Objectives: To study the clinical spectrum of psoriasis and the incidence in the general population and to identify risk factors associated with the occurrence of psoriasis.
Design: Prospective cohort study with nested case-control analysis.
Setting: The data source was the United Kingdom General Practice Research Database containing computerized clinical information entered by general practitioners (GPs).
Patients: The study population comprised patients receiving a first-ever diagnosis of psoriasis between January 1, 1996, and December 31, 1997, and free of cancer.
Interventions: Diagnosis of psoriasis was validated in a random sample of 14% of all ascertained cases requesting confirmation by the GPs. Nested case-control analysis included 3994 cases of psoriasis and a random sample of 10 000 controls frequency matched to cases by age, sex, and calendar year.
Main outcome measures: Incidence rate of psoriasis and estimates of the odds ratio (OR) and 95% confidence interval (CI) for psoriasis as associated with selected risk factors.
Results: The incidence rate of psoriasis was 14 per 10 000 person-years. Patients with antecedents of skin disorders and skin infection within the last year carried the highest risk of developing psoriasis (OR, 3.6 [95% CI, 3.2-4.1], and OR, 2.1 [95% CI, 1.8-2.4], respectively). Also, smoking was found to be an independent risk factors for psoriasis (OR, 1.4 [95% CI, 1.3-1.6]). We did not find an association between risk of psoriasis and antecedents of stress, diabetes, hypertension, hyperlipidemia, cardiovascular disease, or rheumatoid arthritis.
Conclusions: The incidence rate in our study was higher than those published in other studies, probably owing to our case definition that considered cases recorded by the GPs independently of a specialist confirmation. Our results confirm the association between psoriasis, skin disorders, and smoking.