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A significant rise in titre of IgG/total antibody between acute and convalescent sera - however, a significant rise is very difficult to define and depends greatly on the assay used. In the case of CFT and HAI, it is normally taken as a four-fold or greater increase in titre. The main problem is that diagnosis is usually retrospective because by the time the convalescent serum is taken, the patient had probably recovered.
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Presence of IgM - EIA, RIA, and IF may be are used for the detection of IgM. This offers a rapid means of diagnosis. However, there are many problems with IgM assays, such as interference by rheumatoid factor, re-infection by the virus, and unexplained persistence of IgM years after the primary infection.
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Seroconversion - this is defined as changing from a previously antibody negative state to a positive state e.g. seroconversion against HIV following a needle-stick injury, or against rubella following contact with a known case.
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A single high titre of IgG (or total antibody) - this is a very unreliable means of serological diagnosis since the cut-off is very difficult to define.
Criteria for diagnosing re-infection/re-activation
It is often very difficult to differentiate re-infection/re-activation from a primary infection. Under most circumstances, it is not important to differentiate between a primary infection and re-infection. However, it is very important under certain situations, such as rubella infection in the first trimester of pregnancy: primary infection is associated with a high risk of fetal damage whereas re-infection is not. In general, a sharp large rise in antibody titres is found in re-infection whereas IgM is usually low or absent in cases of re-infection/re-activation.
Serological events following primary infection and reinfection. Note that in reinfection, IgM may be absent or only present transiently at a low level.