Fever is usually defined from 38°C upwards, but that number doesn’t mean the same thing taken under the arm as it does in the rectum or the ear. These are different parts of the same body and each has its own normal range. So there is really only one practical rule: always measure in the same place, with the same thermometer, and note which one you used. A tenth of a degree either way matters far less than comparing apples with oranges.
The reason is that no home thermometer measures the thing that actually matters. The relevant temperature is the core temperature, deep inside the body, and getting at it would take invasive methods such as a catheter. What a clinical thermometer does is measure the peripheral temperature at one specific point and use it as an approximation. According to Spain’s medicines agency, the AEMPS, core temperature in someone without a fever sits between 36.4°C and 37.2°C, slightly above the peripheral reading.
Every part of the body has its own normal range
These are the physiological values the AEMPS gives for each measuring site. Worth looking at before you take fright at a number on its own:
| Measuring site | Physiological temperature |
|---|---|
| Mouth | 35.5 – 37.5°C |
| Rectum | 36.6 – 38.0°C |
| Armpit | 34.7 – 37.3°C |
| Ear canal | 36.4 – 38.0°C |
| Forehead / skin | 35.4 – 37.4°C |
The consequence is obvious: 37.4°C under the arm is clearly high, and 37.4°C in the ear is not. If you tell a doctor “it’s 38” without saying where you measured, you have given them half the information.
As for where you should measure, there is no single consensus. In adults the armpit is usually chosen for comfort and safety. In children, the Spanish association of primary care paediatricians recommends the rectal route in under-fives, and the mouth or armpit as an alternative above that age.
Which type of thermometer you have at home
Clinical thermometers fall into two families depending on how they pick up temperature, and they are held to different accuracy requirements.
Digital contact thermometers. The sensor touches the body. They need to reach thermal equilibrium, so they take time, although many include an algorithm that shortens the wait. They are the most accurate: the standard allows them a maximum error of 0.1°C within the measuring range. They only work in the mouth, rectum or armpit; using one on the forehead or wrist produces a meaningless number. Flexible-tip versions are preferable for babies and small children.
Infrared thermometers. These detect thermal radiation and give a result in seconds. The maximum permitted error is larger, 0.3°C under the ASTM E1965 standard, because more sources of error come into play: the emissivity of the skin, the room temperature, the field of view, earwax, or simply the warmth of the hand holding the device.
- Ear models are barely affected by the surroundings, but the curve of the canal and plugs of wax cause false readings.
- Forehead models are the most sensitive to ambient conditions and to blood flow in the skin. Distance matters: it should never exceed 8 cm, and each manufacturer states its own figure.
One detail almost nobody reads: infrared thermometers work in either direct mode or adjusted mode. In direct mode they show the temperature of the spot they measured. In adjusted mode they measure at the skin but display a figure converted to a different reference site (oral, rectal or axillary). In other words, two devices held against the same forehead can give different numbers and both be right. That information has to appear in the instructions.
Mercury thermometers are no longer an option: selling them to the public has been banned in Spain since 2009, when the European directive restricting mercury measuring devices was transposed. They were replaced by gallium or galinstan models, which are still valid.
The mistakes that skew the reading
Most odd numbers come not from a broken thermometer but from how the reading was taken. What the AEMPS recommends:
- Wait after exercise or a bath. Hard exercise and hot water distort the reading for a good while. Leave it at least an hour.
- Wait after eating, drinking or smoking. Especially if you are measuring in the mouth: 20 to 30 minutes is reasonable.
- Let the thermometer acclimatise. If the device has come out of a cold drawer or a car in the sun and you are at a different temperature, the reading is distorted. This affects infrared models in particular.
- Keep away from heat sources. Fireplaces, ovens, radiators, or direct sunlight on the forehead.
- Clean the area and the device. On the forehead, the skin should be clear of hair, sweat, oil or make-up, and you should avoid scars and broken skin. In the ear, keep the probe clean and, if the model uses them, fit a new cover for every reading.
- Always use the same ear. Readings from the right and the left can differ. Same direction, same depth.
- Dry armpit, arm held against the body. If the sensor isn’t wrapped in skin, the number comes out low.
Another common mistake is trusting devices that are not medical devices at all. After the pandemic, screening infrared thermometers of the kind you see in airports and shopping centres became widespread: they have no medical purpose and are no basis for any decision about your health.
How to tell whether your thermometer is reliable
A clinical thermometer is a medical device and has to meet specific requirements. Before buying, check for:
- A CE mark followed by four digits, the number of the notified body that assessed the product.
- Instructions for use in Spanish, which are mandatory for sale in Spain.
- The measuring site stated on the label: oral, rectal, axillary, aural or forehead.
- The indication range, which under the standard has to cover 34.0°C to 43.0°C.
- On infrared models, the measuring distance and whether it works in direct or adjusted mode.
This is not a theoretical precaution. In its market surveillance campaign, the AEMPS reviewed 174 digital thermometers sold in Spain and ended up ordering 31 of them withdrawn from sale, mostly for incomplete information or labelling that was not in Spanish.
When to see a doctor
The number on the thermometer matters less than the state of the person. Seek medical attention if any of the following applies:
- Fever in a baby under three months old, however mild it looks. That always needs urgent assessment.
- Neck stiffness, severe headache, confusion, unusual drowsiness or seizures.
- A rash that does not fade when you press it with a clear glass.
- Difficulty breathing, very rapid breathing, or blue lips.
- Fever lasting more than three days with no clear cause, or coming back after several days without it.
- Signs of dehydration: very little dark urine, a dry mouth, crying without tears.
- Anyone with a weakened immune system, on cancer treatment, or with poorly controlled chronic illness.
- Severe localised pain alongside the fever: abdomen, ear, throat, or on passing urine.
One last thought: if the thermometer in your bathroom cabinet has no instructions, no complete CE mark, or gives numbers that contradict each other from one reading to the next, it isn’t worth keeping. A device that measures badly is worse than no device at all, because it gives false reassurance.
This article is for information only and does not replace assessment by a healthcare professional. If a fever worries you, speak to your doctor or your pharmacist.
Sources: AEMPS — Market surveillance report on digital clinical thermometers (2022, in Spanish) · MedlinePlus (NIH) — Temperature measurement · BOE — Order PRE/222/2009 on mercury-containing measuring devices (in Spanish) · Regulation (EU) 2017/745 on medical devices · NCBI Bookshelf (NIH) — Physiology, Fever