In this third post in an occasional series questioning some of the basics of paediatric assessment, we will look at measuring temperature in children. It’s one of the few vital signs that parents measure themselves before entering the emergency department. But how accurate is their assessment?
When a neonate is brought into the department, it can make a difference if their temperature is normal or just a few points of a degree over. Those extra digits may mean the difference between a watchful eye, careful safety netting, and a full septic workup. When a 2-year-old is brought in following a seizure, a fever may mean you are less likely to do a full workup. (Editors note: though afebrile febrile seizures can occur).
Everybody’s got the fever, that is something you all know
Fever isn’t such a new thing, fever started long ago
He feels warm to me…
Whenever I was ill as a child, my mother would put her cool hand on my forehead, declare, “You don’t have a fever,” and package me off to school. Was she right to do that? Is a caregiver’s touch enough to determine whether a child is febrile?
A 1984 study found that mothers correctly identified that their child had a fever 52.3% of the time (though at least they were better at recognising when there was no fever, being right 93.9% of the time). When the study authors limited the results to children under two years of age, mothers were much more accurate and picked up 90% of cases. Bonadio et al., reassuringly, found that children with a reported temperature at home but not in the emergency department were highly unlikely to have a serious bacterial illness. However, one African study using mothers and medical students found that they could detect a fever on 4 out of 5 occasions but also overestimated its presence.
Mercury thermometers have fallen by the wayside, and parents have many other ways to check their children’s temperature. Scientists, trying to improve on what nature has given us – the hand – have developed a plastic-encased strip with thermophototropic chemicals that respond to changes in skin temperature. A study of one brand found that it only detected 13 out of 40 children with a fever. Could mother know best?
Most guidelines, including the NICE guidelines, suggest that healthcare providers should consider the parents’ subjective perception valid and take it seriously.
Does how the baby is dressed make a difference?
Most studies that examine this really common question involve small numbers of participants. Bundling a child in layers of swaddling may raise the skin temperature by up to 2.5°C with a minimal rise in rectal temperature.
Does it matter where the thermometer is put?
The current NICE guidelines make the following recommendations:-
In infants under four weeks, measure body temperature with an electronic thermometer in the axilla [2007]
In children aged four weeks to 5 years, use one of the following:-
- electronic thermometer in the axilla
- chemical dot thermometer in the axilla
- infra-red tympanic thermometer
The Royal Children’s Hospital in Melbourne defines a fever as a rectal temperature greater than 38°C.
It can be difficult to measure the temperature in a child, especially a cranky, wriggly one. For most parents of the modern generation, a thermometer up the bottom is not an acceptable way to take it. Several studies have compared different sites to a reference standard to determine the gap between the core and the outside world. Many of these were done in the days of mercury thermometers and showed an unacceptable variation between sites, even when the temperature was repeated at the same site. The most accurate data comes from experimental studies in small groups of children undergoing cardiac surgery.
Axillary
Several environmental and physiological factors may alter the axillary temperature. As the thermometer does not directly overlie the axillary artery, it may cause localized cooling and a falsely low reading. Chemical dot thermometers may differ formal readings by ±0.4 degrees, though they have not been compared with a gold (or even brass) standard. An axillary thermometer has a sensitivity of around 30%. Surprisingly, with such poor sensitivity, NICE recommends it.
Rectal
Whilst rectal temperatures are still taken, especially in neonates, they may lag behind core temperature (as determined by a pulmonary artery or oeseophageal temperature detection) and be an inaccurate indicator of febrile status. It is easy to appreciate how the recorded temperature may vary according to how far the probe has been inserted and in the presence or absence of stool. Rectal temperature is also slow to change in reaction to changes in blood temperature (as demonstrated in anaesthetized cardiac cases).
Oral
Parents can use both simple electronic and pacifier (dummy)- based devices, but the challenge is often getting the child to suckle long enough for a temperature to register.
Tympanic
Theoretically, the tympanic membrane temperature approximates the hypothalamus’s temperature (the centre for thermoregulation). Under correct operating conditions, it certainly appears to be more accurate than axillary or rectal measurements. Improved accuracy can be achieved by pretending the thermometer is an otoscope and pulling back on the pinna similarly, thus allowing the instrument to visualize the tympanic membrane. Those available for purchase and home use do not mention this in their instructions, so caregivers may under-report the fever.
And what about those magic wands we have in hospital?
Those ‘magic wands’ or temporal artery thermometers (TATs), as they are called, work by detecting the heat radiated by the skin over the temporal artery. A meta-analysis published in BMJ Open earlier this year found a significant deviation in the level of agreement between TATs and reference standards in the order of ±1°C.
There is a real challenge when it comes to parsing all of this. So many of the studies compare one technique with another, which is already inaccurate, and only those studies that compare a peripheral method with a true core method hold much validity. Even when this is considered, it seems that most methods overestimate at lower temperatures and underestimate at higher ones. With sensitivities for some devices reported to be as low as 22%, we might miss four out of five children with a fever!
What does all of this mean for me?
Mothers are generally very good at knowing if their child has a fever (as long as the child is under two; otherwise, you might as well flip a coin). Most home methods of determining temperatures, such as forehead strips and off-the-shelf in-ear thermometers, are not sufficiently accurate and so should be considered another data point, not an absolute.
When measuring temperature in hospitals, the methods suggested by NICE are NOT the most accurate. They are compared to a brass rather than gold standard (rectal temp versus oesophageal or PA temp).
It seems (to me, at least) that tympanic membrane thermometry (if performed correctly—pull the pinna back) is the most accurate method until something more high-tech comes along. Perhaps a contactless thermal imaging camera?
Apologies to our American cousins
Given that the scale, first devised by German physicist Daniel Fahrenheit in 1724, is only used in a small part of the world, I’ve stuck to Celsius in this post. If you want to know more about this scientific schism to sound more erudite at your next dinner party, read this article.
References
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