Temperature Measurement

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Temperature Measurement

Description of Body Temperature
Locations for Body Temperature Measurement
Why Temperature is Measured in the Hospital
How Temperature is Measured
When Temperature is Measured
What Abnormal Results of Temperature Measurement Mean


Temperature measurement is the quantification of a person’s body temperature, which is an important indicator of a person’s physiological state. Temperature measurement is done to assess whether body temperature is within a narrow, safe range. An abnormally high temperature is a fever, a sign that the body is mounting an immune response.

Description of Body Temperature

Normal body temperature varies from person to person. Gender, age, recent physical activity, having a meal, and the menstrual cycle all affect body temperature within the normal range. Body temperature measurement also varies based on which part of the body it is taken from. The average normal body temperature is 98.6°F (37°C) when taken orally. However, body temperature may vary slightly more than one degree higher or lower and still be in the normal range. Normal body temperature can range from 97 to 100°F (36.1 to 37.8°C). A temperature higher than normal is considered a fever (hyperthermia). A temperature lower than 96°F is considered a state of hypothermia. Temperature varies with the age of a person. Younger people tend to have higher body temperatures. Temperature also varies by the time of day it is taken. Body temperature is usually lowest in the morning and highest in the evening. Temperature can also be elevated by exercise, stress or strong emotions, eating food, heavy clothing, certain medications, or high room temperature. All of these factors need to be taken into account when temperature is measured because they affect the interpretation of temperature-measurement results.

Locations for Body Temperature Measurement


Temperature is often measured orally by placing a thermometer in the heat pocket under the tongue in the back of the mouth. The mouth is closed and the patient breathes through their nose for several minutes until the temperature is measured. Oral temperatures that are 1 to 1.5°F above a patient’s normal body temperature are considered a fever.


The most accurate method of assessing body temperature is rectally using a glass or electronic digital thermometer. A lubricated electronic probe is inserted about 1to 1.5 inches into the anal canal. Normal rectal temperature is usually 0.5 to 1.0°F higher than oral temperature. A rectal body temperature above 100.5°F is considered a fever in adults. In infants and children, a normal rectal temperature may approach 101°F. Rectal temperature measurement is a convenient alternative for patients who are unable to hold an oral thermometer in a closed mouth due to illness or being unconscious. It is also used for infants or very young children who cannot safely hold a thermometer in their mouth.


Another temperature measurement method sometimes employed by pediatricians is placing a thermometer in the armpit. While less invasive than rectal temperature measurement, this location is the least accurate and takes the longest time to measure. Normal temperature in the armpit tends to be 0.5 to 1.0°F lower than oral temperature.


Thermometers made for the ear can be used to assess the body’s core temperature, which approximates the temperature of the internal organs. Ear thermometers may measure the temperature of the eardrum or the ear canal. Normal ear temperature tends to be about 1.4°F higher than oral temperature.

Why Temperature is Measured in the Hospital

Temperature measurement is done to monitor a person’s body temperature. Body temperature, heart rate, breathing rate, and blood pressure are all considered vital signs. If body temperature is abnormal, it is an important indicator of the physiological state of the body. A fever is the body mounting an immune defense against a foreign invader to help fight infection or disease. A fever can be a critical sign that the body is fighting a battle, such as a post-surgical bacterial infection or cancer. It can also be used to assess whether a treatment is working, such as in antibiotic treatment of infections. The extent of a fever does not necessarily correlate with the severity of the illness. However, temperature measurement is still an important tool used in the hospital to monitor a patient’s health.

Measuring and monitoring body temperature can be done at specific time points in the process of diagnosing and treating illness. Physicians sometimes use repeated temperature measurements to follow patterns in body temperature such as how frequently a fever occurs and how long it lasts. These measurements may provide diagnostic insights into body processes during illness. Temperature measurement is especially important for the management of a critically ill patient, where trends in body temperature are significant. Careful temperature measurement is also essential in the health management of elderly people. Because the elderly may have difficulty mounting a high fever as an immune response against infection, a grade fever is often the only early sign that something is wrong. Elderly people are also more prone to hypothermia than younger individuals.

How Temperature is Measured

Mercury Thermometers

Traditionally temperature was measured orally with a graded glass thermometer containing mercury. The level to which the mercury would rise on the graded scale was an indication of temperature. According to the Environmental Protection Agency (EPA), mercury is a toxic substance that is poisonous to both humans and the environment. The dangers associated with mercury if the thermometer breaks in a patient’s body and the cost of disposing of mercury led to the development of modern mercury-free thermometers that do not pose such health risks.

Electronic Digital Thermometers

Digital thermometers with an electronic probe are far more accurate at measuring body temperature than the old mercury thermometers. They are usually a lightweight plastic and shaped like a broad pencil, with the electronic temperature probe at the tip. A digital temperature display window at the other end measures temperature down to a tenth of a decimal point. Electronic thermometers are designed for use in the mouth, rectum, or armpit. Rectal thermometers may have a colored probe to help distinguish them from silver-tipped oral thermometers. They are accurate and easy to use in the hospital. In addition to antiseptic, disposable protective guards are often used to cover the probe to help prevent the spread of infection between patients.

Infrared Ear Thermometers

Digital ear thermometers use infrared energy to measure body temperature instead of an electronic probe. They are made in different shapes. One design has a small cone-shaped end that is placed within the ear. An infrared beam is then aimed at the eardrum. Ear thermometers only take seconds to measure body temperature, whereas other types of thermometers require minutes for accurate temperature measurement.

Disposable Thermometers

Hospitals often use disposable thermometers to decrease risk of transmitting infection from patient to patient. Disposable thermometers are thin pieces of plastic with a colored grid of dots representing temperature on one end. The color change displayed in the grid is how temperature measurement is visualized. Disposable thermometers are accurate and safe since they contain no glass or mercury. One thermometer can be reused on the same patient until it is no longer needed. Disposable thermometers are designed for use in the mouth, armpit, or rectum. A disposable thermometer in patch form has been designed for use on infants whose temperature needs to be monitored for long periods of time.

When Temperature is Measured

In the hospital routine temperature measurement takes place twice a day. The first measurement is usually done in the morning between 7 and 10 am. The second measurement takes place in the afternoon around 2 pm. If a patient is suspected to have an illness causing fever or is critically ill, temperature measurement may be performed up to four times an hour to closely monitor the situation. Interpretation of temperature measurement is influenced by when the measurement is taken. In the early morning, normal adult body temperature may be as low as 96.4°F (35.8°C). In the evening normal temperature may be as high as 99.1°F (37.3°C).

What Abnormal Results of Temperature Measurement Mean

An abnormally high body temperature means that the patient has a fever. Fever is not an illness itself but rather a defense mechanism of the body to fight disease or infection. Higher body temperatures are less hospitable for most bacteria and viruses, and also allow the body’s immune system to mobilize against disease more readily. However, if the body temperature is raised too high for a prolonged period of time, then fever may pose a threat to the body. In infants and children, a very high fever may occur even in response to minor infections.

Abnormally High Body Temperature Potential Causes

  • Infection by Bacteria, Viruses, or Parasites
  • Medications
  • Response to Surgical Procedures without having an Infection
  • Drugs used during Surgery
  • Metabolic Disorders such as Hyperthyroidism
  • Heat Stroke
  • Extreme Dehydration
  • Cancer
  • Inflammatory Conditions and Autoimmune Disorders
  • Physical Trauma
  • Certain Blood Disorders

Abnormally Low Body Temperature Potential Causes

  • Hypothermia from Cold Exposure
  • Medications
  • Metabolic Disorders such as Hypothyroidism
  • Excessive Alcohol Intake
  • Starvation


Autoimmune Disorders— Disorder in which the immune system mounts a response against some aspect of its own body.

Hypothermia— State of abnormally low body temperature that can be fatal if left untreated.

Hyperthyroidism— Disease of the thyroid gland involving overproduction of thyroid hormones. Hyperthyroidism affects body temperature.

Hypothyroidism— Disease of the thyroid gland involving underproduction of thyroid hormones. Hypothyroidism affects body temperature.

Immune Response— Any response of the immune system against something identified by the immune system as being foreign.

Infrared— A type of energy wave given off as heat.

Physiological State— The status of the normal vital life functions of a living organism.

Vital Signs— The physiological aspects of body function basic to life. They are temperature, pulse, breathing rate, and blood pressure.



Bates Guide to Physical Examination and History Taking, Eighth Edition. Lippincott Williams & Wilkins 2003.

Cecil Essentials of Medicine, Sixth Edition. Saunders 2004.

Harrison’s Principles of Internal Medicine, Sixteenth Edition. McGraw-Hill 2005.

Merck Manual of Diagnosis and Therapy, Eighteenth Edition 2006.

Oxford Textbook of Geriatric Medicine, Second Edition. Oxford University Press 2000.

Maria Basile, PhD