Fever Incidence Is Much Lower in the Morning than the Evening: Boston and US National Triage Data

Abstract

In this observational study, we evaluated time-of-day variation in the incidence of fever that is seen at triage. The observed incidence of fever could change greatly over the day because body temperatures generally rise and fall in a daily cycle, yet fever is identified using a temperature threshold that is unchanging, such as ≥38.0° Celsius (C) (≥100.4° Fahrenheit [F]). Methods: We analyzed 93,225 triage temperature measurements from a Boston emergency department (ED) (2009-2012) and 264,617 triage temperature measurements from the National Hospital Ambulatory Medical Care Survey (NHAMCS, 2002-2010), making this the largest study of body temperature since the mid-1800s. Boston data were investigated exploratorily, while NHAMCS was used to corroborate Boston findings and check whether they generalized. NHAMCS results are nationally representative of United States EDs. Analyses focused on adults. Results: In the Boston ED, the proportion of patients with triage temperatures in the fever range (≥38.0°C, ≥100.4°F) increased 2.5-fold from morning to evening (7:00-8:59 PM vs 7:00-8:59 AM: risk ratio [RR] 2.5, 95% confidence interval [CI], 2.0-3.3). Similar time-of-day changes were observed when investigating alternative definitions of fever: temperatures ≥39.0°C (≥102.2°F) and ≥40.0°C (≥104.0°F) increased 2.4-and 3.6-fold from morning to evening (7:00-8:59 PM vs 7:00-8:59 AM: RRs [95% CIs] 2.4 [1.5-4.3] and 3.6 [1.5-17.7], respectively). Analyses of adult NHAMCS patients provided confirmation, showing mostly similar increases for the same fever definitions and times of day (RRs [95% CIs] 1.8 [1.6-2.1], 1.9 [1.4-2.5], and 2.8 [0.8-9.3], respectively), including after adjusting for 12 potential confounders using multivariable regression (adjusted RRs [95% CIs] 1.8 [1.5-2.1], 1.8 [1.3-2.4], and 2.7 [0.8-9.2], respectively), in age-group analyses (18-64 vs 65+ years), and in several sensitivity analyses. The patterns observed for fever mirror the circadian rhythm of body temperature, which reaches its highest and lowest points at similar times. Conclusion: Fever incidence is lower at morning triages than at evening triages. High fevers are especially rare at morning triage and may warrant special consideration for this reason. Studies should examine whether fever-causing diseases are missed or underappreciated during mornings, especially for sepsis cases and during screenings for infectious disease outbreaks. The daily cycling of fever incidence may result from the circadian rhythm. [

Fevers Are Rarer in the Morning—Could We Be Missing Infectious Disease Cases by Screening for Fever Then?

“…Physiologically, circadian rhythms usually reach temperature low points during mornings, and patients can lack fever signs or can present some signs without reaching cutoffs like ≥100.4°F (≥38.0°C). 1,2,8 Although there is a long history of studying circadian rhythms, their relevance to fever presentation remains little known, partly because the large datasets needed for detailed study have only become available recently, and partly because time-of-day variations in fever had lower importance 9 before COVID-19.…”

“…Summary: Body temperatures are less likely to reach the fever range in the morning, 1,2 but it is unknown how this affects practice during disease outbreaks. We retrospectively investigated fever-range temperatures (≥100.4°F, ≥38.0°C) during seasonal influenza outbreaks and the 2009 H1N1 (swine flu) pandemic, which have recently been used as preparatory models for coronavirus disease 2019 (COVID- 19).…”

“…Methods: Temperatures (n=115,149) were collected during triages at a Boston adult emergency department to monitor outbreaks (September 2009-March 2012). 2,3 We also investigated adult triage temperatures (n=218,574) from a nationally representative study of US emergency department visits (December 2002-December 2010). 4 The thermometer types used were temporal artery (Boston) and a nationally representative sample (national).…”

“…We excluded records missing temperature or time (Boston=1.0%, national=7.5%), or indicating repeated or accidental measurement (repeated ≤15 seconds or temperature <95°F: Boston=18.0%), leaving 93,225 Boston and 202,181 national temperatures for analysis. 2 Highinfluenza activity periods were defined as months that fully exceeded CDC ILINet baseline thresholds in region 1 (Boston analysis; outbreak-period n=6627) or nationally (national analysis; outbreak-period n=29,908). 5 We accounted for the national study's multistage design to obtain nationally representative findings.…”

“…4,7 Anonymity requirements prevented similar analyses for Boston data. 2 Our research extends a previous study that did not analyze outbreaks, but provides more details on methods, summarizes patient demographics, examines selection and other potential biases, demonstrates the robustness of findings to exclusion criteria changes (useful because many exclusions were made in Boston data), and shows that the large morning-evening changes in fever rates occurred on both weekdays and weekends (i.e., regardless of changes in workday schedules and availability of other care options). In both the Boston and US national studies, temperatures measured during mornings were less likely to reach the fever range (≥100.4°F, ≥38.0°C), especially during periods of high influenza activity (seasonal flu and the 2009 H1N1 pandemic).…”