Table 4.

Interaction between smoking and GRS in relation to FEV₁/FVC

Exposure	n	FEV1/FVC
		Intercept*	Smoking effect†	GRS effect‡	GRS×smoking interaction: difference in the effect of GRS per smoking category§	Pinteraction¶
Smoking
Never	1884	0.760	–	−0.003	–	–
Former	839	0.738	−0.022	−0.006	−0.003	0.064
Current	121	0.673	−0.087	−0.012	−0.009	0.017

*The intercept at each smoking category is the FEV₁/FVC value for a subject in that smoking category calculated at the mean value for all continuous variables in the model (GRS, age, age², height, height² and 10 principal components) and at the reference category for all categorical covariates (ie, non-asthmatic, female and residing at Iowa).

†The effect of smoking is obtained by subtracting the intercept value for never smoking from the intercept value for the smoking category in question. For example, for former smokers, 0.738–0.760=−0.022 is the difference in FEV₁/FVC for a former smoker relative to a never smoker calculated at the mean value for all continuous variables (GRS, age, age², height, height² and 10 principal components) and at the reference category for all categorical covariates (ie, non-asthmatic, female and residing at Iowa).

‡The effect for the GRS is the individual slope for that GRS for each exposure category and is interpretable as the difference in FEV₁/FVC per unit increase in the GRS.

§The interaction effect between the GRS and smoking is the difference in the effect estimate for that GRS by smoking category and is calculated as the difference in the slope for the GRS for that smoking category relative to never smokers. For former smokers this difference is −0.006−(−0.003)=−0.003.

¶The p value for interaction between the GRS and each smoking category.

GRS, Genetic Risk Score.