Diagnosing a fever is straightforward - the patient's temperature is taken, if the reading is high, he/she has a fever. It is important to take the person's temperature when they are at rest, because physical activity can warm us up. A person is said to have a fever if: The temperature in the mouth is over 37.7C (99.9F), the temperature in the rectum (anus) is over 37.5-38.3C (100-101F) and the temperature under the arm or inside the ear is over 37.2C (99F). As a fever is a sign rather than a disease, when the doctor has confirmed there is an elevated body temperature, certain diagnostic tests may be ordered, depending on what other signs and symptoms exist, these may include blood tests, urine tests, x-rays and other imaging scans.