Table 4

Results from an analysis of main effect and gene-by-smoking interaction effects of the Z allele in PI MZ and PI MM subjects from the Bergen Norway Case–Control Study using two different methods of adjustment for smoking

		Z allele	p Value	Z×S interaction^*	p Value	Pack-years^†	p Value
Model for FEV₁ (% predicted)	Linear	−1.87	0.62	0.16	0.27	−0.33	<0.001
Model for FEV₁ (% predicted)	Piecewise linear	−5.59	0.14	0.35	0.03	−0.38	<0.001
Model for COPD status	Linear	2.78	0.07	0.97	0.09	1.05	<0.001
Model for COPD status	Piecewise linear	4.87	0.01	0.94	0.01	1.08	<0.001

Regression models adjusted for age, sex and smoking (using either a linear or piecewise linear form of the pack-years variable). The FEV₁ model includes a sample weight adjustment to reflect oversampling of COPD cases.
Results reported as β coefficients for FEV₁ model, ORs for COPD model and their respective p values.
* The Z×S interaction term is of the form (number of copies of Z allele × pack-years) for the linear smoking adjustment and (number of copies of Z allele × first 40 pack-years) for the linear and piecewise linear adjustments, respectively.
† Smoking adjustment for the linear pack-years model is done by including a numerical term for total pack-years of smoking. For the piecewise linear model, the smoking main effect is represented by two variables representing the first 40 pack-years of smoking and subsequent exposure. The reported beta coefficient is for the term representing the first 40 pack-years of smoking.
COPD, chronic obstructive pulmonary disease; FEV₁, forced expiratory volume in 1 s.