VIRAL DIAGNOSIS BY ANTIGEN-DETECTION TECHNIQUES

Background: Diagnosis of viral infections can be obtained in the early stages of a disease by detection of viral antigens directly in the cl inical specimen. This has become an important tool for rapid virus dia gnosis. Methods: Antigens produced during virus infections can be dete cted either in cells collected from the site of infection by immunohis tological investigation or in secretions and blood by solid phase immu noassays (IA). Viruses causing acute respiratory infections can be dia gnosed in cells from the respiratory tract, viruses causing vesicular eruptions in epithelial cells from skin scrapings, rabies virus in ner ve cells of the brain or epithelial cells from skin and cornea and cyt omegalovirus (CMV) matrix antigen, pp65, can be detected in peripheral blood leukocytes (PBL) by immunofluorescence (IF) or immunoperoxidase techniques. The quality of specimens can be easily checked during the reading of results. Some IAs for antigen detection, such as detection of HBsAg and HIV p24 antigen in blood are standardized and sensitive. Others give less sensitive results because of the variation of qualit y of the clinical specimen. The latex agglutination tests are mainly u sed for rapid detection of virus or viral antigens in faeces: rota- an d adenoviruses; the method may not be very sensitive but yields a resu lt within a few minutes. Assays detecting viral nucleic acids are more sensitive than antigen detection tests because of a tremendous amplif ication of gene segments obtained by the polymerase chain reaction (PC R). So far such assays are time consuming and expensive and are mainly used in specific clinical situations. Results: After introduction of specific monoclonal antibodies (Mabs), the antigen detection technique s are increasingly used. The need for quality control, trained staff, and standardized reagents and methods for specimen collection and prep aration is now being appreciated. IF for viral respiratory viruses is used for diagnosis and epidemiological studies all over the world. Lik ewise, IF is still the method most often used for rabies diagnosis. Fo r CMV, the pp65 matrix antigen is shown to be a sensitive marker close ly correlated with clinical symptoms. Its detection by the IF techniqu e has proven to be superior to other techniques for prediction of CMV pneumonia in bone marrow transplant patients. IAs are currently used i n fully automated systems for large scale diagnosis based on antigen d etection in serum specimens. Increase of antibody specificity on the s olid phase by use of Mabs directed against the most abundant viral ant igen in the clinical specimen shortens the reaction time; this has bee n employed in most of the constantly appearing new rapid diagnosis kit s based on the immunoassay principle. Conclusion: Although, in virolog y, more sensitive results are obtained by the gene detection method, P CR-directly in clinical samples, viral antigen detection tests are, af ter the introduction of Mabs for diagnostic purposes, increasingly use d because of their low demand on laboratory equipment, their rapid and early result and relatively low cast. Antigen detection is successful ly used directly in clinical specimens for rapid diagnosis of many vir al infections as well as for identification of tissue culture isolated viruses. With Mab-based IAs the reaction time is shortened and new ra pid, almost 'instant test' kits are appearing on the market.