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A fever is an increase in body temperature. A normal body temperature is around 98.6°F (37°C). In general, a fever is a temperature above 100.4°F (38°C).
Body temperature can vary slightly from person to person and may depend on factors such as time of day and physical activity. The method used to take a temperature can also affect the results.
How fever is defined can vary based on the method used to take a temperature. Check with your doctor for specific fever guidelines.
For children over 3 months of age, fever can be defined as:
In a child younger than 3 months, a fever is an under the arm temperature of 99.4°F (37.4°C) or higher.
The most common cause of fever is an infection caused by bacteria or viruses. Other causes of fever may include heat exposure, cancer, autoimmune disorders, certain medicines, or immunizations.
Children with cancer are at higher risk for infection because cancer and its treatment can weaken the immune system. Check with your doctor about what to do if your child as a fever. Read more about fever and signs of infection in pediatric cancer.
Fever symptoms include:
The best way to take a temperature is to use a thermometer with a digital reading. Digital thermometers are quick and easy to use. Do not use a glass mercury-containing thermometer. Mercury thermometers can be toxic if broken.
An oral (by mouth) temperature reading is usually the best way to measure temperature in children over the age of 4 or 5 years. Children should be old enough to understand and follow instructions.
An axillary temperature is taken by placing the thermometer in the armpit. This method can be used for any age. However, an underarm temperature is not as reliable as other measures. Armpit temperatures are best thought of as a screen for fever, not an exact reading.
Temperature can be taken using a thermometer that measures heat from the temporal artery. This artery runs across the forehead, just under the skin. A temporal artery thermometer can be used for any age. However, it may not be as reliable in infants younger than 3 months old. There are different types of forehead thermometers. Some devices work by moving the thermometer across the surface of the forehead. Other devices use a sensor that does not touch the forehead.
Temperature can be taken using a tympanic thermometer which measures heat from the ear canal. However, this method is often not as reliable. The temperature reading can depend on the position of the thermometer. The shape of the ear canal and the presence of earwax can also affect the reading. Ear thermometers are not reliable in infants younger than 6 months old.
An oral temperature is generally the most accurate and convenient way to check for fever. If a child is too young or has painful mouth sores, take the temperature under the arm.
Oral and underarm temperatures are the two methods recommended for most pediatric cancer patients. Talk to your doctor before using any other method, such as temporal artery (forehead) thermometer or tympanic (ear) thermometer, since they may be less accurate. Avoid pacifier thermometers and forehead strips, as these are not reliable and do not give accurate readings.
IMPORTANT: Taking a temperature rectally (in the bottom) can be a risk for pediatric cancer patients. Cancer patients often have weakened immune systems and may have rashes or sores around the anus and bottom. A rectal thermometer can damage the thin skin of the anal area and increase risk for infection. Therefore, rectal temperatures are not usually used to check for fever in children with cancer.
For infants and young children who do not have cancer and who are otherwise healthy, pediatricians may recommend a rectal temperature. Rectal thermometers often provide the most accurate reading of body temperature. Always follow your doctor’s instructions on how to measure fever.
Most thermometers can be cleaned using soap and water. You can also use rubbing alcohol (70% isopropyl alcohol) to wipe off the surface (rinse off before oral use). Wipe dry with a clean cloth.
Do not boil or soak the thermometer in water. Do not clean in the dishwasher. Always follow the directions from the manufacturer.
Reviewed: June 2020