Relative versus absolute risk of comorbidities in patients with psoriasis

Background: Psoriasis is associated with numerous comorbidities, often reported in terms of relative risk. Both doctors and the general population tend to overestimate the effects of exposures when presented in relative terms, leading to anxiety and potentially poor treatment decisions. Absolute risks might provide a better basis for risk assessment. Objective: To characterize and compare relative and absolute risks of comorbidities in patients with psoriasis. Methods: A systematic review using Medline identified comorbidities associated with psoriasis, their relative risks, and information for calculating absolute risks. Results: The comorbidities associated with psoriasis with the highest relative risk were nonmelanoma skin cancer, melanoma, and lymphoma, with relative risks of 7.5, 6.12, and 3.61, respectively; the attributable risk for these 3 conditions were 0.64, 0.05, and 0.17 per 1000 person-years, respectively. To attribute 1 event of these conditions to psoriasis would require seeing 1551; 20,135; and 5823 patients, respectively. Limitations: Database studies might not fully account for confounders, resulting in overestimates of the risk impact of comorbidities. Conclusions: Presenting attributable risk in the form of the number needed to harm provides a clearer picture of the magnitude of risk and a basis for wiser medical decision making and patient education.