Smoking was positively associated with psoriatic arthritis (PsA) risk in the general population, but negatively associated among patients with psoriasis, a recent study found. Conditioning on a causal intermediate variable (psoriasis) may even reverse the association between smoking and PsA, potentially explaining the smoking paradox for the risk of PsA among patients with psoriasis. Using 1995–2015 data, researchers performed survival analysis to examine the association between smoking and incident PsA in the general population and among patients with psoriasis. They clarified the paradox using mediation analysis and conducted bias sensitivity analyses to evaluate the potential impact of index event bias and quantify its magnitude from uncontrolled/unmeasured confounders. They found:
- Of 6.65 million subjects without PsA at baseline, 225,213 participants had psoriasis and 7,057 developed incident PsA.
- Smoking was associated with an increased risk of PsA in the general population, but with a decreased risk among patients with psoriasis.
- Mediation analysis showed that the effect of smoking on the risk of PsA was mediated almost entirely through its effect on psoriasis.
Nguyen U-S D, Zhang Y, Lu N, et al. Smoking paradox in the development of psoriatic arthritis among patients with psoriasis: A population-based study. [Published online ahead of print November 4, 2017]. Ann Rheum Dis. doi:10.1136/annrheumdis-2017-211625.
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