Sensitivity of rapid immunoassay for influenza A and B in the early phase of the disease

Immunochromatography (IC) tests are often used for the rapid diagnosis of influenza. Once influenza is diagnosed, an anti-influenza drug can be administered. Physicians claim, however, that they are not sufficiently sensitive, especially in the early phase of the disease. The aim of the present study was therefore to analyze the sensitivity of the IC test from the standpoint of virology. To evaluate the sensitivity of the IC test statistically, 736 nasopharyngeal specimens were subjected to the IC test and virus isolation (VI). The IC test (+) specimens were compared with VI (+) specimens in the early phase after fever onset. Amounts of the virus in IC(-)VI(+) specimens were compared with those in IC(+)VI(+) specimens on real-time reverse transcriptase-polymerase chain reaction. The isolated viruses from these specimens were subjected to the IC test at the same dose. The nucleotide sequences of NP genes were compared. The IC test was less sensitive for diagnosis in the early phase of the disease. Amounts of virus were significantly lower in IC(-)VI(+) specimens than in IC(+)VI(+) specimens. No variations were observed in the isolated viruses. The IC test is not sufficiently sensitive owing to the low amount of virus in the nasopharyngeal tract in the early phase of the disease. Influenza should be diagnosed on the basis of clinical symptoms within 1 day after the fever onset.