever, while IgM ELISA becomes more reliable thereafter. A diagnostic approach utilizing RT-PCR in the initial febrile phase and serology in the later phase can significantly improve the accuracy and timeliness of scrub typhus diagnosis.