Study Questions Efficacy of Popular Forehead Thermometer
Company officials say findings skewed by improper use of product
FRIDAY, July 13, 2007 (HealthDay News) -- A new study is calling into question a widely used, noninvasive method of taking an individual's temperature, but company officials claim the findings were skewed by improper use of the product.
Craig Crandall, a research scientist at the Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and his colleagues used a body suit filled with hot water to induce heat stress in 16 individuals, 11 of whom were female. They then measured each subject's temperature using both a temporal thermometer made by Exergen Corp., of Watertown, Mass., and an "ingestible pill telemetry system."
A temporal thermometer computes the core body temperature based on forehead temperature, as measured with an infrared scanner. An ingestible pill telemetry system uses a pill-sized device that is swallowed and transmits, from inside the body, a radio signal that correlates to core body temperature.
Crandall's team found that forehead readings were unreliable indicators of core body temperature. Though the two devices reported the same temperature at the start of the experiment, as the subjects' core body temperature (as measured by the ingestible pill) rose, the temporal thermometer readings actually fell. At 50 minutes, for instance, the internally measured temperature had risen about 0.7 degrees C, on average, while the temporal thermometer reported a decrease of about 0.3 degrees C.
The study is published in the July issue of Medicine & Science in Sports & Exercise.
The temporal thermometer "is no better" than the tried-and-true parental method of feeling a child's forehead, concluded Crandall. In fact, "putting your hand on the forehead, in my opinion, is better, because we know the subject is hot, whereas this device actually said a patient was cooling down. So, the hand could actually be more accurate."
However, Exergen President Dr. Francesco Pompei claims the researchers failed to use the product as intended, and that colored the final results.
"The specific concern we had expressed to them two years ago was that they were using the temporal artery thermometer in a manner for which it was not designed and contrary to the manufacturer's instructions ... It was not that they were doing heat stress studies, but that they were conducting them with artificial heating and cooling apparatus, which greatly distorts the thermophysiology of either a patient or an athlete, to the point of little meaning as a laboratory model for actual patients or athletes," Pompei said in a statement.
Frederick Mueller, director of the National Center for Catastrophic Sports Injury Research at the University of North Carolina, said the study was "very interesting," but noted it involved a very small number of subjects, and so must be repeated with a larger population. He also suggested the authors might have seen different results if they had measured temperature after exercise-induced heating, rather than by artificially raising body temperature with a water suit.
The Exergen temporal thermometer is available in versions for both medical professionals and consumers. About one in three Texas hospitals uses them, as does the medical tent at the Boston Marathon. According to a list provided by Exergen, customers include such institutions as Massachusetts General Hospital, the University of California Medical Center in San Francisco, and New York University Medical Center.
However, there have been few reports to validate their efficacy, according to the study authors, and those that have been published describe conflicting findings.
The Texas study arose after a temporal thermometer failed to recognize a fever of 40 degrees C in a patient at Parkland Memorial Hospital in Dallas, a woman with a severe but undiagnosed infection, explained study co-author Dr. Benjamin Levine.
"I investigated it in our hospital, and it turned out many of the nurses had the same experience," he said. "In other words, they [the nurses] thought there was a fever, and a temporal measurement said they didn't."
According to Crandall, there are several reasons why forehead temperature is not a reliable indicator of core body temperature. For one thing, forehead temperature is based on the temperature of the temporal artery, which lies between the skull and the skin of the forehead. But that artery is not in exactly the same location in every individual, nor is it found at precisely the same depth.
More important, there are any number of factors that can change skin temperature without affecting internal temperature. During periods of temperature transition, such as when an individual is developing a fever or when a fever "breaks," blood flow to the skin changes, raising or lowering skin temperature relative to the core body temperature.
Sweat, especially, can affect skin temperature, Crandall noted. "Sweating cools the skin even if the internal temperature is high. I can get someone's temperature elevated, and if I put water on the face and use a fan, the skin temperature will be cool."
For athletes, "this is a serious problem, especially in the summer, because people get very hot and can develop life-threatening hyperthermia if you don't measure body temperature accurately," Levine added.
The research team wiped sweat off each subject before taking temperatures, but according to manufacturer instructions for the consumer product, "wiping the forehead is not recommended, since the sweating immediately begins again." Instead, Exergen recommends reading the temperature of a sweating individual behind the ear.
"We proved many years ago that perspiration does not stop by wiping, and the skin never dries, even though the tiny droplets of water immediately forming at the pores may not be visible. Unless the skin is dry, readings will always be low. As an educated guess, this is probably the most significant cause of the low [temporal] temperatures reported in the study," Pompei said.
Pompei also noted that the use of the temporal thermometry on sweaty athletes such as those running in the Boston Marathon has been largely investigative to this point.
"Due to the profusely perspiring athletes, we have developed methods of measuring temperatures on such athletes by employing an infrared transparent membrane, which is impermeable to water. When properly used, the film is either attached to the [temporal] thermometer or placed on the subject's forehead in a manner that stops the evaporation of the perspiration, thus rendering the temperature of the skin over the [temporal thermometer] the same as if it were completely dry," he explained. "The impermeable membrane method has not been published nor released for general use with the [temporal] thermometer, since we are still refining the method."
Mueller noted that using the temporal thermometer with sweaty sports athletes is questionable at this point.
"I think people should be concerned, especially in athletic injury research," he said. "If I was a director of a marathon using that method, I would doublecheck and make sure we're doing the right thing."
As a result of internal investigations as well as this study, Levine said, Parkland Hospital has stopped using temporal thermometers.
"If it is really important to measure body temperature," he said, "this device is not the way to do it. If the forehead is hot, and the kid isn't sweating, this device works pretty well. But if you think your child has a fever, and they are sweating, and the device says the skin temperature is normal, I would not be reassured by that."
For more information on hyperthermia, visit the National Institute on Aging.