Modeling smoking in systemic sclerosis: A comparison of different statistical approaches

Objective To determine the effect of different methods of modeling smoking on vascular outcomes in rheumatic diseases. Methods Data from the Canadian Scleroderma Research Group Registry were used. Patients self‐reported their smoking history. Vascular outcomes were severity of Raynaud's phenomenon, presence of finger ulcers, and severity of finger ulcers. Several models were developed to capture the experience of smoking: 1) ever compared to never smoking; 2) current and past smoking compared to never smoking; 3) never, past, and current smoking compared using polynomial contrasts; 4) smoking intensity, duration, and time since cessation assessed separately; and 5) smoking modeled using the Comprehensive Smoking Index (CSI), which integrates intensity, duration, and time since cessation into a single covariate. Results This study included 606 patients, of which 16% were current, 42% were past, and 42% were never smokers. Current and past smokers smoked a mean ± SD of 25 ± 17 and 17 ± 18 pack‐years, respectively. Smoking duration was shorter in past compared to current smokers (18.3 versus 31.7 years). Past smokers reported having stopped smoking approximately mean ± SD 16 ± 12 years prior, although this ranged from 1 to 50 years. Smoking had no effect on vascular outcomes in the simplest model comparing ever to never smokers. Models that isolated past smokers revealed the presence of a healthy smoker bias in that group. The model using the CSI demonstrated a strong negative effect of smoking on vascular outcomes. Conclusion Proper modeling of the effect of smoking is essential in studies of vascular outcomes of rheumatic diseases.