A variety of tests may be used to diagnose mono. In the early stages, a doctor may suspect mono based on a patient's signs and symptoms. A health care professional may order tests to rule out other potential causes of the symptoms, such as strep throat. Numbers of white blood cells called lymphocytes may be elevated in the early stages of mono. Lymphocytes may also look different when examined under a microscope. Blood tests such as the heterophile antibody test and the monospot test can be used to confirm mono diagnosis. These tests measure levels of antibodies in the blood. Antibodies are typically detectable after two to three weeks into the illness. Another potential tell-tale sign of mono infection is abnormal liver function tests