ABSTRACT
The detection of viruses in nasopharyngeal aspirates of patients with acute respiratory disease by IF was developed in 1968 using rabbit antisera and antispecies conjugates and was considered one of the original rapid diagnostic methods. Commercial polyclonal sera are available for most respiratory viruses, but these are being replaced by monoclonal antibodies (mAbs), which have more consistent specificity and therefore are less likely to require absorption before use (25-28). Furthermore, the use of mAbs (now commercially available) increases the intensity of specific fluorescence and reduces nonspecific fluorescence. Immunofluorescence can be divided into direct and indirect assays; in the direct IF assay, the sample (usually a cell deposit by cytospin) is fixed on a microscope, and virus-specific serum labeled with a fluorescent dye is added and any unbound antibody washed away. Bound antibody is then detected by fluorescent microscopy (16). In the indirect method, specific unlabeled antiviral antibody is first bound to any antigen and then a second labeled antiantibody from another species is used to detect the presence of the first (29). This increases the number of bound antibodies and, therefore, increases sensitivity. The prior use of cyto-centrifugation increases the sensitivity of the assay (30,31 ). The results of IF assays are usually available within 2-3 hours of taking the sample. Although simple in principle, IF assays have drawbacks in that the specimen needs to contain an adequate number of cells, the antibodies need to be specific, and background fluorescence occurs frequently. Recently, an indirect IF assay containing a pool of mAbs screening for the panel of influenza A, B, RS virus, parainfluenza viruses 1-3, and adenovirus has been described, with a reported sensitivity of 89% (32).
