Table 4.

Association of Postbronchodilator FEV₁ to SVC Ratio of < 0.7 With Exacerbations and Progression to COPD in Smokers With Normal Spirometry Results^a

Variable	IRR (95% CI)	P Value
Total exacerbations	1.60 (0.97-2.64)	.07
Severe exacerbations	2.60 (1.04-4.89)	.040
	HR (95% CI)
Progression to COPD	3.93 (2.71-5.72)	< .001
	OR (95% CI)
Persistent COPD	5.08 (3.09-8.37)	< .001

For exacerbation analysis, data for 830 participants were available. Zero-inflated negative binomial regression models with postbronchodilator FEV₁/SVC < 0.7 as the main independent variable (exposure) and total exacerbations and severe exacerbations as the dependent variables (outcome) were performed. Models included the following covariates: age, sex, race, current smoking status, smoking pack-years, and diabetes mellitus in the count negative binomial regression and an intercept-only model in the zero component. Follow-up time was included as an offset in the models. For progression to COPD analysis, data for 845 participants were available. Interval-censored proportion hazards regression model for progression to COPD included the following covariates: age, sex, race, smoking status, and smoking pack-years. For progression to persistent COPD analysis, a logistic regression model was created with the same covariates. HR = hazard ratio; IRR = incident rate ratio; SVC = slow vital capacity.

^aNormal spirometry results defined as postbronchodilator FEV₁ ≥ 80% and postbronchodilator FEV₁/FVC ≥ 0.7.