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Rectal Thermometer - Usage and Interpretation of the Readings

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Rectal Thermometer - Usage and Interpretation of the Readings

4 min read


A rectal thermometer is a device that is inserted into the baby’s rectum to check the body’s internal temperature. Read further to know more.

Medically reviewed by

Dr. Kaushal Bhavsar

Published At August 5, 2022
Reviewed AtAugust 4, 2023


A thermometer is used to measure the body’s temperature to detect any illness. A higher body temperature is defined as fever. Fever usually causes no harm, and it is often a sign that the body is fighting against an infection. But higher temperatures require medical help. During the COVID-19 pandemic, parents were much worried about their children’s health and were checking for symptoms of fever from time to time. Hence, it is always important to have a thermometer at home and also know how to handle it. Nowadays, the modern digital thermometer has replaced the traditional mercury thermometer. It is comparatively easier and safer to use.

What Is the Normal Body Temperature?

Before using any thermometer, one should know the normal body temperature in order to find out an abnormal body temperature. The normal temperature varies with each child depending on their age, activity, and time of the day. So, any temperature above 100.4 degrees Fahrenheit or 37 degrees Celsius is considered a fever. It is a sign that a child is battling an infection.

What Are the Different Types of Thermometers?

Traditional glass thermometers containing mercury are not recommended these days. The reason is that there is a risk of glass breakage, and the mercury can be harmful to the baby. So, digital thermometers or electronic thermometers are used nowadays. The digital thermometer gives the quickest and more accurate reading. The digital thermometer is available in various shapes and sizes according to its use. They are:

  • Rectal Thermometer (used in the bottom): It is the most accurate method for infants under three months of age.

  • Oral Thermometer (used in the mouth): It is used in kids between four and five years of age.

  • Axillary Thermometers (used in the armpit): They are the least accurate but may be used for the initial temperature check.

  • Temporal Artery Thermometer: They are used to measure the heat waves on the forehead. They can be used in infants who are three months and older.

  • Ear or Tympanic Thermometer: They measure the heat waves in the eardrum.

Due to their less accuracy, certain thermometers are not preferred. They are as follows::

  • Plastic strip thermometer.

  • Pacifier thermometer.

  • Smartphone temperature applications.

What Is a Rectal Thermometer?

The rectal thermometer is a device that is used to measure the baby’s internal temperature by placing the thermometer into the baby's rectum (end of the colon and also the other end of the anal canal). A rectal thermometer is more accurate and is used in children less than three months of age. Their accuracy is because they are not influenced by factors such as drinking cold and hot drinks as in an oral thermometer. The normal rectal temperature is 99.6 degrees Fahrenheit or 37.5 degrees Celsius.

Why Is Rectal Thermometer Preferred In Infants?

Younger children cannot hold an oral thermometer under their tongue for a sufficient time. An axillary thermometer also takes several minutes to record the temperature, which can also be difficult in infants. As these can make the baby uncomfortable, a rectal thermometer is preferred.

Though rectal thermometers can be used in infancy as well as in adulthood, older children may not prefer rectal temperature checking. One can stop using a rectal thermometer after three months of age provided, the child is cooperative while using other thermometers.

How to Measure the Baby’s Temperature Using a Rectal Thermometer?

Many parents panic when a rectal thermometer is used. They need not worry as it is a completely simple and safe process. The oral and rectal thermometers come in different shapes and sizes. Oral thermometers should not be used rectally as they may injure the baby. Rectal thermometers differ from oral thermometers in that rectal thermometers have a security bulb that is specifically designed to record the rectal temperatures safely. A rectal thermometer can be used to record the temperature in the following ways:

  • The tip of the thermometer is washed with soap and rinsed thoroughly.

  • Lubricants such as petroleum jelly are applied on one end of the thermometer and also to the child’s bottom.

  • Now, the child is made to lay down across the parent’s lap such that the child’s belly is facing downwards.

  • Another position is the legs facing upwards towards the chest and held at the back of the thighs with one hand.

  • Now with the other hand, the lubricated thermometer is gently inserted into the child’s anal opening. It is gently inserted to about half to one inch or about 1.25 centimeters to 2.5 centimeters.

  • If one feels any resistance, then stop proceeding.

  • One should wait until they can hear a beep sound or other signal.

  • The reading and the time of the day (morning or evening) are noted.

  • Then, the thermometer is removed and disinfected with soap and water or rubbed with alcohol.

  • One should always accompany the child while using a thermometer.

  • A rectal thermometer should be kept labeled so that others do not use it orally.

How to Interpret the Readings of a Rectal Thermometer?

Generally, 98.6 degrees Fahrenheit or 37 degrees Celsius is considered to be the normal body temperature. This may however vary due to certain reasons. They are:

  • Age: In children over six months, the temperature can vary by one to two degrees.

  • Time of the Day: Highest temperature is recorded during the evening.

  • Type of Thermometer: A rectal and ear thermometer is 0.5 to 1 degree Fahrenheit higher than the oral thermometer.

The armpit and forehead temperature is 0.5 to 1 degree Fahrenheit lower than the temperature measured using an oral thermometer. The difference between oral and rectal temperature is that the rectal temperature is 0.5 to 1 degree Fahrenheit higher than the oral temperature.

Sometimes, body temperature can also be raised when one is active, or is in areas with high temperature or high humidity, during teething (emergence of teeth through the gums of infants and young children). in the child, wearing heavy clothes, and so on.

When Should One Call the Doctor?

One must seek medical help right away under the following conditions:

  • If the baby is under three months of age or older and has a fever of 100.4 degrees Fahrenheit or higher.

  • If the child has breathing difficulty.

  • Fever that lasts longer than 48 hours.

  • The fever goes away but comes back.

  • Seizures (a sudden, uncontrolled shot of electrical activity in the brain).

When Should One Not Use a Rectal Thermometer?

In case the child is constipated, using a rectal thermometer might stimulate a bowel movement. In such situations, one must talk with the child’s pediatrician (child doctor) and then use a rectal thermometer.


Be it any thermometer, one must always have a good knowledge before using it. Proper guidance should be taken from either the health care professionals or the inter-professional persons like nurses and midwives before using them on the child. It is always advisable to have a digital thermometer at home. Young kids may have a fever at any time, and it is always mandatory to monitor the child’s body temperature because very high temperatures may lead to seizures. To prevent these conditions, monitor the child's temperature regularly, and if one notices a high temperature, the child should be immediately taken to a healthcare professional.

Dr. Kaushal Bhavsar
Dr. Kaushal Bhavsar

Pulmonology (Asthma Doctors)


rectal temperaturebody temperature
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