Clinical Thermometers and Children's Health

Copyright 2006 Peter Nisbet
Body temperature is usually measured by indirectly measuring the temperature of the blood. Its accuracy is dependent on both the equipment used and the area of the body used for the measurement. Several factors can affect the accuracy, among them recent exercise, how wrapped up (or ‘bundled’) a young child has been and even the environmental temperature.
Rectal temperature is the most accurate of the common methods, but I do not recommend untrained people use this method with children. It can also be inaccurate since rectal temperature responds very slowly to quick changes in core body temperature. For these reasons I shall not dwell on this method. Children also frequently object to it.
The best option for children under three months old is probably an axillary reading. If you want one thermometer to do all jobs, I would go for an ordinary digital thermometer and restrict the measurements to armpit and oral.
An armpit (axillary) measurement is the easiest and most familiar method, but is the most inaccurate. The measuring area of the thermometer has to be directly over the axillary artery, and this is not easy to achieve with squirming children. If the child is older, oral measurement is simple and more accurate than the armpit.
The most accurate method is by use of a modern tympanic thermometer, which measures temperature in the ear canal. This is very close to core temperature and modern types are not affected by ear wax. They are more accurate than rectal measurements, but not recommended for children under three months. However, please bear in mind that a parent is not expected to make an accurate measurement: just sufficient to decide if a doctor should be called. As I state below any temperature of 38C (100.4F) or above warrants this.
Types Of Thermometer
Glass thermometers have been largely superseded by digital types, and are much safer to use with children and just as accurate. They are designed for specific uses such as tympanic, rectal and oral/axillary. Were I just starting up a family again, I would go the expense of a tympanic type, but my kids are now grown up and there is no need, so I use a standard digital thermometer for oral and armpit use.
There are plastic strip or dot thermometers available, which are stuck to the skin and change colour to indicate temperature. These are very inaccurate and useful only to indicate if a child’s temperature is higher than normal or not. They may indicate a fever, but not what the temperature actually is. They do the same job as the ‘hand on the forehead’.
You can also get thermometers shaped like baby’s pacifiers, or ‘dummies’. These are also inaccurate since they need to stay in the mouth for a certain period of time which, as most mothers know, is just about impossible!
Of all the types of thermometer on the market, the ear, or tympanic, type is the most accurate. However, most parents will find a standard digital thermometer to be financially more viable and able to meet most parents’ requirements. Once a doctor has been called, he will be able to make a much more accurate measurement. Your job is to be equipped to carry out the initial measurement which indicates to you whether or not you should call a doctor.
If your child’s temperature reaches 38C (100.4F) you should call a doctor. I personally would call a doctor rather than take the child to the surgery since there may be people there at special risk from whatever disease your child may have. Remember, a disease is often at its most infectious before the symptoms appear.
How To Use A Thermometer
Always read the manufacturer’s instructions. There may be calibration instructions which are required before the first (sometimes every) use, and operating instructions. Many emit a signal when the measurement is complete, and some allow you to store readings. Some require the previous measurement to be cleared before making another. There are also usually specific cleaning and sterilising instructions.
The instructions usually tell you when not to measure a child’s temperature, such as immediately after exercise, after a bath or consuming hot or cold food or drink.
Oral Readings: You are measuring the temperature of the blood in the lower surface of the tongue. Wait at least 15 minutes after eating and drinking then place the end of the thermometer under the tongue and ask the child to hold it steady with the tongue and lips, but not to bite. The measurement will be complete after 3 – 4 minutes.
Axillary Readings: Here you are measuring the blood temperature in the axillary artery. You can locate the position of this artery with your fingertips. It is best to remove the clothing first, then place the tip of the thermometer over the artery. Fold the child’s arm across the chest to hold the thermometer steady. Again, 3 – 4 minutes should be enough, though a digital thermometer will normally beep when measurement is complete.
Rectal readings (if necessary): lubricate the tip of the thermometer with a suitable lubricant. If the instructions do not recommend one, your pharmacist will help. Lie the child face down on a suitable surface (changing table, bed or your lap with a small child), place your hand firmly near the base of the spine to hold him still then gently insert the thermometer probe about an inch into the anus (there should be no resistance – if there is then stop. Do not try to force the thermometer). Cup your hand round the child’s bottom with the thermometer held between the fingers to keep it still until the reading has been completed.