Antigen negative for influenza A virus, which is a no problem situation. Antigen of influenza A virus can sometimes be detected in ancillary tests. It is also possible to apply in the same amount in clinical practice the detection of antibodies to influenza A virus in the serum, which can be performed by complement binding test or hemagglutination inhibition test. However, the diagnosis of influenza A is mostly based on clinical symptoms combined with epidemiological data and the history of close contact with similar patients, which means that it is mostly a clinical diagnosis. If the influenza A virus antigen can be detected and is negative, it means that influenza A virus infection is most likely not present.