Approach to fever by age and immune status

A febrile child walks into your emergency department or clinic. The thermometer reads 39.2 °C. Before you reach for an antipyretic or a guideline table, look at the child. Are they toxic? Are they perfusing? Are they interactive? That single glance, paired with age and immune status, frames the entire encounter. [4] [5]

This hub page owns the presentation logic for fever across childhood. Sister pages take the deep disease algorithms: undifferentiated fever and fever without a source, paediatric sepsis, meningitis and encephalitis, bacteraemia, and fever in the returned traveller. [6] [8]

Overview & Definition

Picture two children in adjacent cubicles. A three-year-old is running at 39.5 °C, drinking juice, and telling you about her favourite cartoon. Beside her lies a two-week-old at 38.2 °C who has stopped feeding and whose mother says "this is not my baby." The lower temperature is the sicker child. That contrast is the whole point of an age- and immune-status-aware approach. [1] [4]

Fever is an elevated core body temperature set by the hypothalamic thermostat in response to pyrogens. In practice, fever in a child is usually defined as a rectal temperature of 38 °C or higher. Axillary, tympanic and oral readings can mislead, especially in young infants, because they measure skin or canal temperature rather than core. A single falsely low tympanic reading in a febrile neonate is a classic exam trap. [4] [5]

Fever differs from hyperthermia, where heat gain overwhelms dissipation without a change in the hypothalamic set-point — think bundling, heat stroke, or malignant hyperthermia. Antipyretics reset the thermostat, so they work in fever and do little in hyperthermia. [12]

The decisive clinical question is never "how high is the fever?" but "how threatened is this child, and what is their SBI risk by age and immune status?" De and colleagues showed that body temperature lacks accuracy for detecting serious bacterial infection in febrile episodes. [4]

References14Show ledgerHide ledger

1. [1]Biondi EA Prevalence of Bacteremia and Bacterial Meningitis in Febrile Neonates and Infants in the Second Month of Life: A Systematic Review and Meta-analysis JAMA Netw Open, 2019.PMID 30901044
2. [2]Gomez B Validation of the Step-by-Step Approach in the Management of Young Febrile Infants Pediatrics, 2016.PMID 27382134
3. [3]Kuppermann N A Clinical Prediction Rule to Identify Febrile Infants 60 Days and Younger at Low Risk for Serious Bacterial Infections JAMA Pediatr, 2019.PMID 30776077
4. [4]De S Lack of Accuracy of Body Temperature for Detecting Serious Bacterial Infection in Febrile Episodes Pediatr Infect Dis J, 2015.PMID 26065864
5. [5]Fleming S Normal ranges of heart rate and respiratory rate in children from birth to 18 years of age: a systematic review of observational studies Lancet, 2011.PMID 21411136
6. [6]Schlapbach LJ International Consensus Criteria for Pediatric Sepsis and Septic Shock JAMA, 2024.PMID 38245889
7. [7]Sanchez-Pinto LN Development and Validation of the Phoenix Criteria for Pediatric Sepsis and Septic Shock JAMA, 2024.PMID 38245897
8. [8]Weiss SL Surviving Sepsis Campaign International Guidelines for the Management of Sepsis and Septic Shock in Children 2026 Pediatr Crit Care Med, 2026.PMID 41869844
9. [9]Lehrnbecher T Guideline for the Management of Fever and Neutropenia in Pediatric Patients With Cancer and Hematopoietic Cell Transplantation Recipients: 2023 Update J Clin Oncol, 2023.PMID 36689694
10. [10]Mills E Association between caregiver concern for clinical deterioration and critical illness in children presenting to hospital: a prospective cohort study Lancet Child Adolesc Health, 2025.PMID 40451224
11. [11]Burvenich R Effectiveness of safety-netting approaches for acutely ill children: a network meta-analysis Br J Gen Pract, 2025.PMID 39117428
12. [12]Hay AD Paracetamol plus ibuprofen for the treatment of fever in children (PITCH): randomised controlled trial BMJ, 2008.PMID 18765450
13. [13]Mahajan P Serious Bacterial Infections in Young Febrile Infants With Positive Urinalysis Results Pediatrics, 2022.PMID 36097858
14. [14]Burstein B Prediction of Bacteremia and Bacterial Meningitis Among Febrile Infants Aged 28 Days or Younger JAMA, 2026.PMID 41359314