Giving the Rectal Thermometer its Due

One of the cardinal features of many infectious diseases is the presence of fever. Fever, usually defined as a temperature exceeding 38.3 degrees Centigrade (100.94 degrees Fahrenheit), is a defense mechanism employed to stifle the growth of microbes who prefer to grow at lower temperatures and augment the immune system which heightens its activities at higher temperatures. 

Fever also has a major diagnostic role as it is used to discriminate between those who are more ill and those who are less ill. Fever is also part of the criteria for employing the concept "systemic inflammatory response syndrome" to a patient--which can trigger an extensive evaluation for an inciting infection.

With that context, it becomes clear that accurately determining if fever is present in a patient is an important task. Sometimes fever can be obvious and felt directly by touch, however tactile temperature taking is fraught with error therefore thermometers are employed. While it is well known that peripheral temperatures obtained in the mouth, ear, arm pit or forehead can be inaccurate and subject to manipulation (think of the school child who sticks a thermometer in a hot liquid before putting it in his mouth) they generally are the primary means of temperature assessment employed in healthcare settings. The gold standard of temperature measurement is more invasive and would involve measuring the temperature of the blood, the bladder, the esophagus, or the rectum directly. 

Rectal temperatures, being the least invasive of the three, is employed variably to determine temperatures. Usually small children and critically ill adults may have their temperature measured in this manner. A new study, however, provides evidence that this form of temperature measurement should be employed more liberally if accurate temperature measurement will make a difference in clinical management. 

In this study, which was a systematic review and meta-analysis, 75 studies were pooled and revealed that peripheral thermometers level of agreement with central thermometers was unacceptable and could be off by 1-2 degrees Centigrade at extreme temperatures (higher or lower). The sensitivity for detecting fever, furthermore, was just 64%--not too much better than flipping a coin. Specificity, meaning the reliability of a peripheral thermometer's detection of a febrile range temperature, was good at 96%. Translation: if your oral thermometer detects a fever, it is likely a real fever but if it reads normal, fever may still be present. 

To me, the results of this study should lower the threshold for obtaining a rectal temperature in those patients in whom the result would change management (after a peripheral temperature is reported as normal). This does not mean that every parent or grandparent should subject children to routine rectal temperature measurement because in that setting -- an ill-appearing child -- temperature adds little to the treatment (fluids, acetaminophen, ibuprofen). The other implication is the need to better calibrate peripheral thermometers prior to use and the need for innovation in this market to spare all our rectums.