Fever Defined

Here’s what to look for when taking your child’s temperature

The natural question that begins most discussions involving fever is: Exactly what temperature is considered a fever? You'd think you could get a straight answer to such a seemingly straightforward and commonly asked question, but chances are good that if you asked around you'd get a wide range of answers starting anywhere from 99°F on up to 100.4°F or more. That's because several factors go into defining fever.

It's a Matter of Degree

Normal body temperature is generally defined as 98.6°F. A rectal temperature of 100.4°F or higher is typically used to define a fever in newborns. How the numbers that fall in between are interpreted tends to vary considerably. In part, that's because body temperature fluctuates normally over the course of any given day — generally rising a bit in the afternoon and evening. The degree of variation is thought to increase with age; while a child's or adult's temperature can vary as much as 2°F (1.1°C) over the course of the day, you should expect your newborn's range of normal to be much smaller.

Using Thermometers for Good Measure

Back in the days when we were kids, pretty much everyone used a glass mercury-containing thermometer. While you probably aren't interested in the long-winded discussion of the potential dangers of broken glass and the risks associated with exposure to spilled mercury, suffice it to say, these thermometers are not just out of style, they are no longer recommended. From a practical standpoint, it's also a good thing; we have yet to talk to anyone who found mercury thermometers easy to read. In contrast, the readily available digital thermometers currently in vogue are (almost) foolproof — put them in the right place and the numbers just appear.

Hot Spots — Fever by Location

The easiest way to categorize thermometers is according to the part of the body where the temperature is taken (axillary = underarm, tympanic = ear, oral/pacifier = mouth, temperature strips = forehead, and rectal = self-explanatory). Temperature strips, albeit temptingly easy, are generally frowned on as being notoriously inaccurate. Even when taken using some of the more accepted temperature-taking routes, such as placing a thermometer in a child's mouth or armpit, the resulting temperatures can vary quite a bit. That is why the definition of fever usually includes some reference to the location.