Accurate diagnosis of influenza is critical for clinical management, infection control, and public health actions to minimize the burden of the disease.
Rapid influenza diagnostic tests (RIDTs) that detect the influenza virus nucleoprotein (NP) antigen are widely used in clinical practice for diagnosing influenza because they are simple to use and provide results within 15 minutes.
A collaboration between the US Centers of Disease Control (CDC; Atlanta, GA, USA), and the Medical College of Wisconsin (MCW; Milwaukee, WI, USA) evaluated RIDTs commercially available during the 2011–2012 influenza season. The CDC provided 16 influenza A and seven influenza B viruses to MCW. The concentration of influenza virus NP antigen was measured as μg/mL using isotope dilution tandem mass spectrometry. Swab samples or mock nasal wash specimens were prepared from several dilutions of each virus in saline.
The numbers of RIDTs that were positive, defined as at least two positive results of the three tests performed, at each dilution for each of the 23 influenza viruses were compared. The RIDTs overall had fewer positive results with viruses that had the lowest stock NP concentrations of less than 2 µg/mL. One RIDT, the SAS FluAlert Influenza A (SA Scientific; San Antonio, TX, USA) did not uniformly detect influenza A (H1N1)pdm09 (pH1N1) viruses or other influenza A viruses at high concentrations. Four RIDTs detected the majority of influenza B viruses in the third dilution samples, whereas only one RIDT, the BD Directigen EZ Flu A+B (Becton, Dickinson and Company, Franklin Lakes, NJ, USA) detected at least 50% of all influenza A viruses in the third dilution samples.
Clinicians should be aware of the variability of RIDTs when interpreting negative results and should collect test samples using methods that can maximize the concentration of virus antigen in the sample, such as collecting adequate specimens using appropriate methods in the first 24 to 72 hours after illness onset. The study was published on November 2, 2012, in the Morbidity & Mortality Weekly Report.