Table 7.

Interpretation of Influenza Testing Results on Respiratory Specimens

Test and Characteristics	Low Influenza Activitya		High Influenza Activityb
Rapid influenza diagnostic test (antigen detection: immunoassay or immunofluorescence assay) • Low to moderate sensitivity • High specificity ➢ Should not be used for testing of patients with progressive illness and hospitalized patients	Negative result NPV is high: ➢ Likely to be a true-negative result if an upper respiratory tract specimen was collected <4 days after illness onset ➢ If epidemiologically linked to an influenza outbreak, consider confirming with molecular assay	Positive result PPV is low: ➢ Likely to be a false-positive result ➢ Confirm with molecular assay	Negative result NPV is low: ➢ May be a false-negative result, especially if upper respiratory tract specimen was collected >4 days after illness onset, cannot exclude influenza virus infection ➢ Do not withhold antiviral treatment if clinically indicated ➢ Confirm with molecular assay	Positive result PPV is high: ➢ Likely to be a true-positive result
Molecular assay (nucleic acid detection: rapid molecular assayc, multiplex PCR, RT-PCR) • High sensitivity • Very high specificity ➢ Can be used for both outpatients and hospitalized patients ➢ RT-PCR assays should be used for hospitalized patients	Negative result NPV is high: ➢ Very likely to be a true-negative result, especially if an upper respiratory tract specimen was collected <4 days after illness onset	Positive result PPV is low: ➢ False-positive result is possible	Negative result NPV is low: ➢ May be a true-negative result in a patient without lower respiratory tract disease ➢ Consider potential for a false-negative result, especially if an upper respiratory tract specimen was collected in a hospitalized patient ➢ For hospitalized patients on mechanical ventilation who tested negative on upper respiratory tract specimens, collect lower respiratory tract specimens (endotracheal aspirate, BAL fluid) for testing	Positive result PPV is high: ➢ Likely to be a true-positive result

Abbreviations: BAL, bronchoalveolar lavage; NPV, negative predictive value; PCR, polymerase chain reaction; PPV, positive predictive value; RT-PCR, reverse-transcription polymerase chain reaction.

^aLow influenza activity refers to periods when influenza viruses are not known to be circulating or are only sporadically detected among the patient population being tested, such as during the summer months in the United States.

^bHigh influenza activity refers to periods when influenza viruses are known to be circulating among the patient population being tested, such as during the winter months in the United States.

^cRapid molecular assays may have lower sensitivity and specificity compared to other molecular assays (eg, RT-PCR).