🌡️ How to Check Temperature Correctly: A Guide to Thermometers & Techniques

A thermometer reading is only as reliable as the technique used to obtain it. Using the wrong method, a poorly placed device, or even a hot drink a few minutes before can give you a false sense of security — or unnecessary alarm. Dr. Ravi Sishir Reddy explains the different types of thermometers available, where and how to use each one, and the common mistakes that can lead to inaccurate readings.

1. Types of Thermometers at a Glance

• Digital stick (multi‑use) thermometers: Used orally, rectally, or under the armpit. They provide an accurate reading within 30‑60 seconds and are the most versatile for home use.
• Tympanic (ear) thermometers: Use infrared to measure the temperature of the eardrum. Fast and convenient, but must be positioned correctly in the ear canal.
• Temporal artery (forehead) thermometers: Also infrared; swept across the forehead or placed on the temple. Very easy to use and good for children, but can be affected by sweating and environmental temperature.
• Non‑contact infrared thermometers: Measure temperature from a distance (often pointed at the forehead). Quick and hygienic for screening, but less accurate for clinical decision‑making than a digital stick or ear thermometer.
• Glass mercury thermometers: No longer recommended due to the risk of breakage and mercury toxicity. They have been largely replaced by digital devices. If you still have one, replace it.

2. How to Take an Oral Temperature

The oral method is reliable for adults and children over 4‑5 years who can hold the thermometer securely under the tongue.

• Wait at least 15‑30 minutes after eating, drinking hot or cold beverages, or smoking before taking an oral temperature. These activities can temporarily alter the mouth temperature.
• Place the thermometer tip under the tongue, toward the back of the mouth, in one of the sublingual pockets (the areas at the base of the tongue on either side).
• Close the lips gently around the thermometer — do not bite. Breathe through the nose.
• Hold the thermometer in place until it beeps (usually 30‑60 seconds).
• Clean the thermometer with soap and water or an alcohol wipe after use.

Normal oral temperature is about 97.6°F – 99.6°F (36.5°C – 37.5°C), averaging 98.6°F (37°C). A reading of ≥100.4°F (38°C) indicates a fever.

3. Taking a Rectal Temperature (Gold Standard for Infants)

Rectal measurement is the most accurate method for infants under 3 months and is also used in older adults when an exact core temperature is needed.

• Use a designated rectal thermometer (clearly labelled to avoid cross‑use) with a flexible tip.
• Apply a small amount of water‑soluble lubricant (petroleum jelly) to the tip.
• For an infant: lay them on their back, lift their legs toward the chest as if changing a diaper, and gently insert the thermometer about 1‑2 cm (½‑¾ inch) into the rectum. For an older child or adult: lie on one side with knees pulled up.
• Hold the thermometer in place until it beeps. Do not force it if there is resistance.
• Normal rectal temperature is about 0.5°F – 1°F (0.3°C – 0.6°C) higher than oral temperature. A reading ≥100.4°F (38°C) indicates a fever.

4. Armpit (Axillary) Temperature: Convenient but Less Accurate

The axillary method is widely used in India and is acceptable for initial screening, but it is the least accurate of the common methods.

• Make sure the armpit is dry. Sweat can cause a falsely low reading.
• Place the thermometer tip in the centre of the armpit and press the arm firmly against the body to create a sealed pocket.
• Wait for the beep (typically 60‑90 seconds).
• Normal axillary temperature is about 0.5°F – 1°F (0.3°C – 0.6°C) lower than oral temperature. A reading ≥99°F (37.2°C) may indicate a fever.

Dr. Reddy advises that if an axillary reading is borderline or does not match your symptoms, confirm it with an oral or ear reading.

5. Tympanic (Ear) and Temporal (Forehead) Thermometers

• Tympanic: Pull the outer ear gently back and up (for adults) or back and down (for children under 1 year) to straighten the ear canal. Insert the probe snugly and press the button. Earwax or a bent ear canal can cause falsely low readings. Check both ears; if they differ, use the higher reading.
• Temporal artery: Place the sensor on the centre of the forehead and slide it slowly toward the top of the ear, keeping contact with the skin. Sweat, drafts, or a hat can affect the reading. Temporal thermometers are convenient but can be less accurate when the patient has been outdoors in extreme temperatures.

6. Common Mistakes That Lead to Inaccurate Readings

• Measuring oral temperature immediately after a hot or cold drink.
• Not placing the thermometer correctly under the tongue (it should be in the sublingual pocket, not resting on top of the tongue).
• Using a dry armpit without adequate skin contact.
• Failing to clean the thermometer between uses, especially when used in different sites.
• Using an ear thermometer in an ear filled with wax.
• Not waiting for the device to beep before removing it.
• Comparing readings from two different types of thermometers — always note the method alongside the reading.
• Using an oral thermometer rectally without proper labelling and cleaning — this can spread bacteria.