What is the role of the heterophile test in the diagnosis of infectious mononucleosis (IM)?

Updated: May 24, 2019
  • Author: Michael S Omori, MD; Chief Editor: Gil Z Shlamovitz, MD, FACEP  more...
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The heterophile test is the most common and specific test to confirm the diagnosis of infectious mononucleosis.

Paul and Bunnell first described the presence of heterophile antibody in patients with infectious mononucleosis in 1932. The heterophile antibody is an immunoglobulin M (IgM) antibody produced by infected B lymphocytes.

It is not directed against Epstein-Barr virus (EBV) or EBV-infected cells, but it is a result of the infection and the subsequent transformation of the B cell to a plasmacytoid state.

In the heterophile test, human blood is first absorbed by a guinea pig kidney. Then, it is tested for agglutination activity that is directed against horse, sheep, or cow erythrocytes. Successive dilutions yield a titer; a titer of 40 or greater is considered a positive test.

The heterophile antibody is present in 40-60% of patients with infectious mononucleosis in the first week of the illness and in 80-90% of cases by the third or fourth week.

Repeated testing may be indicated in patients with the clinical syndrome who tested negative early in the course of the illness. As few as 50% of patients who are younger than 4 years may develop the heterophile antibody.

The heterophile response usually persists for 3 months, although it may be present for as long as a year following onset. While the heterophile test is a quantitative assay, the various Monospot tests are qualitative.

Monospot tests are slightly more sensitive (85%) than the heterophile assay, but false-positive findings may occur in children or in patients with other viral illnesses.

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