Investigations

Screening tests:

• full blood count -
  • leucocytosis of between 10,000 and 20,000 cells/mm³
• blood film - reveals a lymphocytosis with many atypical activated T lymphocytes (mononucleosis cells).
  • peaks in the second or third week of illness (2)
  • the diagnosis is likely when atypical lymphocytosis is ≥ 20 % or when atypical lymphocytosis is ≥10 % and ≥ 50 % lymphocytes (1), but the number of atypical lymphocytes may vary between 0-90% of the total lymphocyte count (2)
  • when the cut-off point of the abnormal number of lymphocytes is increased, the sensitivity of the test decreases (more false negative results) but the specificity increases (less false positive results) (1)
• positive Paul Bunnell reaction - IgM heterophile antibodies that agglutinate sheep erythrocytes.
  • seen in around 90% of cases (2)
  • heterophile antibodies usually peak during the second week and decrease rapidly after the fourth week (2,3)
  • false negative rate is high (25%) when blood is taken in the first week but the rate reduces to 5% if blood is taken in the third week of illness (1)
  • is less sensitive in patients younger than 12 years (detects around 25-50% of infections) (1)
  • repeat testing is done after 5-7 days in patients with a negative initial test (1)
• liver function test – abnormal in around 80% of patients with mild to moderate elevation of transaminases, alkaline phosphatase and bilirubin (2).

Specific tests:

• more sensitive tests include – detection of viral capsid antigen (VCA)-IgG and VCA-IgM (1):
  • useful in patients with typical clinical features of IM but a negative heterophile antibody test (1)
  • IgM antibodies to Epstein Barr viral capsid antigen (VCA) - detectable early in the course of the disease but transient.
  • IgG EBV VCA - appear soon after IgM. They persist for life at a stable or slowly declining level and so may also be used as markers of previous EBV exposure.
  • these tests are useful in diagnosing patients who have negative heterophile antibody test results(1)
  • when the results are negative, these tests are better than heterophile antibody tests in excluding infectious mononucleosis caused by EBV (1)
  • antibodies to EBV nuclear antigen - detectable from about 4 months after infection. Persist throughout life.
• the isolation of EBV is difficult and rarely performed.
• serologic tests are the methods of choice to come to an unequivocal diagnostic conclusion, while real-time polymerase chain reaction testing plays a minor role in diagnosis. (4)

Note:

• Hoagland's criteria for the diagnosis of infectious mononucleosis (1):
  • this includes the following features in the presence of fever, pharyngitis, and adenopathy, and confirmed by a positive serologic test
    • at least 50 per cent lymphocytes
    • at least 10 per cent atypical lymphocytes .
  • these criteria are most useful for research purposes (1)
  • only about 50% of patients with symptoms suggestive of infectious mononucleosis and a positive heterophile antibody test meet all of Hoagland's criteria (1)

References:

1. Mark H. Ebell. Epstein-Barr Virus Infectious Mononucleosis. Am Fam Physician 2004;70:1279-87,1289-90.
2. Charles PGP. Infectious mononucleosis. Australian Family Physician 2003;32(10)
3. Center for Disease Control (CDC) 2006. National Center for Infectious Diseases - Epstein-Barr virus and Infectious Mononucleosis
4. Niller HH, Bauer G. Epstein-Barr virus: clinical diagnostics. Methods Mol Biol. 2017;1532:33-55.