Hot forehead, chills… There are well-known signs to know if you have a fever. But how much? How to measure it without a thermometer? With his phone? Practical advice.
The temperature in the human body is not uniform and it is different in each part of our body. Our body has two very different zones that work in opposite ways: the internal zone which includes the brain, heart and lungs and the peripheral zone which includes the legs, arms and skin. When you have a fever, the blood carries heat from the peripheral zone to the internal zone. Depending on where we measure the temperature, we will not have the same number.
In practice, fever can be measured with a thermometer in the rectal area. “It is the only temperature related to the deep zone, easily accessible and therefore the most reliable in terms of precision. Indeed, the veins at the anus are in direct communication with the heart and the heat transition time is short. The rectal temperature is therefore the reference temperature to define fever which starts at 38°C. The thermometer should have a soft, moistened or lubricated probe to be comfortable“, explains Dr François Corrard, pediatrician.
Under the armpit: temperature has less value
The temperature can also be measured in peripheral areas: under the armpit, on the forehead or in the ear, but these measurements have less value, because they only measure the surface temperature which is not necessarily that of the interior zone and therefore the fever. “All of these electronic and infrared thermometers use algorithms to correct the measurements and try to approximate the rectal temperature. This electronic management depends on each manufacturer and remains a trade secret., he adds. Remember that fever is a useful means of defense to fight against microbes, that it is generally not dangerous and does not require treatment if it is not accompanied by discomfort.
What if we don’t have a thermometer?
When the temperature rises, the body will take heat at skin level, limbs to concentrate it inward. If we take the temperature at the periphery, it will be lower than the central temperature. And vice versa when the temperature drops. If you don’t have a thermometer, you can consider other signs:
The higher the temperature rises, the colder we will be.
The higher the temperature rises, the colder we will be. “When you have a high temperature around 40°C, your feet and hands are frozen and your armpits are cold.“. Fever can also be accompanied by different signs which correspond to the defense system put in place by the body:
- the drop in energy, motivations,
- loss of appetite,
- asthenia
- irritable behavior. “These changes in behavior which are not linked to fever but can occur at the same time and alert you to the presence of it.
Is the forehead systematically hot?
No. But the forehead, which belongs to the peripheral zone, therefore more or less hot in case of fever, can be a benchmark when we do not have the possibility of measuring fever. “In children, touching the forehead with their mother’s hand can detect the presence of fever. in 80% of cases, but does not allow you to know its height“, underlines Dr Corrard.
Can you tell if you have a fever with your phone?
Connected tools such as the phone or watch take skin temperature from sensors of different types (electronic, infrared, etc.). The assessment of fever is thus approximate since it is interested in peripheral temperature like infrared thermometers. “Here we measure the peripheral temperature different from the internal temperature. This makes it very easy to have a figure but its value for detecting fever and measuring its importance is approximate“, concludes Dr Corrard.
Thanks to Dr François Corrard, pediatrician, Combs-la-Ville.