Because of the cyclic variation of body temperature, an exact definition of pyrexia (or fever) is difficult. The human body temperature is 36.1°C upon waking. It increases to 37.2°C (or more) between 6pm and 10pm and then drops to a minimum between 2am and 4am.
Pathologic fever patterns also tend to follow a normal pattern and tend to be highest (to 'spike') in the evening. Many patients with febrile disease have relatively normal temperatures in the early morning hours.
There is a difference in temperature readings when comparing oral, rectal and core temperatures. This difference is significant and needs to be considered.
Oral temperature is 0.4⁰C lower than rectal temperature. Rectal temperature has the best correlation with core temperature of all the measurement methods.
Pyrexia in pregnancy
Pregnant women also have fluctuating temperatures, and pregnancy results in a progressive and significant increase in endogenous heat production.
Following spontaneous vaginal delivery, 6–9% of patients will have a temperature of 38°C or more, but only a third of these will have a microbial infection.
In the past, there has been a tendency to disregard the importance of an increased temperature the first 24 hours of labour. However:
- a temperature of 41.4°C can lead to convulsion
- irreversible brain damage occurs at 42.2°C
- a temperature of 45.5°C is probably incompatible with life
- a high temperature in pregnant women can damage the fetus.
Therefore, it is important that an increase in temperature is carefully monitored.
Whilst one recorded temperature of 38°C is not sufficient to diagnose pyrexia, a patient presenting with a temperature of 38°C or above needs to be carefully assessed and monitored.
The use of antipyretics in febrile pregnant women is imperative to help prevent intrauterine hyperthermia and possible fetal damage.
Although not all infants with invasive bacterial infection are febrile, fever is a particularly important sign for approximately half of newborn infants with sepsis and meningitis.
Fever, particularly in non-immune women with malaria, is associated with miscarriage and preterm labour.
Most infections (and fevers) in obstetrics and gynaecology reflect a disease process that develops under clinical observation.
The primary exception to this is in relation to sexually transmitted diseases.