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  • Age; injection substance use.

  • Localizing symptoms; weight loss; joint pain.

  • Immunosuppression or neutropenia; history of cancer, risk of COVID-19.

  • Medications.

  • Travel.


The average normal oral body temperature taken in mid-morning is 36.7°C (range 36–37.4°C). This range includes a mean and 2 standard deviations, thus encompassing 95% of a normal population (normal diurnal temperature variation is 0.5–1°C).

The normal rectal or vaginal temperature is 0.5°C higher than the oral temperature, and the axillary temperature is 0.5°C lower. However, a normal body temperature based on a peripheral thermometer (tympanic membrane, temporal artery, axillary, oral) does not always exclude the presence of a fever. To exclude a fever, a rectal temperature is more reliable than an oral temperature (particularly in patients who breathe through their mouth, who are tachypneic, or who are in an ICU setting where a rectal temperature probe can be placed to detect fever). Wearable digital thermometers may detect early mild increased temperature in patients with low WBC counts. One study found that infrared thermography of the inner canthi or whole face most accurately determined fever. In an urban emergency department, the proportion of patients with temperatures in the febrile range (38.0°C or higher, 100.4°F or higher) at triage increased 2.5-fold from morning to evening (7:00–8:59 PM vs 7:00–8:59 AM: risk ratio, 2.5; 95% CI, 2.0–3.3).

Fever is a regulated rise to a new “set point” of body temperature in the hypothalamus induced by pyrogenic cytokines. These cytokines include interleukin-1 (IL-1), tumor necrosis factor, interferon-gamma, and interleukin-6 (IL-6). The elevation in temperature results from either increased heat production (eg, shivering) or decreased heat loss (eg, peripheral vasoconstriction). Hyperthermia—not mediated by cytokines—occurs when body metabolic heat production (as in thyroid storm) or environmental heat load exceeds normal heat loss capacity or when there is impaired heat loss (eg, heat stroke). Body temperature in cytokine-induced fever seldom exceeds 41.1°C unless there is structural damage to hypothalamic regulatory centers; body temperature in hyperthermia may rise to levels (more than 41.1°C) capable of producing irreversible protein denaturation and resultant brain damage; no diurnal variation is observed.


A. Fever

Fever as a symptom provides important information about the presence of illness—particularly infections—and about changes in the clinical status of the patient. Fever may be more predictive of bacteremia in elderly patients. The fever pattern, however, is of marginal value for most specific diagnoses except for the relapsing fever of malaria, borreliosis, and occasional cases of lymphoma, especially Hodgkin disease. Furthermore, the degree of temperature elevation does not necessarily correspond to the severity of the illness. For example, patients with community-acquired pneumonia who were subsequently found to have afebrile bacteremia exhibited the highest 28-day mortality rate. Fever with rash and eosinophilia defines the drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome.


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