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 allelep ValueZ×S interaction*p ValuePack-yearsp Value
Model for FEV1 (% predicted)Linear−1.870.620.160.27−0.33<0.001
Piecewise linear−5.590.140.350.03−0.38<0.001
Model for COPD statusLinear2.780.070.970.091.05<0.001
Piecewise linear4.870.010.940.011.08<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.