Measurement and normal variation
When a patient has or is suspected of having a fever, that person's body temperature is measured using a thermometer.
At a first glance, fever is present if:
- Temperature in the anus (rectum/rectal) or in the ear (otic) is at or over 38.0 °C (100.4 °F)
- Temperature in the mouth (oral) is at or over 37.5 °C (99.5 °F)
- Temperature under the arm (axillary) is at or over 37.2 °C (99.0 °F)
The common oral measurement of normal human body temperatureis 36.8±0.7 °C (98.2±1.3 °F). This means that any oral temperature between 36.1 and 37.5 °C (96.9 and 99.5 °F) is likely to be normal.
However, there are many variations in normal body temperature, and this needs to be considered when measuring for fever. The values given are for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (22.7 to 24.4 °C or 73 to 76 °F), during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes.
Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest around 6 p.m. (assuming the subject follows the prevalent pattern, i.e, sleeping at nighttime and staying awake during daytime). Therefore, an oral temperature of 37.2 °C (99.0 °F) would strictly be a fever in the morning, but not in the afternoon. An oral body temperature reading up to 37.5 °C (99.5 °F) in the early/late afternoon or early/late evening also wouldn't be a fever. Normal body temperature may differ as much as 1 °F (0.6 °C) between individuals or from day to day. In women, temperature differs at various points in the menstrual cycle, and this can be used as part of fertility awareness (although temperature is only one of the variables). Temperature is increased after eating, and psychological factors also influence body temperature.
There are different locations where temperature can be measured, and these differ in temperature variability. Tympanic membrane thermometers measure radiant heat energy from the tympanic membrane (infrared). These may be very convenient, but may also show more variability.
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.
Fever is usually accompanied by sickness behavior which consists of lethargy, depression,anorexia, sleepiness, hyperalgesia, and the inability to concentrate.
Types
According to one common rule of thumb, fever is generally classified for convenience as an anal (core) temperature of:
Fever classification
| Grade | °C | °F |
|---|---|---|
| low grade | 38–39 | 100.4–102.2 |
| moderate | 39–40 | 102.2–104.0 |
| high-grade | 40–41.1 | 104.0–106.0 |
| hyperpyrexia | >41.1 | >106.0 |
The last is a medical emergency because it approaches the upper limit compatible with human life. If the temperature is taken by another route (mouth, ear, armpit), then the reading needs to be converted to the equivalent core body temperature.
- Pel-Ebstein fever: A specific kind of fever associated with Hodgkin's lymphoma, being high for one week and low for the next week and so on. However, there is some debate as to whether this pattern truly exists.
- Continuous fever: Temperature remains above normal throughout the day and does not fluctuate more than 1 °C in 24 hours, e.g. lobar pneumonia, typhoid, urinary tract infection, brucellosis, ortyphus. Typhoid fever may show a specific fever pattern, with a slow stepwise increase and a high plateau.
- Intermittent fever: Elevated temperature is present only for some hours of the day and becomes normal for remaining hours, e.g. malaria, kala-azar, pyaemia, or septicemia. In malaria, there may be a fever with a periodicity of 24 hours (quotidian), 48 hours (tertian fever), or 72 hours (quartan fever, indicating Plasmodium malariae). These patterns may be less clear in travelers.
- Remittent fever: Temperature remains above normal throughout the day and fluctuates more than 1 °C in 24 hours, e.g. infective endocarditis.
Causes
Fever is a common symptom of many medical conditions:
- Infectious disease, e.g. influenza, common cold, HIV, malaria, infectious mononucleosis, orgastroenteritis
- Various skin inflammations, e.g. boils, pimples, acne, or abscess
- Immunological diseases, e.g. lupus erythematosus, sarcoidosis, inflammatory bowel diseases
- Tissue destruction, which can occur in hemolysis, surgery, infarction, crush syndrome,rhabdomyolysis, cerebral hemorrhage, etc.
- Drug fever
- directly caused by the drug, e.g. lamictal, progesterone, or chemotherapeutics causingtumor necrosis
- as an adverse reaction to drugs, e.g. antibiotics or sulfa drugs.
- after drug discontinuation, e.g. heroin or fentanyl withdrawal
- Reaction to incompatible blood products
- Cancers, most commonly renal cancer and leukemia and lymphomas
- Metabolic disorders, e.g. gout or porphyria
- Thrombo-embolic processes, e.g. pulmonary embolism or deep venous thrombosis
Persistent fever which cannot be explained after repeated routine clinical inquiries, is called fever of unknown origin.
Usefulness of fever
- increased mobility of leukocytes
- enhanced leukocytes phagocytosis
- endotoxin effects decreased
- increased proliferation of T Cells
- enhanced activity of interferon