Use a rectal thermometer to take your cat’s temperature. An oral thermometer can be used in a pinch, but the bulb is more likely to break off. Before inserting the thermometer into the rectum, shake the mercury column down below 99° F (37.2°C) and lubricate the tip of the thermometer with any nontoxic, greasy substance (petroleum jelly, lubricant jelly, vegetable oil). Place your cat on a table or other level platform, hold your cat’s tail up with one hand, and insert the thermometer into the rectum with a firm, gentle push.
You may feel some resistance to the thermometer just after you pass it through the anus; this is due the cat’s strong internal anal sphincter muscle. When you encounter this resistance just continue to push gently but firmly until the muscle relaxes and allows the thermometer to pass, or rotate the thermometer gently. This is most easily done with the cat standing, but can be done while he or she sits or lies down. How far you need to insert the thermometer to get an accurate rectal temperature depends on the size of the cat—an inch to an inch and one-half (2.5 to 3.75 cm) is usually sufficient.
If you feel the thermometer go into a fecal mass when you insert it, try again. The thermometer should be left two or three minutes although many thermometers will register an accurate temperature in about one minute. (Helpful information if you have an uncooperative cat!)
RESTRAINT WHILE TAKING A CAT’S TEMPERATURE
To read the thermometer, roll it back and forth between your fingers until you can see the thin mercury column inside. The point where the column stops is the temperature. Each large mark indicates one degree, each small mark two-tenths of a degree. Normal is usually 101.0 to 102.5°F (38.3 to 39.2°C).