Reasons for false negative: No suitable specimens were collected in different courses of disease; viruses from collected specimens were not enough to be detected; it is not sensitive enough for the test reagent; the nucleic acid extraction is not efficient. False positive: There are cross contamination during nucleic acid extraction and PCR products contamination; it is also related to specimen type and sampling method. Broncho-alveolar lavage fluid is better than swabs and can be collected simultaneously with nasopharyngeal swabs.