PURPOSE: Although abnormal FEV1/FVC ratio is considered the gold standard for diagnosing obstruction in asthma, the prevalence of abnormal FEV1/FVC ration in physician-diagnosed asthma is not well characterized. Further, the role of RV/TLC ratio >35% has not been explored well in the evaluation of subjects with asthma.
METHODS: Lung function data in subjects with physician-diagnosed asthma were collected from Jan 2008 to December 2010. Subjects with abnormal DLCO were excluded to avoid inclusion of COPD. Predictors for abnormal RV/TLC ratio were determined using general linear model. Ethnicity and gender were used as fixed factors and interval predictors with significant co-linearity were excluded from the model. A p value of <0.05 was considered statistically significant.
RESULTS: Of the 392 patients, 270 (70%) were women and 59% were current smokers. The ethnicity was Caucasian in 50%, Hispanic in 37% and African American in 13%. The mean age was 48 (SD 13.6), BMI was 29.5 (SD 12) and mean pack year smoking was 11 (SD 13). Abnormal FEV1/FVC ration was observed in 79 (20%), abnormal FEV1 was present in 97 (25%), significant reversibility in 124 (32%) and abnormal RV/TLC ratio in 225 (57%). The model explained 92% of the variation. There was no interaction between gender and race. The significant predictors of abnormal RV/TLC ratio were: female gender, African American race, age, FEV1, FEV1/FVC ratio, peak flow ERV, RV and TLC. The BMI, FEV1 reversibility and smoking had no impact on RV/TLC ratio.
CONCLUSIONS: RV/TLC ratio was a better criterion for diagnosing obstruction in asthma. Air trapping and bronchodilator response appears to be mediated by independent patho-physiologic mechanism in asthma.
CLINICAL IMPLICATIONS: The Lack of sensitivity for FEV1/FVC ratio as a diagnostic test may explain the discordance between asthma symptoms and abnormal FEV1/FVC ratio. Symptom score may better correlate with abnormal RV/TLC ratio. The the role of abnormal FEV1/FVC ratio as a gold standard for obstruction needs reevaluation.
DISCLOSURE: Jose Joseph: Other: Autthor for a topic in UpTodate
The following authors have nothing to disclose: Andres Escobar, M. Bashir, Dale Claes
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