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Infectious mononucleosis, commonly called mono, refers to an infection usually caused by the Epstein-Barr virus (EBV). The mono test detects proteins in the blood called heterophile antibodies that are produced by the immune system in response to an EBV infection.
Infectious mononucleosis is characterized by a particular set of symptoms that most often affects adolescents. People who have mono often have a fever, sore throat, swollen glands, and fatigue. Many will also have an enlarged spleen, and a few may have an enlarged liver. Symptoms of the infection usually arise about one month after the initial infection and may last for several weeks. The associated fatigue may last for several months. Mono is usually a self-limiting condition; the symptoms resolve without any specific treatment.
Epstein-Barr virus is very common and very contagious. According to the Centers for Disease Control and Prevention (CDC), most people in the United States are infected by EBV at some point in their lives. The virus is present in the saliva of an infected person and is easily spread from person to person through close contact such as kissing and through sharing utensils or cups.
Most of the time, EBV infection occurs in childhood and causes few or no symptoms. However, it can cause symptoms associated with infectious mononucleosis in about 25% of teens and young adults, according to the CDC. Mono can affect anyone at any age, but its prevalence is highest in populations of young people, such as students in high schools or colleges, or in the military.
The mono test is 71% to 90% accurate and may be used as an initial test for diagnosing infectious mononucleosis. However, the test does have a 25% false-negative rate due to the fact that some people infected with EBV do not produce the heterophile antibodies that the mono test is designed to detect. If a mono test is negative and suspicion it still high, then a test specific for EBV antibodies is usually performed.
A complete blood count (CBC) and blood smear are usually also performed, as mono is also characterized by a high white blood cell (WBC) count and the presence of atypical white blood cells (usually reported as reactive lymphocytes) as seen on a blood smear.
According to the CDC, examples of other causes of mono include:
Cytomegalovirus (CMV)
Hepatitis A, hepatitis B or hepatitis C
HIV
Rubella
Toxoplasmosis
The mononucleosis test is used to help determine whether a person with symptoms has infectious mononucleosis (mono). The test is used to detect proteins in the blood called heterophile antibodies that are produced by the immune system in response to an Epstein-Barr virus (EBV) infection, the most common cause of mono. (For more on mono, see the "What is being tested?" section.)
A mono test is frequently ordered along with a complete blood count (CBC). The CBC is used to determine whether the number of white blood cells (WBCs) is elevated and whether a significant number of reactive lymphocytes are present. Mono is characterized by the presence of atypical white blood cells.
If the mono test is initially negative but the healthcare practitioner still suspects mono, a repeat test done a week or so later may be used to determine whether heterophile antibodies have developed. If the mono test is persistently negative, a test specific for EBV antibodies may be used to help confirm or rule out the presence of an EBV infection. A strep test may also be ordered along with a mono test to determine whether a person's sore throat is due to strep throat (group A streptococcal infection) instead of or in addition to mononucleosis.
The mono test is primarily ordered when a person, especially a teen or young adult, has symptoms that a healthcare practitioner suspects are due to infectious mononucleosis. Symptoms can sometimes be confused with those of a cold or the flu. Some of the more common signs and symptoms of mono include:
Fever
Headache
Sore throat
Swollen glands in the neck and/or armpits
Extreme weakness or fatigue
Some people may experience additional signs and symptoms such as:
Stomach pain
Enlarged liver and/or spleen
Rash
The test may be repeated when it is initially negative but suspicion of mono remains high.
A positive mono test with an increased number of white blood cells and reactive lymphocytes on a blood smear in the presence of symptoms associated with mono indicates a likely diagnosis of infectious mononucleosis.
A negative mono test requires careful interpretation.
If symptoms and reactive lymphocytes are present but the mono test is negative, then it may be too early to detect the heterophile antibodies or the affected person may be one of a small percentage of people who do not make heterophile antibodies. The mono test may be repeated at a later time and/or tests specific for EBV antibodies may be performed to help confirm or rule out the mononucleosis diagnosis.
Most infants and young children will not make heterophile antibodies, so they will have negative mono tests even when infected with EBV. This population is rarely tested, however, because they do not usually have symptoms of infectious mononucleosis.
People with negative mono tests and few or no reactive lymphocytes may be infected by another microorganism that is causing symptoms, such as a cytomegalovirus (CMV) or toxoplasmosis. If the infection occurs during pregnancy, it can be important to determine the cause since some of these other infections have been associated with pregnancy complications and injury to the fetus. It is also important to identify strep throat when present because it requires prompt treatment with antibiotics. (Since EBV is a virus, it cannot be treated with antibiotics.)
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