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.
Prospective cohort study with nested case-control analysis.
The data source was the United Kingdom General Practice Research Database containing computerized clinical information entered by general practitioners (GPs).
The study population comprised patients receiving a first-ever diagnosis of psoriasis between January 1, 1996, and December 31, 1997, and free of cancer.
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.
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.
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.