Age; injection substance use.
Localizing symptoms; weight loss; joint pain.
Immunosuppression or neutropenia; history of cancer, risk of COVID-19 (see Chapter 32).
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 intensive care unit 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 white blood cell counts. One study found that infrared thermography of the inner canthi or whole face most accurately determined fever.
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 (TNF), 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). Body temperature in cytokine-induced fever seldom exceeds 41.1°C unless there is structural damage to hypothalamic regulatory centers.
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.
In general, the febrile response tends to be greater in children than in adults. In older persons, neonates, and persons receiving certain medications (eg, nonsteroidal anti-inflammatory drugs [NSAIDs], corticosteroids), a normal temperature or even hypothermia may be observed. Markedly elevated body temperature may result in profound metabolic disturbances. High temperature during the first trimester of pregnancy may cause birth defects, such as anencephaly. Fever increases insulin requirements and alters the metabolism and disposition of drugs used for the treatment of the diverse diseases associated with fever.