What Causes a False Positive Monospot Test?

What Causes a False Positive Monospot Test?

Patient Presentation
A 10-year-old male came went to his private medical doctor’s office with a sore throat.
He was noted to have very large tonsils and was placed on clindamycin and prednisone.
A few days later he developed a low-grade fever. He was seen again and referred to an otolaryngologist for large tonsils and a tonsillectomy and adenoidectomy were performed that day.
He continued to be febrile with a sore throat and several days later went to the local emergency room where laboratory testing revealed markedly elevated liver function tests.
He was transferred to a regional children’s hospital for further evaluation.
The past medical history revealed a healthy child who was diagnosed with an Epstein-Barr infection 2 years previously based upon a positive monospot test.
The family history was non-contributory.
The review of systems was positive for fatigue and jaundice.
The pertinent physical exam showed normal vital signs except for a fever to 100.6° F.
He had scleral icterus and a yellow hue to his skin.
HEENT exam showed cervical lymph nodes that were < 0.5 cm in size bilaterally. Pharyngeal erythema was noted along with bilateral eschar in the tonsillar vaults.
Heart and lung examination were normal.
Abdominal examination revealed a liver 4 cm below the right costal margin that was firm with no obvious splenomegaly or masses.
Neurological examination showed a male who was very sleepy but arousable and aware of his surroundings and situation.
He had bilateral asterixis of his ankles.
An extensive laboratory evaluation and work-up was completed.
The pathological examination of the tonsils and adenoids, a bone marrow biopsy and a liver biopsy were all compatible with
the diagnosis of acute Epstein-Barr virus infection causing hemophagocytic lymphohistiocytosis.
Chemotherapy was begun and the patient began responding to treatment.

Almost all humans are infected with Epstein Barr virus by adulthood.
After infection, EBV’s viral genome is retained in a small fraction of B lymphocytes for the patient’s lifetime.
Carriers who are healthy have ~1-50 infected cells per million leukocytes. The virus tends to stay in the cellular cytoplasm and does not express many proteins.
Patients who appear to have EBV-induced disease have a much higher viral loads and the viral-infected cells express many proteins which appear to lead to disease.
The major EBV strains are types A and B. EBV is associated with oncogenicity (particularly lymphomas and carcinomas), drug resistance and other diseases.

Learning Point
A monospot is a test for heterophil antibody agglutination of red blood cells of animals.
Timing is important for the test.
Positive testing appears usually within 1 week of the onset of symptoms with a peak at 2-5 weeks and then waning usually over 3 months.
Waning can occur for as much as 1 year.
During week 1 after symptoms begin, there is a 25% false negative rate. During week 2, there is a 5-10% false negative rate. By week 3 there is a 5% false negative rate.
Age is also important for the test. Young children and the elderly often will be falsely negative.

Other testing for Epstein Barr virus is available including indirect fluorescent antibody testing or enzyme linked immunosorbent assays. These are often more difficult to perform and are costlier. In-situ hybridization is also available for various biopsy specimens.

False positive causes for Monospot tests include:

  • Pre-analytical laboratory problem (the most common cause, usually because of the problems noted above)
  • Infectious
    • Cytomegalovirus
    • Hepatitis
    • Influenza
    • Malaria
    • Rubella
    • Varicella
  • Leukemia/Lymphoma (especially Burkitt’s lymphoma)
  • Pancreatic cancer
  • Rheumatoid arthritis
  • Serum sickness
  • Systemic lupus erythematosus

Questions for Further Discussion
1. What are other viral causes of hemophagocytic lymphohistiocytosis?
2. What specific cancer types are associated with Epstein-Barr virus?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at the National Guideline Clearinghouse and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for this topic: Infectious Mononucleosis.

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

Bakerman’s ABC’s of Interpretive Laboratory Data. 4th Edit. Interpretive Laboratory Data Inc. Scottsdale, AZ. 2002: 387-388.

Gulley ML, Tang W. Laboratory Assays for Epstein-Barr Virus-Related Disease.
J Mol Diagn. 2008 Jul;10(4):279-92.

Moses S. Monospot. FamilyPracticeNotebook.com.
Available from the Internet at http://www.fpnotebook.com/ID/Lab/Mnspt.htm (rev. 11/32008, cited 12/8/08).

ACGME Competencies Highlighted by Case

  • Patient Care
    1. When interacting with patients and their families, the health care professional communicates effectively and demonstrates caring and respectful behaviors.
    2. Essential and accurate information about the patients’ is gathered.
    3. Informed decisions about diagnostic and therapeutic interventions based on patient information and preferences, up-to-date scientific evidence, and clinical judgment is made.
    4. Patient management plans are developed and carried out.
    7. All medical and invasive procedures considered essential for the area of practice are competently performed.
    9. Patient-focused care is provided by working with health care professionals, including those from other disciplines.

  • Medical Knowledge
    10. An investigatory and analytic thinking approach to the clinical situation is demonstrated.
    11. Basic and clinically supportive sciences appropriate to their discipline are known and applied.

  • Practice Based Learning and Improvement
    13. Information about other populations of patients, especially the larger population from which this patient is drawn, is obtained and used.

    Donna M. D’Alessandro, MD
    Professor of Pediatrics, University of Iowa Children’s Hospital