Multicentre, longitudinal, observational, prospective study (2 years of follow-up, annual check-ups).

Patients with PsA of recent onset (defined as within 2 years of evolution since onset of symptoms attributable to the disease).

Inclusion criteria:

• 1.

  Adults of both sexes, 18 years of age or older.

• 2.

  Compliance of the CASPAR classification criteria.12

• 3.

  PsA with a duration of under 2 years since onset of symptoms.

• 4.

  Patients attended in rheumatology departments of the centres which participated in the study.

• 5.

  Able to effectively communicate with the people conducting the study and to complete the questionnaires themselves.

Exclusion criteria:

• 1.

  Patients who were participating in the clinical trial of any product under investigation one month prior to visit 0 or during the study.

• 2.

  Very serious or terminal illness.

• 3.

  Patients with any physical disability not attributable to PsA or mental patients.

The centres were invited with reference to criteria relating to geographical aspects, services provided in each hospital and the population attended. They were all required to have a dermatologist to assist with data collection. Thirty three centres finally participated, distributed into 21 of the 55 provinces in Spain. The list of participating centres is contained in Appendix A.

Initially all participating centres were to recruit the same number of patients, to be fixed beforehand. However, during the course of the study, due to the different recruitment capacity in the different centres, it was decided that each one of them would include patients in accordance with their capacity.

The baseline visit intention was to reflect the patient situation prior to the evolution of the disease being modified by treatments regulated by the rheumatology departments. The patients who participated were therefore not to have been treated for more than 3 weeks with methotrexate, leflunomide or apremilast, and could not be in treatment with biologic DMARDS. These intervals were fixed according to mean time from treatment initiation until the beginning of treatment response being 4weeks in the case of synthetic DMARDS and 1 week in the case of biologic DMARDS. In cases where the patient had been taking synthetic DMARDS for over 3 weeks, we confirmed with the researcher rheumatologist that the patient sill had not responded at the time of the baseline visit; this was exceptional (6 patients), and for all of them the time which passed from synthetic DMARDS initiation was under 2 months.

If a patient with psoriasis in treatment with synthetic or biologic DMARDS developed PsA and was referred to the rheumatology department for diagnosis and management, they could be included in the study. The intention of the baseline visit reflecting the situation of the patient prior to the course of the disease being changed by treatment prescribed by the rheumatology service would be complied with.

The patients were invited to participate consecutively in one of their regular visits to the rheumatologist. Patient access to the rheumatology services were the normal ones of the participating services (basically from primary care and dermatology consultancies). The recruitment period began in November 2014 and terminated in October 2016.

All the patients gave their informed consent. The participating centres assigned an identification code to each of the participants to maintain data confidentiality in accordance with the law in force (RD 1720/2007 which develops the Organic Law 15/1999, of 13th December, on Personal Data protection). The study was approved by the Research Ethics Committees of the participating centres.

• a)

  Socio-demographic data: date of birth: gender; civil status (married/cohabiting, separated/divorced, widowed, single); educational level (no education, primary education, secondary education, university studies).

• b)

  Employment status: professional activity, coded in accordance with the National Occupation Classification (CNO2011)13; current employment status (retired/pensioner, unemployed, actively employed, housewife, student); employment disability (without temporary or permanent disability; sick leave during the last year, cause and duration in days; changes of employment due to illness in the last year.

• c)

  Family history (father, mother, grandparents, brothers/sisters of: psoriatic arthritis, other inflammatory arthritis, psoriasis, inflammatory intestinal disease.

• d)

  Personal history and comorbidity, year of diagnosis, treatments (using review of medical history): Charlson comorbidity index, adjusted by age14; cardiovascular risk factors (hypertension, diabetes mellitus, hyperlipidaemia); cardiovascular events (myocardial infarction, stroke, peripheral vascular disease, ischaemic vascular disease); intestinal inflammatory disease (Crohn's disease, ulcerous colitis or other inflammatory intestinal disease); infectious diseases (tuberculosis, HIV or other infectious diseases); neoplasms and location; fibromyalgia, depression.

• e)

  Anthropometric data: weight, height, BMI, waist measurement, hip measurement and waist to hip measurement.

• f)

  Lifestyle: tobacco habit (patients who state they have smoked at least 100cigarettes in their life and at time of visit they smoke every day or several days, are classified as “current smoker”. Patients who state they have smoked under 100 cigarettes in their whole life and at the time of visit they do not smoke at all are classified as “ex-smokers”. Patients who state they have never smoked 100 cigarettes are defined as “non smokers”. This classification is the one used in the Health Data and Disability System of the Centres for the Control and Prevention of Diseases (https://www.cdc.gov/nchs/nhis/tobacco/tobacco_recodes.htm). Alcohol consumption (measured in units of standard drink per week and evaluated using the Systematic Interview of Alcohol Consumption).15 Physical activity (assessed with the short form version of the International Physical Activity Questionnaire [IPAQ]).16

• g)

  Use of health services in the last year (through review of medical files): hospitalizations (Yes/No) and reason; orthopaedic surgery (Yes/No) and type; number of visits to: primary care, rheumatology, emergency departments, other specialties.

• h)

  Clinical situation at diagnosis of PsA: year of presentation of PsA symptoms, date of a PsA diagnosis, origin of patient (primary care, rheumatology, dermatology, others) clinical form (axial, peripheral, mixed) and joint pattern (oligoarticular, polyarticular, distal, mutilant, spondilytical).

• i)

  Joint involvement and back pain: number of painful joints (NAD28 and NAD68); number of swollen joints (NAT28 and NAT66); chest expansion, distance between tongue base and wall; modified Schöber test; lateral flexion of spine; intermalleolar distance; finger floor distance, cervical rotation, assessment of the patient of night time pain in the back during the previous week on a scale of 0 (no pain) to 10 (very intense); patient assessment of back pain in general during the last week on a scale of 0 (no pain to 10 (very intense); BASDAI index.17

• j)

  Pain and overall assessment of the disease during the last week: evaluation of overall pain provided by the patient on a scale of 0 (no pain) to 10 (very intense); overall evaluation of the disease provided by the patient on a scale of 0 (they feel very well) to 10 (they feel very bad); overall evaluation of the disease provided by the physician on a scale of 0 (level of minimum activity) to 10 (level of maximum activity).

• k)

  Enthesitis, dactylitis and uveítis: extended version of the MASES18 index; presence of dactylitis (Yes/No) and number of fingers affected; presence of uveitis, from the previous visit of the study until the current one (Yes/No) and laterality.

• l)

  Skin and nail involvement (assessed by a dermatologist): cutaneous psoriasis (Yes/No); year of presentation of the psoriasis; clinical type (psoriasis vulgar [in plaques], in drops, erythrodermic, general pustular, localized pustular, inverted, others); special locations (psoriasis of the scalp, nail bed, palmoplantar, intergluteal fold and/or perianal, palmoplantar pustular, mucous membrane involvement); treatment for psoriasis and year of initiation (topical treatment, phototherapy, retinoids, methotrexate, cyclosporine, etanercept, infliximab, adalimumab, ustekinumab, others). Body surface area (BSA) affected by psoriasis; PASI index19; nail diseases (number of fingers and toes affected in each hand and each foot).

• m)

  Functional situation and quality of life: HAQ20 questionnaire, PsAID21 questionnaire, BASFI22 index, SF-3623questionnaire.

• n)

  Radiographic assessment: BASRI24,25 index, Steinbrocker method modified for PsA.26

• o)

  Lab analysis determinations: ESR, PCR, rheumatoid factor, HLA B27, blood parameters, glucose, uric acid, total proteins, albumin, bilirubin, liver function tests, (GOT, GPT y GGT), total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides.

• p)

  PsA treatment, starting date, end date, reason for withdrawal: NSAID, glucocorticoids, synthetic DMARDS (methotrexate, leflunomide, sulfasalazin, apremilast, cyclosporine), biologic DMARDS (adalimumab, etanercept, infliximab, golimumab, ustekinumab, certolizumab, secukinumab). In follow up visits, in the case of DMARDS, it was also noted whether there was therapy compliance (using the question “is the patient complying with treatment?”, with response options of “Yes/no/don’t know”) and for biologic DMARDS the administration guideline was also recorded (standard/optimization regimen).

• q)

  Number of PsA outbreaks and date of last outbreak: in axial skeleton (defined as any inflammatory episode affecting the axial skeleton [chest and/or spine-pelvis] assessed as such by a rheumatologist between the previous and current study visit) and in peripheral joints (defined as any inflammatory joint episode, dactylitic or of the observed by a rheumatologist between the previous and present study visit).

Table 1 lists the measurements made in each study visit. These measurements will allow evaluation to be made of the disease through indexes, such as the Disease Activity for Psoriatic Arthritis (DPSAA)27 or the Psoriatic Arthritis Disease Activity Score (PASDAS).28

We initially planned to recruit 295 patients, assuming that there could be up to 25% of losses throughout follow-up. This sample size would make it possible to detect a relative risk of >2.30 as significant, exposure of 50%, confidence interval of 95% and statistical power of 80%.

The final sample size was 211patients.

Data will be monitored online throughout the study, to correct incongruence or data which are missing. Furthermore, 60% of baseline visits will be monitored in situ.

Periodical meetings will be made of researchers with study coordinators, so as to resolve doubts and maintain uniformity of data collection. Whenever in doubt, the researchers may contact the coordinators throughout fieldwork.