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How to tell if you have a fever

It is considered a fever when the temperature under your armpit, i.e., the axillary temperature is higher than 100.4ºF (38ºC), as it is easy for your body to reach a temperature of 99.5ºF (37.5ºC) when it’s very hot or when you have many layers of clothes on.

The safest way to find out if you have a fever is using a thermometer to measure your temperature and not just place your hand on your forehead or nape. 

If the temperature of your armpit goes over 102.2ºF (39ºC), please go straight to the emergency department, as a high fever can cause seizures or put your life in danger.

How to tell if you have a fever

How to correctly measure your temperature

To measure your body temperature correctly, you will need to know how to use each type of thermometer. The most common ones are:

  • Glass or digital thermometer: place the metal tip into the armpit, anus or mouth in direct contact with the skin or the mucous membranes and wait for at least 3 minutes, or until the thermometer makes a sound before you check your temperature;
  • Infrared thermometer: place the tip of the thermometer on your forehead or inside the ear canal and press the button. After the beep, the temperature will come up on the thermometer immediately.

Body temperature should be measured when you have been resting and never after physical exercise or after a shower or bath. This is because you will naturally have a higher temperature in these situations, and so the temperature reading would not be a real assessment of having a fever or not. 

The most common, most comfortable, and safest thermometer to use is the digital thermometer, as it can read the temperature under the armpit and produce a sound when it reaches the body temperature. However, any thermometer is trustworthy, as long as it is used correctly. The only type of thermometer we do not advise is the mercury thermometer, as it can cause poisoning if it breaks.

How to measure a baby’s temperature 

You should measure a baby’s body temperature with a thermometer, just like with an adult. Still, it is best to use faster, more comfortable thermometers such as digital or infrared thermometers. 

The best place to measure the baby’s temperature correctly is the anus, and so it is best to use a digital thermometer with a soft tip so as not to hurt the baby. However, if the parents do not feel comfortable with this, they may measure the temperature in the armpit and confirm that temperature with the anal temperature at the pediatrician’s office.

What temperature is considered a fever in adults?

Average body temperature varies between 95.7ºF (35.4ºC) and 98.9ºF (37.2ºC) when it is an axillary temperature, but this can increase when there is a flu or an infection that causes fever. The main body temperature variations include:

  • Slightly raised body temperature, known as "subfebrile": between 99.5ºF (37.5ºC) and 100.4ºF (38ºC). In these cases, generally, other symptoms appear, such as shivering, shaking, or facial redness. In these situations either take off a layer of clothing, take a warm bath or drink water;
  • Fever: when the axillary temperature is above 100.4ºF (38ºC). In adults, taking a 1000 mg tablet of paracetamol may be recommended. If you are feverish, make sure to keep only one layer of clothes on and place cold compresses on the forehead. If the temperature does not go down after three hours, you will need to go to the emergency department; 
  • High fever: this is when the axillary temperature is above 105.8ºF (41ºC), which should be considered a medical emergency and, therefore, a doctor should be seen as soon as possible.

Your body temperature may also be below usual if it is under 95.7ºF (35.4ºC). This generally happens when you are exposed to cold temperatures for a long time, and it is known as hypothermia. In these cases, you will need to remove the source of the cold temperature and put on several layers of clothing, drink hot tea, and warm up the environment, if possible.

How to tell if you have a fever

What temperature is considered a fever in babies and children?

Babies and children have a slightly different body temperature to adults, as it usually varies between 96.8ºF (36ºC) and 98.6ºF (37ºC). The main temperature variations in children are:

  • Slightly raised temperature: between 98.8ºF (37.1ºC) and 99.5ºF (37.5ºC). In these cases, remove a layer of clothing and give a warm bath;
  • Fever: anal temperature above 100.04ºF (37.8ºC) or axillary temperature above 100.4ºF (38ºC). In these cases, the parents should speak to a pediatrician to ask him to supervise the use of medication for reducing the fever or whether they need to go to the emergency department;
  • Low body temperature (hypothermia): temperature below 95.9ºF (35.5ºC). In these cases, make sure to put on another layer of clothes on the child and avoid drafts. If the body temperature does not go up in 30 minutes, go to the emergency department.

Variations in babies’ and children’s body temperatures are not always due to illness or infection, as the body temperature may vary according to the number of clothes worn, teeth erupting, a reaction to a vaccine, or due to the heat of the environment.

When should you to take remedies to lower a fever 

Removing extra clothing and taking a warm bath are good ways to lower body temperature. Still, when this is not sufficient, the doctor may recommend you take an antipyretic to reduce the fever. The medication most people take in these situations is paracetamol, which can be take up to three times a day, in intervals of 6 to 8 hours. 

Babies and children should only have medication for fever under a pediatrician’s supervision as dosages vary according to age and weight.

Bibliography >

  • POTTER, Patrícia A.; PERRY, Anne G.. Fundamentos de Enfermagem. 9.ed. Rio de Janeiro: Elsevier, 2017. 205-217.
  • NHS. High temperature (fever) in adults. Available on: <https://www.nhs.uk/conditions/fever-in-adults/>. Access in 19 Jun 2020
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