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An important number of viruses (measles, rubella, human parvovirus B19, herpes simplex, varicella zoster, enteroviruses, dengue and others) can cause exanthematic diseases. Laboratory diagnosis of exanthems due to viral infections include the identification of the disease-causing virus (isolation, antigen detection or nucleic acid detection) and the determination of a specific serological response (seroconversion or specific IgM).
The aim of virus isolation is the recovering of infectious viruses, being useful for molecular epidemiological studies. It is of application for measles, rubella, enterovirus, herpes simplex and varicella-zoster. However, due to its methodological complications, it is not of application in most laboratories. For some viruses, isolation is not practical, because there is no cell substrate available (Epstein-Barr, parvovirus B19). An alternative to viral isolation is the detection of antigens on clinical specimens (dengue). Thirdly, nucleic acid detection is a very useful approach for diagnosing viral exanthems, specially when multiplex assays recognising measles, rubella and parvovirus B19, or enterovirus and herpesvirus, are used.
The best tool for the serological diagnosis is the detection of IgM, since it provides a rapid diagnosis during the first days of the disease. However it has some drawbacks when used to diagnose viral rashes (the presence of rheumatoid factor and specific IgG; the multiple reactivity due to polyclonal stimulation of memory lymphocytes; the cross-reactivity between viruses causing a similar clinical picture; the IgM synthesis in herpesvirus reactivations). In such cases, confirmatory serological approaches (establishment of seroprofiles, or avidity assays) are necessary.
Both direct and indirect methods must be considered as complementary and the results obtained for each one should be evaluated in the context of the clinical profile and epidemiological history.