The protocol for a patient-driven online prospective European observational cohort aiming to determine risk factors for the development of psoriatic arthritis among people living with psoriasis: the HIPPOCRATES Prospective Observational Study.

Background
Up to one-third of people living with psoriasis develop psoriatic arthritis (PsA), and the majority have active psoriasis prior to the development of arthritis. Clinical risk factors, such as nail involvement, in conjunction with novel blood biomarkers, could improve PsA risk monitoring and early diagnosis.
Objectives
The aim of the HIPPOCRATES Prospective Observational Study (HPOS-www.hpos.study) is to follow a cohort living with psoriasis and identify risk factors for the development of PsA.
Design
HPOS is a patient-driven online prospective European observational cohort.
Methods
Adult participants with psoriasis but with no prior diagnosis of PsA are eligible. Participants are invited to provide consent and join the study online. They complete a semi-structured questionnaire to collect data on demographics, psoriasis, comorbidities, risk factors for PsA, and the Psoriasis Epidemiology Screening Tool screening questionnaire. Follow-up is conducted through a questionnaire every 6 months. The primary outcome is the new onset of PsA confirmed by a diagnosis from their doctor. The study will also collect peripheral blood samples from a subset of participants for biomarker identification.
Ethics
This study follows the principles of the Declaration of Helsinki. To date, ethical approval has been granted by independent ethical committees in 10 countries.
Discussion
Studying a cohort of individuals with psoriasis will allow us to identify risk factors for arthritis development and to develop a risk calculator. This can support focused efforts on screening, patient education, and even studies looking to delay or prevent the onset of arthritis. This study, run via remote online data collection, provides an efficient way to recruit a large cohort (25,000) across multiple countries. However, challenges have had to be addressed with some key changes in study design, ethical review, and recruitment strategies required for each individual country.
Trial Registration
HPOS, Clinicaltrials.gov ID: NCT05858528, IRAS number 325080; https://clinicaltrials.gov/study/NCT05858528?locStr=United%20Kingdom&country=United%20Kingdom&cond=Psoriasis&term=HPOS&aggFilters=status%3Anot%20rec&rank=1.

Plain language summary
The HIPPOCRATES prospective observational study (HPOS)
The HPOS Study, part of the HIPPOCRATES project, aims to find out what signs or factors can show which people with psoriasis might later develop Psoriatic Arthritis (PsA). PsA is a type of inflammatory arthritis that is related to the skin condition psoriasis. It occurs in about 1–2% of the general population but can develop in up to 30% of people who already have skin or nail psoriasis. Diagnosing PsA early can be difficult because symptoms can be vague or inconsistent, which means treatment often starts only after joint damage has already happened. By learning more about how psoriasis develops into PsA, researchers hope to find new ways to treat the disease earlier—or even prevent or delay it. The HPOS Study is an observational study that uses online questionnaires. Adults (aged 18 or older) who have psoriasis but not PsA can take part. Participants fill out a questionnaire every six months for three years. These questionnaires collect information about age, psoriasis details, lifestyle and health factors, early joint symptoms (using the PEST questionnaire), daily function, treatment satisfaction, disease impact, fatigue, and mental health. If early signs of PsA appear, participants are advised to contact a doctor for assessment. The study plans to recruit 25,000 people across 14 European countries (including the UK, Ireland, France, Germany, and others) and expects that around 675 participants will develop PsA each year. A smaller group of 3,000 participants will also provide a small finger-prick blood sample, which will help researchers look for blood markers that might predict PsA development. HPOS is the first large-scale European study to track how psoriasis progresses to PsA. The findings could lead to a “risk calculator” that helps doctors identify people at high risk of developing PsA earlier.