Performance of five FDA-approved rapid antigen tests in the detection of 2009 H1N1 influenza A virus

Rapid antigen tests are commonly used by clinicians for rapid, simple, point-of-care testing. Five rapid antigen tests were shown to have low sensitivity (40.3-58.8%) when compared to real-time RT-PCR using nasal wash specimens from patients with influenza-like-illness (N=167) that were collected previously and confirmed as 2009 pandemic influenza A (H1N1)-positive by PCR. Rapid antigen test sensitivity correlated with virus levels in nasal secretions when comparisons were made to cycle threshold (C_T) values obtained from real-time RT-PCR. When C_T values are <25 (equating to viral concentrations of >10⁴TCID₅₀/ml) sensitivity for all five rapid antigen kits was high (range: 83-94% positive); however, when C_T values are >30 (10²TCID₅₀/ml), sensitivities of only 16-18% were observed for four of five rapid antigen kits. The Directigen EZ Flu A+B test detected more positive samples (35%) at lower viral concentrations with C_T values >30 when compared with other commercial kits (P=0.05). Rapid antigen test results must be interpreted with caution, and negative specimens may need confirmation by sensitive molecular assays.