For a febrile patient, fever may be diagnosed by oral testing. This is done by placing the disposable or digital thermometer underneath the tongue to one side and closing the mouth over it. After a couple of minutes or a few beeps from the thermometer, remove the thermometer and read the temperature recorded. Remember to wash the thermometer with soap water and pat it dry.

For very small babies or patients unable to hold the thermometer, this invasive technique is used. First apply Vaseline or petroleum jelly on the bulb of the thermometer to insert it easily into the anus. Keep the baby face down and slowly insert the bulb of the thermometer into its anus by spreading its legs apart. Do not thrust it into the rectum, but keep it close to the anal opening. The thermometer must be inserted only o.5 inches inside. Hold it firmly by catching hold of the baby together for a couple of minutes. Remove it and read the temperature. Wash and dry the thermometer.Never use a rectal thermometer for oral testing later.

Take the thermometer and place it at the center of the armpit. Press the arm against the body and keep still for a couple of minute. Now remove the thermometer and read the temperature. An axillary or armpit reading is always 1°F lower than the oral temperature.

The probe of the thermometer should be cleaned before using it for tympanic or ear testing in order to avoid risk of infection. For babies who are less than a year old, the earlobe is pulled down and back and the thermometer is gently inserted to wards the eardrum. For older babies, the ear lobe is pulled up and back and the thermometer is moved close to the eardrum and held there for a couple of minutes. 

To do temporal testing, press the thermometer strip or cup close to the forehead. Ensure that the forehead is dry before testing. Place the thermometer on the forehead for the stipulated time. Temporal thermometers are not as accurate as oral ones. For very young babies or for high temperatures greater than 103°F, other methods are preferred. Being an invasive method, this technique has to be exercised carefully. 

A pacifier thermometer looks like any normal pacifier when placed in the baby’s mouth. Assemble all the parts of the thermometer including the temperature part. Let the baby suckle the pacifier for a couple of minutes. Now remove and record the temperature and clean after use. 

For persistent fevers with unknown cause, blood testing has to be done to identify the type of fever attack. Blood testing is an invasive method where a sample of blood is tested for the drop in platelet count signaling the presence of infection as well as the presence of microorganisms.