It is indicated as a confirmatory test when a physician suspects EBV, typically in the presence of clinical features such as fever, malaise, pharyngitis, tender lymphadenopathy and splenomegaly. In the case of delayed or absent seroconversion, an immunofluorescence test could be used if the diagnosis is in doubt. It has the following characteristics: VCAs of the IgM class, antibodies to EBV early antigen, absent antibodies to EBV nuclear antigen
Usefulness
One source states that the specificity of the test is high, virtually 100%, Another source states that a number of other conditions can cause false positives. Rarely, however, a false positive heterophile antibody test may result from systemic lupus erythematosus, toxoplasmosis, rubella, lymphoma and leukemia. However, the sensitivity is only moderate, so a negative test does not exclude EBV. This lack of sensitivity is especially the case in young children, many of whom will not produce the heterophile antibody at any stage and thus have a false negative test result.
Timing
It will generally not be positive during the 4–6 week incubation period before the onset of symptoms. The highest amount of heterophile antibodies occurs 2 to 5 weeks after the onset of symptoms. If positive, it will remain so for at least six weeks. An elevated heterophile antibody level may persist up to 1 year.
Process
The test relies on the agglutination of horse erythrocytes by heterophile antibodies in patient serum. Heterophile means it reacts with proteins across species lines. Heterophile also can mean that it is an antibody that reacts with antigens other than the antigen that stimulated it. A 20% suspension of horse red cells is used in an isotonic 3–8% sodium citrate formulation. One drop of the patient's serum to be tested is mixed on an opal glass slide with one drop of a particulate suspension of guinea-pig kidney stroma, and a suspension of ox red cell stroma; sera and suspensions are mixed with a wooden applicator 10 times. Ten micro liters of the horse red cell suspension are then added and mixed with each drop of adsorbed serum. The mixture is left undisturbed for one minute. Examine for the presence or absence of red cell agglutination. If stronger with the sera adsorbed with guinea-pig kidney, the test is positive. If stronger with the sera adsorbed with ox red cell stroma, the test is negative. If agglutination is absent in both mixtures, the test is negative. A known 'positive' and 'negative' control serum is tested with each batch of test sera.
