How to Read a Fever

“Feeling your forehead doesn’t even give a good guess about your body temperature,” says Harold Neu, M.D. “You must use a fever thermometer to get an accurate reading.”

There are two basic types of glass fever thermometers, oral and rectal, with the only difference being in the shape of the bulb: thin and long on the oral and short and stubby on the rectal. Rectal temperatures are the most accurate; oral temperatures can be thrown off by breathing through the mouth, smoking, or having just had a drink of something hot or cold. Rectal readings are, in general, one degree Fahrenheit higher than oral temperatures. ( If neither of these methods is convenient, temperature can also be taken by placing an oral thermometer under the armpit for at least two minutes, which will give a reading about one degree Fahrenheit lower than an oral temperature. )

Glass thermometers have several disadvantages:They may break in handling or even in the mouth.

If a thermometer breaks in the mouth, don’t worry; there is only a tiny amount of mercury in the tube. Just be sure to remove any slivers of glass from the mouth. Glass thermometers also need to be shaken down to 96 degrees Fahrenheit in order to allow the body’s true temperature to register. On the other hand, glass thermometers have a big advantage: They are cheap, with most selling for about three dollars.

More convenient and somewhat more expensive ( costing about seven to ten dollars) are newer digital thermometers, which register temperatures accurately within a tenth of a degree. These thermometers are also fast. It takes less than a minute for the temperature to register as compared to three minutes with glass thermometers. Most digital thermometers run on a “button” (or hearing-aid type) battery that boasts a two- to three-year life under normal use.

Pharmacies generally carry a selection of both glass and digital thermometers.