A rectal thermometer is the most accurate way to take a temperature.

To take a rectal temperature:

• Apply a lubricant jelly or petroleum jelly, such as Vaseline, on the bulb of the thermometer, so that you can insert it easily.
• Choose a quiet place so that the child won't be distracted or move around too much. When measuring the temperature of babies or small children:
  • Place your child face up with legs bent toward the chest with your hand on the back of the thighs or holding the feet as if you were changing a diaper.
  • Place the child face down on your lap or on a flat covered or padded surface, such as a bed.
• Gently insert the bulb end of the rectal thermometer about 1.25 cm (0.5 in.) to 2.5 cm (1 in.) into the anal canal. Don't force it into the rectum. Hold the thermometer in place with two fingers close to the anal opening (not near the end of the thermometer).
• Leave the thermometer in place for the required amount of time, usually a minute. Some digital thermometers give a series of short beeps when the reading is done. Time yourself with a watch or clock.
• Remove the thermometer and read it.
• Clean a digital thermometer with cool, soapy water, and rinse it off before putting it away.
• After a thermometer has been used to take a rectal temperature, do not use it to take an oral temperature.