What is the definition of fever (pyrexia)?

From the Guidelines

Fever is typically defined as a temperature of 100.4°F (38°C) or higher, as stated in the most recent and highest quality study 1. This definition is crucial in the context of pediatric patients, where fever can be a symptom of a serious infection. The evaluation and management of febrile children is evolving, and the introduction of vaccines such as Haemophilus influenzae type b and Streptococcus pneumoniae has decreased the incidence of serious bacterial infections 1. However, the challenge remains to differentiate between children with a benign, self-limited viral infection and those with a serious bacterial infection. The reliability of other methods of temperature measurements is lower and must be considered in the context of the clinical setting, as noted in 1. Key considerations in the initial assessment of the febrile pediatric patient include the accuracy of the temperature measurement, the presence of localizing signs, and the child's immunization status 1. Fever without a source, or fever without a focus, has specific criteria, including acute onset, duration of less than 1 week, and absence of localizing signs 1. The definition of fever varies between studies, but the generally used definition is a rectal temperature of greater than or equal to 38.0 C (100.4 F), as documented in the clinical setting or at home within the past 24 hours 1. In the context of urinary tract infections, fever is defined as a temperature of at least 38.0°C (100.4°F) 1. Overall, the definition of fever as a temperature of 100.4°F (38°C) or higher is a critical component of the evaluation and management of febrile children, and is supported by the most recent and highest quality evidence 1.

Some key points to consider when evaluating a febrile child include:

• The child's age and immunization status
• The presence of localizing signs or symptoms
• The accuracy of the temperature measurement
• The child's overall appearance and behavior
• The presence of any underlying medical conditions

It is essential to approach each febrile child with a thorough evaluation and consideration of these factors to provide appropriate care and management. The most recent study 1 provides the highest quality evidence for the definition of fever, and should be used as the basis for clinical decision-making. By prioritizing the most recent and highest quality evidence, clinicians can provide the best possible care for febrile children and improve outcomes in terms of morbidity, mortality, and quality of life.

From the Research

Definition of Fever

• Fever is often defined as a temperature greater than a certain threshold, but this definition ignores the importance of the anatomic site at which temperature measurements are taken, as well as the diurnal oscillations that characterize body temperature 2
• The concept of fever is complex and involves the role of exogenous and endogenous pyrogens, thermoregulation, and the biological activities of pyrogenic cytokines 2

Measurement of Fever

• Body temperature can be measured at different anatomic sites, and the definition of fever should take into account the normal temperature range for each site 2
• Diurnal oscillations in body temperature should also be considered when defining fever 2

Treatment of Fever

• Combination treatment with ibuprofen and acetaminophen may be beneficial for sustained fever reduction in children older than 6 months 3
• Alternating therapy with paracetamol and ibuprofen may result in a lower mean temperature at one hour after the second dose, and may also result in fewer children remaining or becoming febrile for up to three hours after it is given 4
• Combined therapy with paracetamol and ibuprofen can result in a lower mean temperature at one hour after treatment, and probably also results in a lower mean temperature at four hours, and in fewer children remaining or becoming febrile for at least four hours after treatment 4
• Paracetamol/Ibuprofen combination may be more effective in patients with bacterial fever one hour after its administration 5
• Dual therapy (alternating or combined) may be superior to single therapies for treating fever in children, with combined therapy being superior to acetaminophen, and alternating therapy being comparable to ibuprofen 6