Diagnostic performance of near-patient testing for influenza

BACKGROUND: Rapid diagnosis of influenza is important for controlling outbreaks and starting antiviral therapy. Direct antigen detection (DAD) is rapid, but lacks sensitivity, whereas nucleic acid amplification testing (NAT) is more sensitive, but also more time-consuming. OBJECTIVES: To evaluate the performance of a rapid isothermal NAT and two DADs. STUDY DESIGN: During February–May 2014, we tested 211 consecutive patients with influenza-like illness using a commercial isothermal NAT (Alere™ Influenza A&B) as well as the DAD Sofia(®) Influenza A + B and BinaxNOW(®) Influenza A&B for detection of influenza-A and -B virus. RespiFinder-22(®) a commercial multiplex NAT served as reference test. Serial 10-fold dilutions of influenza-A and -B cell culture supernatants were examined. Another 225 patient samples were tested during December 2014–February 2015. RESULTS: Compared to RespiFinder-22(®), the isothermal NAT Alere™ Influenza A&B, and the DAD Sofia(®) Influenza A + B and BinaxNOW(®) Influenza A&B had sensitivities of 77.8%, 59.3% and 29.6%, and specificities of 99.5%, 98.9% and 100%, respectively, for the first 211 patient samples. Alere™ Influenza A&B showed 85.7% sensitivity and 100% specificity in the second cohort. Isothermal NAT was 10-100-fold more sensitive compared to DAD for influenza virus culture supernatants with a lower limit of detection of 5000–50,000 copies/mL. The average turn-around time (TAT) of isothermal NAT and DADs was 15 min, but increased to 110 min for Alere™ Influenza A&B, 30 min for BinaxNOW(®) Influenza A&B, and 45 min for Sofia(®) Influenza A + B, when analyzing batches of 6 samples. CONCLUSION: Simple sample processing and a TAT of 15 min render isothermal NAT Alere™ Influenza A&B suitable for sequential near-patient testing, but the TAT advantage is lost when testing of larger series.