The rapid diagnosis of viral infections.

283 © The laboratory diagnosis of viral infections traditionally has been considered a lengthy process, providing results that may have minimal clinical impact. Specimens inoculated into cell culture tubes typically require incubation for several days prior to development of cytopathic effect or detection by methods such as hemadsorption. Culture tubes are often observed for 2 weeks or longer before reporting negative results. However, significant advances during the past 10 to 15 years in the development of viral diagnostic tests provide rapid and more useful results. Cell culture incubation times have been shortened significantly with the use of centrifugation enhanced shell vials. Methodologies that detect viral antigens or nucleic acids directly in patient specimens can be completed in minutes or hours. Decreased sensitivity and speci-ficity may accompany improved time to detection. The costs required to perform newer, rapid tests versus traditional culture methods are additional considerations. Rapid tests may have unique specimen requirements. All of these issues must be weighed carefully when considering which tests to offer. Respiratory Viruses Rapid tests have had more impact on the detection of respiratory viruses than any other group of viruses. Indeed, studies have demonstrated a number of positive outcomes of rapid detection of respiratory viruses in hospitalized patients, including decreased length of hospitalization, patient isolation, costs, and better use of antibiotics. 1,2 Respiratory viruses that are frequently identified by both traditional and rapid methods include influenza virus, respiratory syncytial virus (RSV), adenovirus, and parainfluenza virus. All of these viruses will grow in commonly available cell lines in tube cultures and are usually detectable after several days of incubation. However, influenza and parainfluenza viruses may not produce discernible cytopathic effect (CPE), requiring blind hemadsorption or immuno-fluorescence staining of cells. Respiratory syncytial virus is more labile than other respiratory viruses. Titers decrease rapidly during specimen transport, even at 4°C. Culture is therefore a relatively insensitive test for RSV. Non-culture methods perform well in comparison. Rapid non-culture tests used routinely to detect respiratory viruses include direct immunofluorescence assay (DFA) and anti-gen/enzyme assays employing solid membrane surfaces. Centrifugation-enhanced shell vials provide rapid culture detection of respiratory viruses. The DFA procedure utilizes fluorochrome (usually fluorescein isothiocyanate)-labeled monoclonal anti-bodies for the staining of viral antigens within infected cells. Following excitation, fluorochromes emit light at specific wavelengths. Fluorescein isothiocyanate appears apple green when excited by ultraviolet light. The DFA procedure can be performed on a variety of respiratory specimens including swabs …