[A case of acute pneumonia with prolonged false-positive results using an influenza test kit]

Influenza (flu) is a prominent infectious disease that worsens the general prognosis of older adults. We herein report a case of a clinically odd and rare reaction on an instant diagnostic kit for influenza. An 81-year-old man with a fever and rhinorrhea consulted our hospital. He had a history of dementia of Alzheimer's type and rheumatoid arthritis and had been treated with oral prednisolone (10 mg/day). Instant diagnostic test kit A using exudation from the upper pharynx showed positivity for antigen of flu A virus, and computed tomography indicated acute pneumonia. Immediately after the diagnosis, 150 mg/day of oseltamivir was started for 5 days. However, a high fever over 38.0°C persisted, and flu A antigen from the upper pharynx was repeatedly detected using test kit A. Despite subsequent oral treatment with 100 mg/day of amantadine and single venous infusion of 300 mg/day of peramivir, the high fever continued, and the detection of C-reactive protein in the serum as well as flu A antigen in the upper pharynx persisted. We suspected test failure, and the results of another test kit (kit B) were indeed negative. Furthermore, polymerase chain reaction performed by two independent laboratories failed to detect flu gene fragments. We concluded that the patient did not have the flu, and results of test kit A had been a false positive. The patient was successfully treated with ABPC/SBT infusions. We should consider the implications of diagnosing flu using instant test kits.