Acute febrile illness (AFI) is common for people seeking care globally and represents a spectrum of infectious and non-infectious etiologies with significant geographical variations. However, AFI research follows a heterogeneous methodology and reporting, making it challenging for individual fever studies interpretation and comparison and thus generating evidence-based decisions. Herein, we aim to conduct a pilot study investigating the impact of considering different case definitions of fever as inclusion criteria among febrile patients attending two Emergency Departments (ED) in Rio de Janeiro, Brazil, from October 28, 2018, to March 29, 2019. Secondarily, we explored the impact of the recent use of antipyretics in measured temperature at the arrival of the EDs. A prospective study was conducted, and we screened all consecutive non-severe patients who presented to the EDs. The inclusion criterion was the self-report of fever regardless of the temperature measured. A logistic regression model was built to investigate the factors associated with a febrile presentation at the arrival of EDs. We triaged 1551 subjects, and 374 [24.1% (95% CI 22-26)] had a history of fever, of whom 248 (66.3%), 115 (30.7%), and 11 (2.9%) had a temperature < 37.5 °C, ≥ 37.5 °C, and no temperature measurement registered at arrival, respectively. 198/374 (53%) had a suspicion of infection, and the site of infection varied according to the height of temperate at presentation. The antibiotic prescription was more likely in those who had a temperature ≥37.5 °C compared to those with a temperature <37.5 °C [crude OR: 2 (95% 1.1-3.5)]. 249/374 (66.6%) subjects reported self-administering an antipyretic within 2h [IQR 2-6] before EDs. Recent antipyretic uptake did not influence the measured temperature at presentation. In conclusion, we suggest harmonizing the case definition of fever in AFI research for a history of fever regardless of temperature cut-off would enable a complete picture of the different clinical fever phenotypes, thus improving our understanding of the global burden of febrile illness.