Mononucleosis is an infectious disease that is often referred to simply as “mono.” This condition occurs primarily in adolescents and young adults. Its symptoms include swollen glands, sore throat, fever and general weakness, making it easy to mistake mono for other illnesses. Mono, however, lasts for much longer than most colds and fevers, and can persist for several months at a time. It is caused by the Epstein-Barr virus, but not everyone with the virus will get mononucleosis.

Doctors usually recommend getting a mononucleosis test when a teenager or young adult has flu-like symptoms that persist for some time. Occasionally, the test may be administered more than once. This is most common if the doctor strongly believes that the patient has mono and thinks there is a high chance of a false negative. Patients who have symptoms but test negative on a mononucleosis test can also take an EBV antibodies test to look for the body's response to the Epstein-Barr virus. 

People who have negative results and do not show results on an EBV test may be suffering from infection by a microorganism that causes similar symptoms. These include toxoplasmosis and cytometgalovirus. Detecting infections correctly is especially important in pregnant patients, who can suffer from complications or pass the infection on to their babies. After about the fourth week of infection, the mono test loses its effectiveness, but other tests for this illness may remain appropriate.