Temperature is one of the simplest measurements to take at home, yet readings are easily affected by the type of thermometer, where it is placed and what the person has just been doing. This guide explains how to measure temperature reliably and how to think about a fever sensibly.
A digital stick thermometer is inexpensive, quick and suitable for most households. Infrared ear and forehead thermometers are convenient but are more sensitive to technique, such as the angle of the probe or sweat on the forehead.
Glass mercury thermometers are being phased out in many countries because a broken thermometer creates a risk from glass and mercury. If one is still used at home, handle it carefully and keep it away from children. Do not use a mercury thermometer in the mouth of a young child or a person who may bite.
Place the tip under the tongue, towards the back, and keep the lips closed until the thermometer signals. Wait around 15 to 30 minutes after eating, drinking hot or cold drinks, or smoking. Oral readings are not suitable for young children, or for anyone who is very drowsy, confused, or breathing through the mouth.
Place the tip in the centre of a dry armpit, with the arm held firmly against the body until the reading is complete. This method is safe and easy but tends to read lower than an oral measurement and is less precise.
Follow the device instructions closely. For ear thermometers, a clean probe and the correct positioning in the ear canal are important; earwax can affect the result. Forehead readings can be affected by sweating, a fan or air conditioning, or recent time in the sun.
For babies and young children, ask your doctor or nurse which method they recommend. A temperature of 38.0 °C (100.4 °F) or higher in a baby younger than 3 months requires prompt medical assessment, because young infants can become seriously unwell with few other signs.
Normal body temperature varies slightly from person to person and through the day. A temperature of 38 °C (100.4 °F) or above is commonly used to describe a fever, but the number should always be considered alongside how the person looks and behaves.
Fever is a response to an illness — most often an infection — rather than an illness in itself. Medicines such as paracetamol can lower the temperature and make the person more comfortable, but they do not treat the cause. A temperature that falls after medicine does not mean the illness is necessarily mild.
Older adults and people with weakened immunity may develop serious infections with only a mild rise in temperature, or none at all. In these people, new confusion, reduced eating and drinking, or unusual weakness can be more important than the thermometer reading. Our guide on warning signs in an older person explains this further.
In Sri Lanka, a fever may be caused by dengue, leptospirosis, respiratory infections, urinary infections and many other conditions. It is not possible to tell the cause from the temperature alone. A fever lasting more than a day or two, particularly during dengue season, should be discussed with a doctor, who may request blood tests.
In dengue, the period around the time the fever settles can be the most dangerous. Do not assume a patient is recovering simply because the temperature has come down. Avoid taking anti-inflammatory painkillers for a fever unless a doctor has advised them, because some can be harmful in dengue. Detailed information is available on our English Health Education guide to dengue warning signs.
Contact a doctor if a fever continues for more than a couple of days, keeps returning, or occurs in a person with a long-term illness, a weakened immune system, recent surgery or a recent hospital stay.
Seek urgent or emergency care if fever occurs with:
In Sri Lanka, call 1990 Suwa Seriya for an emergency ambulance, or seek immediate care at an appropriate hospital.
Call to discuss whether a doctor review at home would be appropriate.
0 727 725 725Published by: Sineth Hospitals
Medically reviewed by: Dr. Seneth Gajasinghe, MBBS (Colombo), MD (Colombo), Medical Director, Sineth Hospitals
Last reviewed:
How unwell a person looks and behaves matters more than the exact number on the thermometer. If you are worried, seek medical advice.
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