Study Population
Data were retrieved in May 2016, and, in total, 5438 patients with moderate to severe psoriasis were registered in the PsoReg at this time. PsoReg, initiated in 2006, is the national registry for systemic treatment of psoriasis in Sweden. Patients are registered at local, regional, and university hospitals, as well as in private practices and at treatment centers initiated by the Swedish Psoriasis Patient Organization (PSO) [18]. To be included in the PsoReg, patients need to be diagnosed with moderate to severe psoriasis and treated with, or considered for, systemic treatment by a specialist in dermatology. Thus, patients with mild psoriasis who are treated in primary care are not eligible for inclusion in the register. Nationwide, 65% of all biologically treated psoriasis patients and approximately 45% of all systemically treated psoriasis patients are estimated to be included in the PsoReg.
Definition of Variables
PASI is a combined score consisting of several dimensions of psoriasis, and is based on four areas: head, arms, trunk, and legs. Furthermore, for each body area, three plaque characteristics are assessed by the degree of erythema (redness), induration (thickness), and desquamation (scaling). The scores of the clinical signs in each area are summed and are finally weighted according to the area’s proportion of the body, before being converted to the final score, which ranged from 0 to a theoretical maximum of 72.
For each patient, information on sex, age, body mass index (BMI), disease duration, diagnosis of psoriatic arthritis (PsA), smoking status, PASI score, and the season in which assessment of the PASI score was carried out was retrieved from the PsoReg.
Statistical Analysis
Patient characteristics were analyzed at enrolment to examine differences between women and men. Continuous variables (age, disease duration, and BMI) were analyzed using the Student’s t test, while categorical variables (smoking, PsA, obesity, and season of the PASI evaluation) were tested using the Chi square test. The difference in the aggregated PASI score was analyzed using a Mann–Whitney–Wilcoxon test and a multivariable linear regression. At enrolment, a kernel-smoothing estimation was used to plot the difference in PASI score for both men and women, according to age at enrolment.
The differences in the independent PASI components between women and men were first investigated using the Mann–Whitney–Wilcoxon test separately for the different assessments: degree of involvement, erythema, induration and desquamation, and within each body region (head, trunk, arms, and legs). To be able to adjust for potential confounders and effect modifiers, a multiple linear regression was used to analyze the weighted aggregated PASI score, while multiple ordinal logistic regressions were used to analyze the different components of the PASI assessment. For each body area, four different regressions were fitted, where the score of the plaque characteristics (erythema, induration, and desquamation) and degree of skin involvement were outcomes. Age (continuous), sex (dichotomous), BMI (continuous), disease duration (continuous), PsA (dichotomous), smoking status (dichotomous), and season (categorical) were included as independent variables. The season variable was categorized into four different periods; winter (December, January, February), spring (March, April and May), summer (June, July, August), and autumn (September, October, November). The assumption of proportionality in the ordinal logistic regressions was tested by estimating the odds ratios in logistic regressions to make sure that the estimates did not vary between the two methods. Furthermore, different thresholds were also tested for the various categories to ensure that the results were stable.
To illustrate differences between sexes in the treatment of psoriasis on a national level, independent from the PsoReg, aggregated information on all dispensed prescriptions for (biologic treatment) ustekinumab (Anatomical Therapeutic Chemical [ATC] code L04AC05) and (topical treatment) calcipotriol (± corticosteroid; ATC codes D05AX02, D05AX52) was collected from the Prescribed Drug Register (PDR). Both substances are indicated for psoriasis and PsA, but not for other indications.
All statistical analyses were performed using SAS version 9.3 (SAS Institute, Inc., Cary, NC, USA). A p value ≤0.05 was considered to be statistically significant.
Ethics
This research was approved by the Umeå Ethical Review Board, and patients were recruited into the study after informed consent had been obtained.