• Paul Walsh, Christina Overmyer, Christine Hancock, Jacquelyn Heffner, Nicholas Walker, Thienphuc Nguyen, Lucas Shanholtzer, Enrique Caldera, James Pusavat, Eli Mordechai, Martin E Adelson, and Kathryn T Iacono.
• Department of Emergency Medicine, University of California Davis, , Sacramento, California, USA.
• Emerg Med J. 2014 Feb 1; 31 (2): 153-9.

ObjectiveTo measure the performance characteristics of an immunochromatographic rapid antigen test for respiratory syncytial virus (RSV) and determine how its interpretation should be contextualised in patients presenting to the emergency department (ED) with bronchiolitis.DesignDiagnostic accuracy study of a rapid RSV test.SettingCounty hospital ED.InterventionWe took paired nasal samples from consecutively enrolled infants with bronchiolitis and tested them with a rapid immunochromatographic antigen test and reverse transcriptase PCR gold standard.Outcome MeasuresSensitivity, specificity, the effect of point prevalence, clinical findings and overall context on predictive values. We used these to construct a graphical contextual model to show how the results of RSV antigen tests from infants presenting within 24 h should influence interpretation of subsequent antigen tests.ResultsWe analysed 607 patients. The sensitivity and specificity for immunochromatographic testing was 79.4% (95% CI 73.9% to 84.2%) and 67.1% (95% CI 61.9% to 72%) respectively. We found little evidence of spectrum bias. In our contextual model the best predictor of a positive RT-PCR test was a positive antigen test OR 5.47 (95% CI 3.65 to 8.18) and the number of other infants having positive tests within 24 h OR 1.48 (95% CI 1.26 to 1.72) per infant. Increasing numbers presenting to the ED with bronchiolitis in a given day increases the probability of RSV infection.ConclusionsThe RSV antigen test we examined had modest performance characteristics. The results of the antigen test should be interpreted in the context of the results of previous tests.

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