Smoking quantity determines disease activity and function in Chinese patients with ankylosing spondylitis

The objective of this study was to systemically and comprehensively evaluate the associations between smoking and disease outcomes in patients with ankylosing spondylitis (AS). Information on smoking, clinical features, and sociodemographic characteristics was collected by a questionnaire administered directly to the patient. Group differences were analyzed by t test or chi-square test. Logistic regression analysis was conducted with the Bath AS Disease Activity Index (BASDAI), Bath AS Functional Index (BASFI), C-reactive protein, and erythrocyte sedimentation rate as the dependent variables and different stratification of smoking duration, smoking intensity, and cumulative smoking as independent variables. In order to compare our results with previous studies, meta-analysis was performed to calculate standardized mean difference (SMD) for relationship between outcomes and smoking status. A total of 1178 AS patients were analyzed. Compared with non-smokers, the risk of having active disease (BASDAI ≥ 4) was higher in patients who smoked at least 15 years, or 15 cigarettes per day, or 15 pack-years (OR = 1.70 [1.06, 2.73], 1.75 [1.08, 2.82], and 1.97 [1.06, 3.67], respectively); and smokers had increasing risk of BASDAI ≥ 4 with increasing years of smoking, or cigarettes per day, or pack-years (p_-trend = 0.010, 0.008 and 0.006, respectively). The risk of having active disease was higher in patients who smoked at least 15 cigarettes per day or 15 pack-years (OR = 1.74 [1.06, 2.84] and 2.89 [1.56, 5.35], respectively), with increasing number of cigarettes per day and pack-years. Smokers had an increased risk of BASFI ≥ 4 (p_-trend = 0.040 and 0.007, respectively). By meta-analysis, current, former and ever smokers had significantly higher BASDAI (SMD = 0.34 [0.18, 0.48], 0.10 [0.01, 0.19], and 0.27 [0.20, 0.34], respectively) and BASFI (SMD = 0.35 [0.16, 0.55], 0.30 [0.22, 0.39], and 0.35 [0.21, 0.50], respectively) compared to non-smokers. Smoking is a risk factor for greater disease activity and worse functioning in AS patients.