Background: The FEV1/forced vital capacity (FVC) ratio is used as a criterion for airflow obstruction; however, the test characteristics of spirometry in the diagnosis of asthma are not well established. The accuracy of a test depends on the pretest probability of disease.
Objective: We wanted to estimate the FEV1/FVC ratio z score threshold with optimal accuracy for the diagnosis of asthma for different pretest probabilities.
Methods: Asthmatic patients enrolled in 4 trials from the Asthma Clinical Research Centers were included in this analysis. Measured and predicted FEV1/FVC ratios were obtained, with calculation of z scores for each participant. Across a range of asthma prevalences and z score thresholds, the overall diagnostic accuracy was calculated.
Results: One thousand six hundred eight participants were included (mean age, 39 years; 71% female; 61% white). The mean FEV1 percent predicted value was 83% (SD, 15%). In a symptomatic population with 50% pretest probability of asthma, optimal accuracy (68%) is achieved with a z score threshold of -1.0 (16th percentile), corresponding to a 6 percentage point reduction from the predicted ratio. However, in a screening population with a 5% pretest probability of asthma, the optimum z score is -2.0 (second percentile), corresponding to a 12 percentage point reduction from the predicted ratio. These findings were not altered by markers of disease control.
Conclusion: Reduction of the FEV1/FVC ratio can support the diagnosis of asthma; however, the ratio is neither sensitive nor specific enough for diagnostic accuracy. When interpreting spirometric results, consideration of the pretest probability is an important consideration in the diagnosis of asthma based on airflow limitation.
Keywords: Asthma; accuracy; airflow obstruction; spirometry; z score.
Copyright © 2015 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.