7.2 Temperature Measurement

Routes and Their Normal Ranges

Body temperature reflects the balance between heat the body produces and heat it loses. The number you get depends on where you measure, so always document the route.

Normal Temperature by Route

Memory anchor: rectal is about 1 degree F higher than oral; axillary is about 1 degree F lower than oral. So if an oral reading would be 98.6, the same patient reads roughly 99.6 rectally and 97.6 axillary.

Oral Temperature

Use for alert, cooperative adults who can hold the probe under the tongue. Do not use it for unconscious patients, those receiving oxygen by mask, confused patients who may bite, infants, or anyone with a mouth injury. After hot or cold food, drink, or smoking, wait 15-30 minutes, otherwise the reading is falsely high or low. Place the probe under the tongue to one side (the sublingual pocket), have the patient close the lips, and read at the beep.

Rectal, Axillary, and Electronic Methods

Rectal Temperature

The most accurate route because it samples a closed core cavity, but it is the most invasive. Use it when the oral route is impossible, but avoid it with rectal surgery, diarrhea, hemorrhoids, patients on blood thinners (bleeding risk), and cardiac patients (insertion can stimulate the vagus nerve and slow the heart).

Procedure essentials: provide privacy, glove, lubricate the probe cover, position the patient side-lying (left Sims' position), insert only 1/2 to 1 inch in an adult, and hold it in place the entire time. Never walk away from a rectal thermometer.

Axillary Temperature

The least accurate route but the least invasive, useful when both oral and rectal are contraindicated, or for screening. Dry the armpit, center the probe in the axilla, hold the arm snug against the body, and label the result "axillary."

Electronic and Disposable Thermometers

For a tympanic reading, pull the ear up and back in adults (down and back in infants under 1 year) to straighten the canal.

Naming Abnormal Temperatures

Report immediately any temperature above 101 degrees F or below 97 degrees F, a sudden change from baseline, or a normal number paired with the patient feeling hot, chilled, or sweaty.

Fahrenheit, Celsius, and Common Test Traps

Most United States facilities chart in Fahrenheit, but you may see Celsius on imported equipment or in education materials, so know the anchor points: 98.6 F equals 37.0 C, 100.4 F (the fever line) equals 38.0 C, and 104 F (hyperthermia) equals 40.0 C. You will not be asked to do hand calculations on the skills exam, but a written knowledge test may pair the two scales, so memorize those three matched values rather than a formula.

Probe Covers, Cleaning, and Infection Control

Every oral, rectal, and tympanic reading uses a fresh disposable probe cover, which is both an accuracy and an infection-control step. Rectal and oral probes are color-coded, usually red for rectal and blue or green for oral, and they are never interchanged. After a rectal temperature, remove gloves and perform hand hygiene before touching anything else, because the perineal area is a reservoir for bacteria such as E. coli. A common written-test trap asks what to do if the same digital unit must serve both routes; the answer is to use the dedicated color-coded probe and a new cover, not to wipe and reuse.

A Worked Scenario

A confused resident on oxygen by mask needs a temperature. The CNA should not choose the oral route: the patient cannot reliably hold the probe, and the mask interferes. Tympanic or temporal artery is fast and safe here; axillary is acceptable but slower and least accurate. If the nurse needs a true core reading and there is no rectal contraindication, the rectal route is most accurate, but the CNA confirms it is allowed first.

Why Temperature Trends Matter

A single number means little without context. A resident whose temperature climbs from a baseline 97.8 F to 99.8 F over a shift may be developing an infection even though 99.8 F is still inside the "normal" band. This is why you compare to baseline and report a rising trend, not just a number that crosses a fixed line. Document the time with every reading so the nurse can see the slope, and never alter or estimate a temperature to make it look normal.