Rectal temperature may not be accurate assessment of core body temperature

In one study, other methods of recording fever were more accurate than rectal thermometers, the basis of monitoring pediatric fever since the mid-fifties.

Issue: July 1997

WASHINGTON, D.C. — In a study guaranteed to make parents a bit less squeamish about taking their children's temperature, ear temperatures obtained with a standard infrared 1-second thermometer were a more accurate reflection of core body temperature — especially when core temperature was changing, as it does during a fever — than rectal temperatures measured with a standard mercury thermometer.

The first commercial infrared thermometers were introduced in 1986. They detected infrared emissions from the eardrum and the ear canal surrounding it, measure the ear temperature and calculate body temperature based on that reading.

Differing results

However, results from one device may not match results from another. "Different manufacturers take that reading and do things with it. One of them may add 0.8° and say it's a rectal equivalent. Another will add 0.3° and say it's an oral equivalent. Another company might add 1.2° and say it's core temperature. But what they all do is measure temperature in the ear," said James M. Chamberlain, MD, associate professor of pediatrics at George Washington University and associate medical director at Children's Hospital emergency department.

These offsets are conveniences designed "to make readings more comparable to what people are used to dealing with, which are rectal or oral readings." In fact, the actual reading, without the offsets, would probably be a more accurate measure of body temperature.

Chamberlain conducted the study with Susan Verghese, MD, of Children's National Medical Center, who presented the results at the Pediatric Societies' Annual Meeting here.

The two-part study first examined 40 patients with fever in the emergency department at Children's Hospital. Rectal and ear temperatures were taken at the same time. The patients then received acetaminophen (Tylenol, McNeil) to lower the fever. An hour later, researchers measured the temperatures again and compared the difference between rectal and ear measurements and found that there was no consistent relationship. The ear and rectal temperature differences changed dramatically over time.

"They were all over the place; some of them went up, some went down, some stayed the same," said Chamberlain. The ear temperatures were reported to be accurate, however, since they had been taken three times.

The second phase of the study involved 10 children undergoing open heart surgery. The children's bodies were cooled to a temperature of about 25° C during surgery and then were warmed after surgery, causing a fever spike. Researchers used an indwelling contact thermistor in the esophagus to measure body temperature — "That's the gold standard," according to Chamberlain. In addition, researchers also inserted a rectal probe, a tympanic membrane thermometer in the left ear and an infrared ear thermometer in the right ear of each child.

Rectal reading different

All readings matched in all the children except for the rectal reading, and all children showed exactly the same pattern. "Rectal was way off; at times it was 6° C off from any other body sites. When you have the fever spike after surgery, when the temperature goes up to 39° C, [the rectal probe] never exhibited that response. It was too slow too respond, because the blood supply to the rectum is poor," Chamberlain said. "With those patients we showed that it's the rectal temperature that is inaccurate, not the ear temperature; ear temperature perfectly tracked core [temperature]. The results were highly statistically significant."

Why rectal?

"We've known since 1954 that rectal temperature does not accurately track core but people have just ignored it," Chamberlain said. "Unfortunately, we have 20 years of studies on using rectal temperature and the risk of urinary tract infections and bacterial diseases when you get a rectal temperature above 39° C. So a lot of clinicians still depend on the rectal temperature in young infants, between birth and 3 months, where the other clinical signs aren't reliable."

Further research using an ear thermometer for fever measurement could eliminate the use of the rectal thermometers completely, according to Chamberlain.

"There's no reason to use a rectal thermometer. I don't know why anyone ever chose that site," he said.

For more information:

Verghese ST, Chamberlain JM. Ear-rectal temperature differences: the effect of time and comparison to core body. Presented at the 37th annual meeting of the Ambulatory Pediatric Association. May 2-6. Washington, DC.