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.