A 2-year-old boy with fever and lymphadenopathy (Page 2/4)

Prepared by...
Panuwong Sansamranjai, M.D.
Thanyawee Puthanakit, M.D.
Department of Pediatrics, Chiang Mai University
Diagnosis : INFECTIOUS MONONUCLEOSIS
 
What laboratory test should be performed to confirm diagnosis ?
1. Complete blood count with peripheral blood smear; patients with IM typically have an absolute lymphocytosis (>50% lymphocytes, with total lymphocytosis >5000/uL), and prominent atypical lymphocytes, so called Downey cells (> 10% of total leukocytes)
The peripheral blood smear demonstrated Downey cells, large lymphocytes with irregular oviod, folded nucleus and abundant, moderately basophilic agraular cytoplasm. The nuclearchromatin is coarse. Some had small indistinct nucleoli. (Figure 1)
 
Figure1: The peripheral blood smear demonstrated Downey cells,
hallmark of infectious mononucleosis.
   
2. Detection of heterophile antibodies: The heterophil antibody is IgM which had differential absorption. It causes agglutination of sheep and horse erythrocytes but not guinea pig kidney antigen extract. The EBV heterophile antibody titers are highest during the first 4 weeks of disease. False negative reactions occur in 10% of adults and older children and often negative in children younger than 4 years of age.
   
3. EBV-specific laboratory test (Figure 2)
Antibodies to several antigen complexes may be measured. These antigens are the viral capsid antigen (VCA), the early antigen (EA) and the EBV nuclear antigen (EBNA). Testing for anti-VCA antibody is useful for identifying acute infection. Because serum antibody to EBNA is not present until several weeks to months after onset of infection, a positive anti-EBNA antibody excludes primary infection.
 
Figure 2: Schematic representation of the antibodies to variuos Epstein-Barr virus antigens in patients with infectious mononucleosis.
 
Copy from Leach CT., Sumaya CV. Epstein-Barr Virus . In: Feigin RD, Cherry JD, Demmler GJ, Kaplan SL. Textbook of Pediatric Infectious Diseases. 5th ed. Philadelphia: Saunders; 2004. p.1945.
 
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