Haematology

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Notes

A negative result does not exclude a diagnosis of Infectious Mononucleosis. Consider sending for EBV serology if clinically indicated.

Purpose of IM test:

Infectious mononucleosis (IM) is caused by the Epstein-Barr virus, which is a member of the herpes virus family. The virus is spread via saliva, and has an incubation period of four to six weeks. Symptoms of IM are fever, sore throat and swollen lymph glands. The condition is usually accompanied by large ‘atypical’ lymphocytes on peripheral blood film. Diagnosis of IM is made based on the presence of heterophile antibodies, also called Paul-Bunnell antibodies. Heterophile antigens are IgM in class, which agglutinate erythrocytes from different species including bovine, camel, horse, goat, and sheep. They are present in 89-90% of acute IM cases and can be detected in 60-70% of patients during the first week of clinical illness. Heterophile antibody screening tests may also be positive in other viral infections, autoimmune disease and haematological malignancies, but do not appear to be positive in primary bacterial infection. The tests have low sensitivity and low negative predictive value in young children and are not useful under the age of four.

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Reference range

Normally negative

Last updated

Wed, 04 Feb 2026 14:02:09 GMT