Table 3.

Measures interaction between HLA-DRB1 shared epitope (SE) containing alleles (1 or 2 copies vs. none) and smoking in rheumatoid arthritis risk in African Americans^*

All RA
Measures of Additive Interaction	Ever smoking /SE	≥ 10 pack-years /SE
Attributable proportion† (AP) (95% CI)	0.47 (0.14 to 0.80)	0.58 (0.16 to 0.99)
P-value for interaction	Padd = 0.006	Padd = 0.007
Relative excess risk due to interaction (95% CI)	2.04 (−0.36 to 4.43)	4.86 (−3.08 to 12.80)
Synergy Index (95% CI)	2.55 (0.99 to 6.60)	2.88 (0.92 to 9.04)
Measures of Multiplicative Interaction
P-value of product term	Pmult = 0.17	Pmult = 0.38
Anti-CCP Antibody Positive RA
Measures of Additive Interaction
Attributable proportion† (AP) (95% CI)	0.53 (0.22 to 0.83)	0.63 (0.25 to 1.00)
P-value for interaction	Padd = 0.001	Padd = 0.001
Relative excess risk due to interaction (95% CI)	3.16 (−0.22 to 6.54)	7.16 (−3.76 to 18.08)
Synergy Index (95% CI)	2.70 (1.13 to 6.45)	3.17 (1.04 to 9.62)
Measures of Multiplicative Interaction
P-value of product term	Pmult = 0.13	Pmult = 0.36
Anti-CCP Antibody Negative RA
Measures of Additive Interaction
Attributable proportion (AP)† (95% CI)	0.04 (−0.77 to 0.86)	0.24 (−0.69 to 1.17)
P-value for interaction	Padd = 0.92	Padd = 0.61
Relative excess risk due to interaction (95% CI)	0.08 (−1.39 to 1.54)	0.81 (−3.17 to 4.80)
Synergy Index (95% CI)	1.11 (0.15 to 8.38)	1.52 (0.26 to 9.04)
Measures of Multiplicative Interaction
P-value of product term	Pmult = 0.96	Pmult = 0.87

^*Additive interaction examined by assessing for evidence of departure from additivity in age- and sex-adjusted models; Multiplicative interaction examined by modeling SE-smoking product term. Smoking examined as ever vs. never smoking and heavy smoking (≥ 10 pack-years) vs. other (never or < 10 pack-years).

^†AP = 0 corresponds to no interaction and an AP = 1.0 corresponds to ‘complete’ additive interaction