Your doctor will ask you about your medical history and current symptoms. He or she will want to know about recent exposure to anyone with mononucleosis or mono-like symptoms.
During a physical exam, your doctor will look for signs of mononucleosis. These include:
A reddened throat with enlarged tonsils
Swollen lymph nodes in the neck and elsewhere
An enlarged spleen
A red rash, usually on the chest
Your doctor also will do blood tests to help make the diagnosis. The results of these blood tests may not be abnormal until the person has been ill for a week.
Two types of blood tests help to make the diagnosis:
Differential white blood cell count. This test measures levels of different types of white blood cells. In the first few weeks of mononucleosis, the number of lymphocytes (a type of white blood cell) is quite high. There also are large numbers of lymphocytes that look unusual, called "atypical lymphocytes".
Heterophil tests. Mononucleosis causes white blood cells to make an unusual kind of antibody called heterophil antibody. Heterophil tests measure levels of heterophil antibody.