Body temperature is one of the core vital signs, used in early warning scores, sepsis screening and infection monitoring. Yet temperature is often measured inaccurately, with the wrong device, poor technique or at the wrong site. A missed fever can delay sepsis treatment; a missed low temperature can hide serious illness, especially in older adults and newborns.
Normal temperature
Normal core temperature is typically around 36.5–37.5°C, but varies by person, time of day (lower in early morning), site of measurement, age and activity. Older adults often run slightly lower and may not mount a fever when infected.
Methods compared
| Site | Notes |
|---|---|
| Oral | Reliable if done correctly; place under the tongue in the sublingual pocket, lips closed. Wait 15–30 minutes after eating, drinking or smoking. Not for young children, confused or unconscious patients. |
| Axillary | Safe and widely used, especially for babies and children under 4 weeks (NICE recommends electronic axillary thermometers for infants under 4 weeks). Tends to read lower than core. Place in the centre of a dry armpit, arm held close. |
| Tympanic (ear) | Quick; used for older infants, children and adults. Pull the ear gently back (and up for adults) to straighten the canal. Earwax and poor technique reduce accuracy. |
| Temporal artery (forehead) | Non-invasive; accuracy varies by device and technique; affected by sweating and environment. |
| Rectal | Close to core, but rarely used routinely now due to discomfort, infection risk and risk of injury; avoid in neutropenic patients. |
| Core (bladder, oesophageal, pulmonary artery) | Used in critical care and theatres. |
Tips for accuracy
- Use the same site and device for trend monitoring and document the site.
- Follow the manufacturer’s instructions and use probe covers.
- Check devices are calibrated and batteries are working.
- Don’t measure immediately after a bath, exercise or under heavy blankets.
- Mercury glass thermometers are being phased out; use digital alternatives.
Fever
- Commonly defined as 38°C or above.
- Infants under 3 months with a temperature of 38°C or higher are considered high risk for serious illness and need urgent assessment.
- In neutropenic patients (e.g. on chemotherapy), fever is a medical emergency: neutropenic sepsis. Give antibiotics urgently per protocol.
- In children with sickle cell disease, fever is an emergency.
- In malaria-endemic areas, test febrile patients for malaria.
- Fever height doesn’t reliably indicate severity; look at the whole patient.
Hypothermia and low temperature
- Below 36°C should prompt attention; below 35°C is hypothermia.
- A low temperature can be a sign of sepsis, especially in older adults, newborns and immunocompromised patients.
- Newborns lose heat rapidly; keep them warm with skin-to-skin contact. See neonatal sepsis.
- Peri-operative hypothermia increases infection and bleeding risk.
- Older adults in cold homes are vulnerable in winter.
Escalation
Temperature is part of NEWS2 and paediatric early warning scores. Combine with other signs and escalate according to your score and clinical concern. See sepsis recognition.
Based on NICE NG143 (Fever in under 5s), RCP NEWS2 and clinical skills guidance. Educational content: follow local policies and device instructions.