Factors influencing the diagnostic accuracy of the rapid influenza antigen detection test (RIADT): a cross-sectional study.

Objective: To evaluate the diagnostic accuracy of the rapid influenza antigen detection test (RIADT) and determine which symptoms are relevant to results.
Design: Single-centre, cross-sectional study.
Setting: Primary care centre, Tokyo, Japan.
Participants: 82 consecutive outpatients presenting with upper respiratory symptoms and fever ≥37°C at any time from symptom onset, between December 2010 and April 2011.
Main Outcome Measures: Results of history and physical examination including age, sex, temperature, time of test from symptom onset, vaccination record and current symptoms (sore throat, arthralgia and/or myalgia, headache, chills, cough and/or throat phlegm, nasal discharge) were recorded. The RIADT and a fully automated respiratory virus nucleic acid test (Verigene Respiratory Virus Plus; VRV), the latter being the gold standard, were performed. Patients were divided into four groups: false negative (FN), RIADT- and VRV+; true positive (TP), RIADT+ and VRV+; true negative (TN), RIADT- and VRV-; and false positive, RIADT+ and VRV-. Groups were compared regarding age, sex, temperature, time of test from symptom onset, vaccination record and symptoms.
Results: RIADT sensitivity, specificity, positive predictive value and negative predictive value were 72.9% (95% CI 61.5% to 84.2%), 91.3% (79.7% to 102.8%), 95.6% (89.5% to 101.6%) and 56.8% (40.8% to 72.7%), respectively. Time from symptom onset to test was shorter for the FN group than the TP group (p=0.009). No significant differences were detected for the other factors assessed. Results revealed higher temperatures for FN than TN patients (p=0.043), and more FN than TN patients had chills (p=0.058).
Conclusions: The RIADT sensitivity was low, due to early administration of the test. In the epidemic season, the RIADT should not be used for suspected influenza until 12 h after symptom onset. A positive RIADT firmly supports the influenza diagnosis; a negative result does not confirm its absence. High fever and chills might indicate influenza, but additional tests are sometimes necessary.