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Q How long does the Monospot remain positive after glandular fever?

A The Monospot test (a quick version of the Paul Bunnell test) is not strictly a diagnostic test for glandular fever as it actually detects antibodies directed against sheep and horse red cells, called heterophile antibodies.

Heterophile antibodies occur during EBV infection but are not specific for EBV infection, as they also occur in patients with rheumatic diseases, lymphoma, hepatitis and other viral infections, serum sickness and in some healthy individuals. They do not occur in CMV or toxoplasma infections.

EBV-induced heterophile antibodies can be identified by specific absorption by bovine red cells but not guinea pig kidney cells. Heterophile antibodies are IgM class antibodies.

Some 80-90 per cent of patients with EBV infection are positive by the third week of infection. Retesting may be required, as 15 per cent who are negative if testing is carried out in the first week will be positive when retested two to three weeks after the onset.

Antibodies persist usually for three-six months but may be positive for up to 12 months.

False negative results occur in 10-15 per cent of cases, but in children under the age of four the false negative rate is up to 80 per cent, making the test useless in small children.

Where there is a strong suspicion of EBV infection but the Monospot test is negative, it is more useful to undertake testing for EBV-specific antibodies. IgM antibodies to viral capsid antigen (VCA) appear first, followed by IgG antibodies to VCA.

IgG antibodies to Epstein-Barr nuclear antigen (EBNA) appear two-four weeks after onset and persist for life. Rare failure to produce antibodies to EBNA has been associated with chronic EBV disease.

Acute infection with EBV can be diagnosed by detection of IgM to VCA, in the absence of antibodies to EBNA. In a convalescent sample at four to six weeks the presence of IgM and IgG to VCA and IgG to EBNA is absolutely diagnostic of recent acute EBV infection.

Antibodies are also detected to early antigen (EA) in acute infection: these antibodies disappear over time in most patients.

Dr Gavin Spickett is consultant clinical immunologist at the Royal Victoria Infirmary, Newcastle

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