Korat-Scrub Typhus Score: A Clinical Tool for Predicting Scrub Typhus in Patients With Acute Undifferentiated Febrile Illness

Abstract

Objective: Scrub typhus is a major cause of acute undifferentiated febrile illness (AUFI) in the Asia–Pacific region. Nonspecific presentation, limited diagnostic accuracy and delayed treatment increase morbidity and mortality. This study aimed to develop a simple clinical score for early diagnosis in adults with AUFI. Methods: A cross-sectional study of adults (≥ 18 years) with AUFI was conducted at a tertiary hospital in northeastern Thailand from June 2021 to May 2022. The Korat–Scrub Typhus Score was developed by analysing all clinically relevant variables using multivariable logistic regression with backward stepwise selection. Model performance was assessed using the Hosmer–Lemeshow goodness-of-fit test and the area under the receiver operating characteristic curve (AUC), with internal validation via 1000 bootstrap replications. Results: Among 366 AUFI patients, 75 had scrub typhus. Most were male (66.7%), with mean age of 53.6 ± 17.6 years and median illness duration of 4 days (IQR 2–7). Five predictors were identified: outdoor activities, rash (excluding eschar), headache, abnormal chest radiograph findings and aspartate aminotransferase ≥ 100 U/L. The Korat-Scrub Typhus Score (0–13) stratified risk into low (< 5), moderate (5–6) and high (> 6) groups, with likelihood ratios of 0.55 (95% CI, 0.43–0.71), 2.20 (95% CI, 1.37–3.53) and 9.22 (95% CI, 4.19–20.25), respectively. The model showed good calibration, acceptable discrimination (AUC = 0.783, 95% CI 0.725–0.842) and minimal overfitting after internal validation. Conclusion: Scrub typhus is a common cause of AUFI. The Korat-Scrub Typhus Score uses simple clinical and laboratory data to support early diagnosis and timely treatment.