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Subject: Respiratory Culture, Rule out Viral Pathogens
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Most respiratory viral syndromes are relatively mild and self-limited. Occasionally, severe disease develops for which specific diagnosis is needed to optimize therapeutic and management decisions. Viral culture may be used to provide isolates for antiviral susceptibility testing or further characterization for epidemiologic reasons. This test may be ordered to make a specific diagnosis for severe seasonal respiratory viral illness. Agents cultured typically include influenza virus A and B, RSV, and parainfluenza virus types 1, 2, and 3; adenovirus may be included.
Specimens should be collected according to general recommendations for virus culture of the specimen type.
Specimens should be collected early in acute infection.
Nasopharyngeal specimens are often optimal for diagnosis.
RSV is a fastidious virus and should be delivered to the laboratory as quickly as possible. This virus may not survive freezing if prolonged transport is needed. Most specimens should be placed in a viral transport medium and transported on wet ice (4°C).
Specimens for molecular diagnostic, DFA, EIA, or other diagnostic testing for respiratory viruses should be submitted according to laboratory recommendations.
Nasopharyngeal wash specimens, or swabs, are usually associated with the best sensitivity of culture.
Specimens are typically inoculated onto several different types of cell lines to help ensure growth of all of the target pathogens. Shell vial cultures may be inoculated in addition to tube cultures. Positive cultures are determined by cytopathic effect or by staining with virus-specific tagged monoclonal antibodies.
Shell vial cultures may be positive within 48–72 hours.
Most respiratory viral pathogens can be detected in tube cultures by blind staining with immunologic reagents after 7 days of inoculation.
Expected results: Negative.
Positive results: Positive cultures for specific viruses indicate active infection with that agent.
Negative results: Negative viral culture significantly decreases the possibility but does not rule out respiratory virus infection. Molecular diagnostic techniques should be considered in patients with severe disease.
Negative cultures may be caused by numerous factors, including poor specimen collection technique, collection of specimens after acute disease when virus concentrations are below the level of detection, or infection caused by a viral pathogen not included in the respiratory virus culture panel used.
Coinfection with two or more respiratory viral pathogens is relatively common. The impact of coinfection with specific viral pathogens, such as human metapneumovirus, awaits further study.