Temperature readings vary from one healthy person to another and are influenced by factors such as age, gender, and ambient temperature. Temperature readings vary even in the same person at different times. Therefore, normal body temperatures can only be given as ranges.

Readings from different parts of the body also differ from each other. For example, rectal temperatures tend to be higher than oral (mouth) temperatures, and ear and armpit temperatures tend to be lower than oral temperatures. Here are ranges of average temperatures of healthy people taken from different body sites in various studies (Sund-Levander et al 2004):

Mouth            33.2°C to 38.2°C
Armpit           35.5°C to 37.0°C
Rectal           34.4°C–37.8°C.

Because of the wide variation in “normal” temperatures, Age Smart instructions suggest that you find out your normal temperature ranges by taking your temperature several times when you are feeling healthy. Guidelines issued by NICE (National Institute for Health and Clinical Excellence) suggest that fever should be defined as “an elevation of body temperature above the normal daily variation” (NICE 2007).

National Institute for Health and Clinical Excellence (NICE). National Collaborating Centre for Women’s and Children’s Health. Feverish illness in children. Ed. Welsh A. RCOG Press at the Royal College of Obstetricians and Gynaecologists. 2007. http://guidance.nice.org.uk/CG160

Sund-Levander M, Grodzinsky E, Loyd D, Wahren LK. Errors in body temperature assessment related to individual variation, measuring technique and equipment. Int J Nurs Pract. 2004;10(5):216-23.

The differences are small, and many healthy children have temperatures similar to temperatures in adults and vice versa.

One large study found that, on average, body temperature gets lower as people get older (Chamberlain et al 1995), but other studies do not agree (Herzog & Coyne 1993, Stern et al 2009). So the assumption that children in general have higher normal body temperatures than adults may not be true.

Chamberlain JM, Terndrup TE, Alexander DT, et al. Determination of normal ear temperature with an infrared emission detection thermometer. Ann Emerg Med. 1995;25(1):15-20.
Herzog LW, Coyne LJ. What is fever? Normal temperature in infants less than 3 months old. Clin Pediatr (Phila). 1993;32(3):142-6.
Stern G, Beel J, Suki B, et al. Long-range correlations in rectal temperature fluctuations of healthy infants during maturation. PLoS One. 2009;4(7):e6431.

The tops of “normal” temperature ranges, which are based on statistical sampling of healthy people, are not the same as fever as defined by medical criteria. The Age Smart color scale is not meant to show whether temperatures are within the normal range. It is intended to alert users to temperatures that they should worry about based on the most conservative criteria. Very young babies may not run high fevers when they have infections, so even a temperature that is only a little higher than normal may be a sign of infection in a little baby. In little babies, even a small rise in temperature must be taken seriously (Jain et al 2014).

You should also keep in mind that fever is only one of the clinical signs of serious illness. Doctors decide whether to treat a patient with fever based on such things as how long the patient has had a fever, whether the patient has had all the important shots (immunizations), whether the patient seems unusually sleepy, whether the patient has a rash, and whether the patient has been vomiting or has no appetite.

Jain S, Cheng J, Alpern ER, et al. Management of febrile neonates in US pediatric emergency departments. Pediatrics. 2014;133(2):187-95.

High fever in children should be a concern. Consult a physician if a child’s temperature is in the elevated (fever) or high temperature (high fever) range, especially if the child has symptoms along with fever.

High fever usually is a sign of infection, which can be either viral or bacterial. In young children, there is a chance that the infection can be meningitis or sepsis, either of which can be very serious. To be on the safe side, if temperature readings are elevated or high, you should consult a doctor immediately. The doctor will ask about symptoms, because without symptoms, even a high temperature may not be cause for alarm.

You may choose to not consult a doctor if the indicator is green and is not accompanied by worrisome signs or symptoms. If you have doubts about the symptoms, you should always consult your physician.

The cutoff of 37.7 is specific to the Thermoscan Infrared Ear thermometer only. Different digital thermometers are programmed to give different readouts from different body sites. Since the Thermoscan Infrared Ear thermometer displays an oral equivalent display reading, the cutoff has been set to a value that would be considered an elevated temperature on an oral thermometer.. For a different thermometer that has been set to display a rectal or underarm reading, the threshold would be different. For a thermometer set to display a rectal temperature equivalent, for example, a reading of 37.7°C on an oral equivalent thermometer would be equivalent to 38°C rectally and the yellow indicator light would be triggered at this temperature.

The standard definitions of fever used in emergency departments generally start at about 38 C (Sullivan 2011, Am Coll Emergency Physicians 2003, NICE 2007, Hoffman et al 2013). Keep in mind that temperature above 38 C is only one of the factors that doctors consider when deciding whether to run diagnostic tests or admit a patient to the hospital. Doctors consider symptoms as well as temperature readings and laboratory tests to decide whether treatment is needed.

American College of Emergency Physicians Clinical Policies Committee; American College of Emergency Physicians Clinical Policies Subcommittee on Pediatric Fever. Clinical policy for children younger than three years presenting to the emergency department with fever. Ann Emerg Med. 2003 Oct;42(4):530-45.
Hoffman RJ, Etwaru K, Dreisinger N, Khokhar A, Husk G. Comparison of temporal artery thermometry and rectal thermometry in febrile pediatric emergency department patients. Pediatr Emerg Care. 2013;29(3):301-4.
National Institute for Health and Clinical Excellence (NICE). National Collaborating Centre for Women’s and Children’s Health. Feverish illness in children. Ed. Welsh A. RCOG Press at the Royal College of Obstetricians and Gynaecologists. 2007. http://guidance.nice.org.uk/CG160
Sullivan JE, Farrar HC, Section on Clinical Pharmacology and Therapeutics, Committee on Drugs. Fever and antipyretic use in children. Pediatrics. 2011;127(3):580-7.

For children in the age range of 0 to 3 months, temperatures above 38 C (rectal) often mean that there is an infection, so they need immediate attention. In very young babies, any temperature elevation should be considered serious (red zone). That is why no yellow zone is given for this age group on the Age Smart scale.

NICE guidelines (NICE 2007) suggest that the cutoff for high fever should be 38.0°C (rectal) for infants less than 3 months of age and 39°C (rectal) for infants between 3 and 6 months of age. When adjusted to an equivalent value for oral readings, these are the same as the Age Smart cutoffs for high fever that are programmed into the Thermoscan Infrared thermometer.

However, the Age Smart scale adds an intermediate elevated temperature range for toddlers between 3 and 36 months to be more conservative, since you would not want to miss a potentially dangerous illness.

National Institute for Health and Clinical Excellence (NICE). National Collaborating Centre for Women’s and Children’s Health. Feverish illness in children. Ed. Welsh A. RCOG Press at the Royal College of Obstetricians and Gynaecologists. 2007. http://guidance.nice.org.uk/CG160

If there are symptoms, such as unusual irritability, unresponsiveness, weakness, vomiting, diarrhea, dry skin, unusual thirst, seizure, loss of appetite, or changes in activity, you should contact a doctor regardless of whether the thermometer shows green, yellow, or red.

If temperature readings are in the red range, you should speak with a doctor whether or not there are symptoms . Someone who has an elevated temperature in the yellow range and also feels sick (or appears to be sick), should speak with a doctor to find out whether to come into the medical office. You may choose not to consult a physician only if the indicator is green and there are no symptoms. When in doubt about symptoms, always consult your physician.