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These types of thermometers are available at pharmacies. Make sure that the digital thermometer you buy is labeled for rectal use. If you plan to use a digital thermometer for both oral and rectal temperatures, then buy two and label them accordingly. Also, never use a mercury thermometer, which is the glass kind that used to be used.  Rectal thermometers have a security bulb specifically designed for safely taking rectal temperatures.  Review usage of your specific thermometer. Familiarization of the thermometer will aid in preventing prolonged insertion into rectum.  Follow and keep the manufacturer's instructions for correct use to ensure safety and accuracy. This can cause an inaccurate reading. Never use the same thermometer that you use in the rectum to take temperatures in other ways, because this can spread bacteria. If you prefer to use a disposable thermometer sleeve, use one of these instead and always dispose of after use and use a new one each time. But, be careful with the sleeves. They may pull off the thermometer as you are taking the temperature. You will need to hold it as you pull the thermometer out when you’re finished. Only insert it about ½ to 1 inch and do not force it in if there is resistance. Keep the thermometer in the baby’s rectum until it indicates that it is finished. Then, remove the thermometer and check the reading. Turn the thermometer on.

Summary:
Purchase a digital rectal thermometer. Be sure that the baby or patient has not had a bath or been swaddled (when babies are wrapped tightly for warmth) in the past 20 minutes. Clean the tip of the rectal thermometer with soapy water or rubbing alcohol. Apply petroleum jelly to the tip of the thermometer to make it easier to insert. Lay your baby on his back and insert the digital thermometer into the rectum.