Authors’ response

Abstract

We thank the writers for their interest. 1 2 We agree that bronchiolitis is a clinical diagnosis; it is because the viral aetiology cannot be determined clinically that testing is contemplated. We also agree that specific treatment is not yet available; although respiratory syncytial virus (RSV) specific drugs are coming.Cohorting is controversial. Because multiple strains of RSV circulate during each season and dual infection does occur, relying on antigen testing to cohort patients risks reinfecting infants with a different RSV strain or another virus.In presenting single positive and negative predictive values, Beattie et al 1 alludes to a key point of our paper. Both RSV and bronchiolitis prevalence are seasonal (sometimes with more than one peak). Predictive values vary with prevalence. For example, a patient using the assay described by the letter writers' 2011 paper 3 who tested positive when RSV prevalence is 10% would have only a 35% chance of actually having RSV. Our modelling addresses the dynamic nature of RSV prevalence.Some antigen tests likely outperform others. However, methodology matters. The letter writers' paper evaluated the antigen test over a four-month period in infants in whom hospital admission was anticipated. This is a legitimate clinical practice, but as a research design, it diminishes external validity and generalisability. When measuring diagnostic performance, one should generally include a broad range of disease severity over a number of seasons (antigenic drift may affect test performance).Although confidence intervals were not presented in the letter writers' 2011 paper, they can be calculated, and for sensitivity, overlap with ours (see online response). Our measurements of specificity do differ. This likely reflects the differences in inclusion criteria.This does not diminish the cohorting papers that were not cited, rather it reflects the focused nature of our research question and (an unrequited) desire to avoid tangential controversies. REFERENCES1 Beattie TF, McLellan K, Templeton K. Near-patienting for RSV in the emergency department. Emerg Med J 2014;31:173-174. 2 Walsh P, Overmyer C, Hancock C, et al. Is the interpretation of rapid antigen testing for respiratory syncytial virus as simple as positive or negative? Emerg Med J 2014;31:153-159. 3 Mills JM, Harper J, Broomfield D, et al. Rapid testing for respiratory syncytial virus in a paediatric emergency department: benefits for infection control and bed management.