Evaluation of interpretation strategies and substantial bronchodilator response in pediatric patients with normal baseline spirometry.

BACKGROUND Controversy exists regarding the best method to interpret pediatric spirometry. There is also controversy regarding the benefit of performing post-bronchodilator spirometry after normal baseline spirometry. This study compares the use of lower limit of normal (LLN) against percent of predicted (PP) in the interpretation of spirometry. We also investigate the occurrence of a substantial bronchodilator response for patients who received post-bronchodilator spirometry. METHODS Spirometric tests performed in the pediatric clinic at San Antonio Military Medical Center were retrospectively reviewed. Results of spirometry were compared using LLN and PP for interpretation. Abnormal spirometry was defined as a low FEV1 or low FEV1/FVC, indicating evidence of airway obstruction. The presence of a substantial bronchodilator response was recorded and the results were analyzed. RESULTS Of 242 tests, 212 normal and 30 abnormal tests were reported using the LLN interpretation strategy. Using the PP interpretation strategy, there was a significant difference in the number of normal (183) and abnormal (59) tests, when compared to the LLN (P < .001). No significant difference between LLN versus PP interpretation strategies was noted in the number of baseline tests, normal or abnormal, that demonstrated a substantial response to bronchodilator. Of the subjects with normal baseline spirometry, 10% (PP) and 12% (LLN) had a substantial bronchodilator response. An abnormal baseline spirometry was more likely to have a substantial response to bronchodilator, compared to normal baseline spirometry (P < .001). CONCLUSIONS The use of LLN for interpretation is more likely to report a test as normal, when compared to the PP interpretation strategy. Although a substantial bronchodilator response is more likely to occur following abnormal baseline spirometry, 10-12% of subjects with normal baseline spirometry showed a substantial bronchodilator response. This suggests that normal baseline spirometry may miss reversible airway obstruction, which is a hallmark of asthma.