Diagnosing Mono - Mono Spot Test

Blood Tests to Diagnose Infectious Mononucleosis

Drawing blood
Drawing blood. Francis Twitty/Getty Images

What is Mono?

Mono is short for mononucleosis or infectious mononucleosis. It is generally caused by the Epstein-Barr virus (EBV), however cytomegalovirus (CMV) can cause a CMV mononucleosis which resembles mono.

Mono typically causes severe fatigue (tiredness or lethargy), swollen tonsils with a very sore throat, swollen lymph nodes, fever, and sometimes body aches. It is most common in adolescents, although many people become infected with EBV and never develop infectious mononucleosis.

If you suspect you have mono, you should see your physician to be tested and rule out other illnesses, such as strep throat, which must be treated with antibiotics.

Diagnosing Mono

Your doctor will look at your symptoms and your age (since people infected with EBV are more likely to develop mono if they are a teenager or young adult), and perform a physical evaluation where they will look in the back of your throat, feel your neck and other areas where you may have swollen lymph nodes, and listens to your lungs. If your doctor suspects mono, they will likely order two distinct blood tests:

  • Complete Blood Count (CBC)
  • Mono Spot Test

If you have mono, your CBC will show an elevated White Blood Count, specifically lymphocytes. Your doctor may tell you that you have lymphocytosis, which just means that your lymphocytes are elevated. You lymphocytes are just a part of your body's immune system. It is natural for these cells to be elevated during an infection.

Mono Spot Test

Testing for EBV is commonly performed by a heterophile antibody test. The most popular heterophile antibody test is the mononucleosis spot test, usually shortened to just mono spot test. The lab can draw a mono spot test at the same time they draw your CBC. Your blood will be drawn from a vein in your arm and while somewhat uncomfortable, like a shot, the pain only lasts for a minute.

Not enough blood is taken to make you anemic, however if you are squeamish, it's best to eat or drink something before having your blood drawn for this test.

Your blood will be analyzed in a laboratory for antibodies. Antibodies are a specific component of your immune system that fight infection. Your body produces these antibodies in response to a virus or other pathogen that it considers a threat. If your body detects EBV, it can develop a type of antibody called heterophile antibodies. If you have heterophile antibodies in your blood, then your mono spot test is "positive." A positive mono spot test accompanied by the symptoms of mono results in a diagnosis of infectious mononucleosis. However, mono spot tests can be false-positive, about 10-15% of the time, particularly in early stages of the illness.

Rarely, people who actually have mono can have a negative mono spot test. This may happen because you waited too long to see a doctor, and the heterophile antibodies rapidly decrease in number after you've been infected for about four weeks.

It may also mean that you are not infected with EBV but have developed "mono" (some doctors believe it is more accurate to call this a "mono-like" infection) from cytomegalovirus.

If your mono spot test is negative but you have all the symptoms of mono, your doctor will likely repeat the mono spot test before doing more extensive antibody tests. These more specific EBV antibody tests are rarely needed.

Since EBV is a virus, not a bacteria, mono cannot be treated with antibiotics, but rather the mono symptoms are treated. Rest and fluids are highly recommended, and in some cases, your doctor may send you home with a prescription to treat symptoms of sore throat and swollen lymph nodes.


Aronson, M.D. & Auwaerter, P.G. (2015). Infectious mononucleosis in adults and adolescents. Accessed on January 24, 2016 from http://www.uptodate.com (subscription required)

CDC. Epstein-Barr Virus and Infectious Mononucleosis. Accessed: August 30, 2011

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