We sought to determine the proportion of bronchiolitis episodes attributable to respiratory syncytial virus (RSV) among ICD‐9 coded infant bronchiolitis episodes which were tested for RSV.

Bronchiolitis healthcare encounters were extracted from Kaiser Permanente Northern California databases for years 2006 to 2009. We used ICD‐9 codes for bronchiolitis to capture bronchiolitis‐related healthcare encounters including hospital admissions (Hospitalization), emergency department visits (EDV), and outpatient visits (OPV). We reported the monthly proportion of RSV‐positive bronchiolitis episodes among tested bronchiolitis episodes. We used logistic regression to assess association between bronchiolitis episodes and patient demographic and health care characteristics. We also used logistic regression to assess association between decision to test and patient demographics and health care characteristics.

Among 10,411 ICD‐9 coded infant bronchiolitis episodes, 29% were RSV tested. Fifty one percent of those tested were RSV positive. Between December and February, and in infants ≤6�months, the proportion of bronchiolitis episodes that were attributable to RSV was 77.2% among hospitalized episodes, 78.3% among EDV episodes, and 60.9% among OPV episodes, respectively. The proportion of RSV‐positive bronchiolitis episodes varied based upon infant age at diagnosis, level of health care service used, and time of the year of the episode.

Estimation of the proportion of ICD‐9 coded bronchiolitis episodes attributable to RSV is more specific when restricting to bronchiolitis episodes during peak months, younger infant age, and those requiring higher level of healthcare.