In this third post in an occasional series questioning some of the basics of paediatric assessment, we are going to look at measuring temperature in children. It’s one of the few vital signs that parents measure themselves before even coming into the emergency department. But just how accurate is their assessment?
When a neonate is brought into the department it can really 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 and careful safety netting and a full septic workup. When a 2-year-old is brought in following a seizure, the presence of a fever may mean you are less likely to do a full work-up. (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 carers touch enough to determine if a child is febrile or not?
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 not a fever, being right 93.9% of the time). When the study authors limited the results to those children less than two years of age then mothers were much more accurate and picked up 90% of cases. Bonadio et al, reassuringly, found that children who had 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 that they also over-estimated the presence of a fever.
Mercury thermometers have fallen by the wayside now and parents have many other means at their disposal to check the temperature of their child. Scientists, trying to improve on what nature has given us – the hand – have come up with a plastic-encased strip with thermophototropic chemicals that respond to changes in skin temperature. A study of one brand found that only detected 13 out of 40 children with a fever. Could mother know best?
Most guidelines, including the NICE guidelines, suggest that the parents’ subjective perception should be considered valid and taken seriously by healthcare providers.
Does how the baby is dressed make a difference?
Most of the studies that look at this really common question involve small numbers of participants. Bundling up a child in layers and 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 4 weeks, measure body temperature with an electronic thermometer in the axilla 
In children aged 4 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 a modern generation, a thermometer up the bottom is not an acceptable way to take it. There are a number of studies that have compared different sites to a reference standard in order to determine the gap between the core and outside world. A lot of these were done in the days of mercury thermometers and showed an unacceptable variation between sites and 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.
The axillary temperature may be altered by a number of environmental and physiological factors. As the thermometer does not directly overlie the axillary artery it may, itself, 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%. With such a poor sensitivity it is surprising that it is recommended by NICE.
Whilst rectal temperatures are still taken, especially in neonates, they may lag behind core temperature (as determined by pulmonary artery or oeseophageal temperature detection) and so 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).
Both simple electronic and pacifier (dummy) based devices are available to parents but the challenge is often getting the child to suckle long enough for a temperature to register.
The tympanic membrane temperature approximates, theoretically, the temperature of the hypothalamus (the centre for thermoregulation). They certainly appear to be more accurate than axillary or rectal measurements under correct operating conditions. Improved accuracy can be achieved by pretending the thermometer is an otoscope and pulling back on the pinna in the same way, thus allowing the instrument to visualize the tympanic membrane. Those available for purchase and home use do not mention this in their instructions and 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 actually 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 though when it comes to parsing all of this. So many of the studies compare one technique with another, already inaccurate technique, that only those studies that compare a peripheral method with a true core method hold much validity. Even when this is taken into account it seems that the majority of methods over-estimate at lower temperatures and under-estimate at higher ones. With sensitivities for some devices reported to be as low as 22% that would mean we might miss four out of five children with a fever!
So what does all of this mean for me at work?
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 just flip a coin. Most home methods of determining temperature 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 it comes to measuring temperature in hospitals the methods suggested by NICE are NOT the most accurate and 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 something like a contactless thermal imaging camera?
Apologies to our American cousins
Given that the scale, first devised by German physicist Daniel Fahrenheit in 1724, is really only used in a small part of the world I’ve stuck to the use of Celsius in this post. If you want to know more about this scientific schism so you can sound more erudite at your next dinner party then read this article.
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