Infectious Mononucleosis Test

Definition

Infectious mononucleosis (IM) tests detect the presence or absence of antibodies in the blood stream directed against proteins of the Epstein-Barr virus (EBV), the cause of IM.

Purpose

Infectious mononucleosis tests are used to diagnose infectious mononucleosis in patients with symptoms compatible with the disease. Initial testing is based on the demonstration of heterophile antibodies produced in infectious mononucleosis. Heterophile antibodies are those that react with the cells from a different (nonhuman) species. A positive result from a rapid slide (Monospot) test for IM specific heterophile antibodies is diagnostic, and no further testing is necessary. The Monospot test will be positive in more than 90% of adults or adolescents with IM, but is more frequently negative in young children. When the Monospot test results are negative, an EBV antibody panel may be needed to differentiate EBV infections from mononucleosis-like illnesses induced by cytomegalovirus, adenovirus, or Toxoplasma gondii. The EBV antibody panel can differentiate persons who have never been infected with EBV, acute infections, and past infections. EBV antibody tests are not needed when the doctor believes that a person has IM and the Monospot test is positive.

Precautions

Blood for this test is collected by venipuncture. The nurse or phlebotomist performing the procedure should observe universal precautions for the prevention of transmission of bloodborne pathogens. False positive Monospot results occur in a small percentage of the patient population. False negative Monospot results occur in 10% to 15% of patients, primarily in children under the age of 10. With the EBV panel false positive results may occur in patients with rheumatoid arthritis, leukemia, lymphoma, or HIV.

Description

The Epstein-Barr Virus (EBV) is one of the most common human viruses, and most of the world's population is infected at sometime during their lives. According to the CDC, when an EBV infection occurs during adolescence or young adulthood, it causes infectious mononucleosis 35-50% of the time. The virus is believed to be transmitted primarily via salivary exchange, including intimate kissing, sharing toothbrushes, cups, or eating utensils.

Symptoms of infectious mononucleosis include fever, sore throat, swollen lymph glands lasting for two to three weeks, and fatigue and a swollen spleen or liver typically lasting for approximately one month. While the infection is rarely fatal and usually resolves in one or two months, the course is more chronic in some persons, and the virus may remain dormant in some throat and blood cells for the rest of the person's life.

The clinical diagnosis of infectious mononucleosis is suggested on the basis of the symptoms of fever, sore throat, swollen lymph glands, and the age of the patient. Laboratory tests are needed for confirmation. Laboratory findings suggestive of infectious mononucleosis include an elevated white blood cell count, an increased lymphocyte count, and the presence of a significant number of atypical lymphocytes (seen when viewing a stained blood smear under the microscope ). Diagnosis is usually made by demonstrating a positive reaction to a rapid slide test (usually referred to as a Monospot test) for the specific heterophile anibodies seen in IM.

Heterophile antibodies may be of two types, called Forssman and nonForssman. Infectious mononucleosis causes production of nonForssman heterophile antibodies. Testing for heterophile antibodies must distinguish these from the Forssman type which are not produced by IM and are present in the blood of many persons without IM. The Monospot test is based upon the principle that IM heterophile antibodies will agglutinate horse red blood cells (because they are nonForssman). First, the serum is mixed with two different antigen suspensions, guinea pig kidney antigen and beef red blood cell stroma, prior to testing with the horse red cells. The guinea pig kidney antigen absorbs (removes) Forssman heterophile antibodies while the beef red cell stroma removes nonForssman IM antibodies. After mixing the serum with these two suspensions, the serum is mixed with the horse red cells. In infectious mononucleosis, agglutination should be seen in the serum mixed with guinea pig antigen. Little or no agglutination should be seen in the serum mixed with the beef red cell stroma.

An EBV antibody panel can be used to confirm a negative Monospot result or rule out a false positive Monospot result. The panel includes four antibody measurements: IgM viral capsid antigen (VCA), IgG VCA, Early Antigen (EA), and Epstein-Barr nuclear antigen (EBNA), which help determine the stage of the patient's EBV infection.

Preparation

To obtain the 2 mLs of blood required for this test, a nurse or phlebotomist ties a tourniquet on the person's upper arm, locates a vein in the inner elbow region, and inserts a needle into that vein. Vacuum action draws the blood through the needle into an attached tube. Collection of the sample takes only a few minutes.

Aftercare

Discomfort or bruising may occur at the puncture site. Applying pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated accordingly.

Complications

There are no risks beyond those of having blood drawn for any other purpose.

Results

Results of the rapid slide test are determined as positive or negative. According to the CDC, the confirmatory diagnosis of EBV infection is summarized as follows:

• Susceptibility: If antibodies to the VCA are not detected.
• Primary infection: If IgM antibody to the VCA is present in the absence of antibody to EBNA. A rising or high IgG antibody titre to the viral capsid antigen and a negative antibody test to EBNA after at least four weeks of illness are strongly suggestive of primary infection.
• Past infection: If antibodies to both the VCA and EBNA are present, then past infection (from four to six months to years earlier) is indicated.
• Reactivation: In the presence of antibodies to EBNA, an elevation of antibodies to early antigen suggests reactivation.
• Chronic EBV infection: Reliable laboratory evidence for continued active EBV infection is very seldom found in patients who have been ill for more than four months.

If the results are difficult to interpret, it may be necessary to retest later, after waiting one to three weeks. The change in the amounts of antibody detected between the two tests can be particularly useful, at times, in helping to make a diagnosis.

The complete blood count (CBC) in a patient with infectious mononucleosis typically reveals:

• a white blood cell count (WBC) of 10,000 to 20,000 cells per microliter
• more than 4,500 lymphocytes per microliter and more than 50% lymphocytes in the differential
• atypical lymphocytes (Downey cells) accounting for more than 10% of total leukocytes

Health care team roles

Physicians order and interpret the IM tests. Nurses or phlebotomists usually draw the blood needed for these tests. Clinical laboratory scientists/medical technologists or clinical laboratory technicians/medical laboratory technicians perform the antibody tests in clinical laboratories. Interpretation of EBV antibody tests is somewhat complex and requires familiarity with EBV testing and access to all the patient's clinical information.

KEY TERMS

Heterophile antibodies— Antibodies created against one species that cross react with another.

Lymphocyte— White blood cell that fights viral and some bacterial infections by direct attack or the production of antibodies.

Resources

BOOKS

Bailey, R. Eugene. "Infectious Mononucleosis." In Current Diagnosis, edited by Rex B. Conn, William Z. Borer, and Jack W. Snyder. Philadelphia: W. B. Saunders, 1997.

Chernecky, Cynthia C., and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures, 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

PERIODICALS

Henle, G., W. Henle, and C. A. Horowitz. "Epstein-Barr Virus Specific Diagnosis: Tests in Infectious Mononucleosis." Human Pathology 5 (1997): 551-558.

OTHER

Centers for Disease Control and Prevention, National Center for Infectious Diseases: Epstein-Barr and Infectious Mononucleosis. Updated Sept 1999. 〈http://www.cdc.gov/ncidod/diseases/ebv.htm〉.