Performance of WHO and CDC influenza-like illness (ILI) case definitions in detecting influenza in the tropics

Abstract

Background: Influenza-like illness (ILI) is defined by the World Health Organization (WHO) as an acute respiratory infection with onset within the past 10 days, presenting with cough and a measured temperature of >=38 °C, and the Centers for Disease Control and Prevention (CDC) as fever >=37.8 °C and cough and/or sore throat. We assessed for the performances of WHO and CDC ILI case definitions in the detection of influenza infections in a tropical country where seasonal patterns are less well-defined. Methods and materials: From June 2016 through Nov 2018, 717 consecutive adults presenting with uncomplicated upper respiratory tract infections (ICD10-AM J00-J06) at the emergency department of a 1600-bed acute hospital in Singapore were screened for 15 major respiratory viruses using a multiplex PCR respiratory virus pathogen panel (Seeplex RV15 ACE Detection), via throat and nasal swabs. Demographic, clinical, and epidemiological data were included to assess for the performances of WHO and CDC ILI case definitions in the detection of influenza infection, using multivariable logistic regression models. We also computed the sensitivities, specificities, and positive likelihood ratios (LR+) of the definitions. Results: The median age of participants was 36 (IQR 28–51) years. Two-thirds (67.8%) had no comorbidities. One-in-five (20.6%) patients tested positive for influenza and 14.4% for rhinovirus. Overall, 14.5% fulfilled the WHO ILI criteria and 20.8% the CDC ILI criteria. Performance of WHO ILI criteria in detecting influenza: sensitivity 38.5%, specificity 91.7%, and LR+ 4.7. Performance of CDC ILI criteria: sensitivity 48.7%, specificity 86.5%, and LR+ 3.6. After adjusting for age, comorbidities, influenza vaccination in the prior 6 months, travel history in preceding 7 days, month and year of illness presentation, patients fulfilling WHO ILI definition were nearly 7 times (Adj OR 6.81, 95%CI 4.22–11.00, p < 0.001) and those fulfilling the CDC ILI definition were 6 times (Adj OR 6.25, 95%CI 4.04–9.67, p < 0.001) as likely to have an influenza Infection respectively, than patients not fulfilling the criteria. Conclusion: The WHO and CDC ILI case definitions can be used to diagnose influenza infections in the tropics, regardless of the month of the year.