Use of rapid influenza diagnostic tests under field conditions as a screening tool during an outbreak of the 2009 novel influenza virus: practical considerations.

BACKGROUND: Rapid influenza diagnostic tests (RIDTs) are used in various settings as a first-line screen of patient specimens. During the initial outbreak of the 2009 novel influenza A/H1N1 virus, the Nebraska Public Health Laboratory (NPHL) adopted a testing algorithm, attempting to maximize the usefulness of RIDTs. However, it became apparent that a high percentage of the positive specimens received from off-site facilities were negative for influenza viruses by the confirmatory test, the Luminex xTAG Respiratory Viral Panel (RVP) molecular assay.

OBJECTIVES: To explore the cause of discrepancies between RIDTs results obtained from on-site facility testing versus confirmatory testing performed at NPHL.

STUDY DESIGN: Specimens (n=336) tested with RIDTs at off-site facilities and screened for high-probability of containing H1N1 were sent to the NPHL for confirmatory testing by RVP.

RESULTS: Of 336 specimens analyzed, 104 were negative for influenza A or B by both RIDT and RVP; 127 were positive by both tests; 102 were positive by RIDT only; and 3 were positive by RVP only. Using the RVP assay as the gold standard, overall RIDT characteristics in this screened population were: sensitivity=97.7% (95%CI: 92.5, 99.3); specificity=48.1% (95%CI: 40.4, 55.8); positive predictive value=54.3% (95%CI: 47.0, 61.4); and negative predicative value=97.1% (95%CI: 90.6, 99.1).

CONCLUSIONS: The results show that the confirmation of RIDT-positive results varied widely by testing site. Possible explanations for the discrepancies in performance characteristics include testing a narrowly defined sample population, test facility characteristics, facility work load, and seasonal timing.