Outcome Measures in Psoriatic Arthritis

Key points

Domains

In 2007, the GRAPPA and Outcome Measures in Rheumatology Clinical Trials (OMERACT) group published consensus on a core set of PsA domains that should be assessed in clinical trials (Fig. 1).¹ The core set included in the inner circle includes activities of global disease, peripheral joint disease, and skin disease, along with measures of impact such as physical function, quality of life, and pain. Items in the outer circle were designated as useful domains to measure but were not to be included

Arthritis

Measures of peripheral joint disease activity are based on tender and swollen joint counts and can be combined with other measures in a composite fashion. The key issue in PsA is that a full 68/66 joint count should be performed, as reduced joint counts designed for RA are not appropriate.¹ This issue is particularly crucial in oligoarthritis whereby disease is not accurately assessed using reduced joint counts.² However, even in patients with polyarthritis, significant proportions of active

Skin

The body surface area (BSA) was the first measure devised to assess psoriasis, and it is based on the rule of nines for burns.⁷ Many subsequent measures are partially based on this instrument, including the psoriasis area and severity index (PASI), in common use to assess skin involvement. PASI includes scores for erythema, induration, and scaling, as well as the BSA on different parts of the body (head, trunk, upper limbs, and lower limbs).⁸ The PASI is the most widely used outcome for

Nails

Several different measures are available to assess nail involvement in psoriasis. In particular, the nail psoriasis severity index (NAPSI) and a modified version (mNAPSI) are the 2 most popular in PsA studies. Both instruments score each nail individually for features of nail bed changes (onycholysis, oil drop, splinter hemorrhages, and nail bed hyperkeratosis) and nail matrix psoriasis (pitting, leukonychia, red spots in lunula, and nail plate crumbling).⁹ The mNAPSI scores the most common

Enthesitis

The Mander enthesitis index (MEI) was the first instrument to assess enthesitis and includes 66 potential sites graded for tenderness from 0 to 3. These sites were identified from examination of patients with ankylosing spondylitis (AS) and subsequently validated in a group comprising mainly patients with AS.¹⁰ The Leeds enthesitis index (LEI) was developed to simplify the MEI and provide a more feasible tool specific to patients with PsA. Researchers used dichotomous scoring for pain rather

Dactylitis

Dactylitis or fusiform swelling of an entire digit is a typical feature of PsA. In most clinical trials measuring dactylitis, a simple count of dactylitis digits sometimes with a grading of tenderness from 0 to 3 has been used.3, 14, 15, 16 Both these measures show responsiveness in trials of TNF inhibitors and are feasible in clinical practice. A more quantitative scoring system, the Leeds dactylitis index (LDI), is also available for use in clinical trials.¹⁷ This scoring system uses a

Axial disease

Just as many measures of peripheral joint involvement were borrowed from RA, potential measures for axial disease activity are typically borrowed from AS. For many years, the Bath AS disease activity index (BASDAI) was the predominant and in some cases the only instrument to assess axial disease.¹⁸ This 6-item patient questionnaire has been tested in multiple axial PsA cohorts and correlates well with other measures of disease activity. Based on these findings, some groups recommend its use in

Patient-reported outcomes: quality of life, function, fatigue, impact of disease

Both function and quality of life are included in the core set for PsA. Measurements of the impact of the disease on these aspects of a patient’s life are commonly reported in interventional trials and cohort studies. The most common measure of physical function is the HAQ, a generic measure used across many forms of arthritis.²⁷ It consists of 20 items measuring impairment of function in 8 domains, and each task is rated from 0 (no difficulty) to 3 (unable to do). The final score ranges from 0

Composite measures

The easiest global measure of PsA is a global disease activity VAS. This measure can be completed by both physician and patient and aim to encapsulate a global picture of psoriatic disease. However, there has historically been a variety of ways to phrase the question given to the patient, leading to a GRAPPA project to standardize this. The concern was that the patient may not be clear on whether they are being asked about their arthritis only, all musculoskeletal symptoms, or the entire

Future considerations/summary

Over the last decade, the evaluation of clinical response in PsA was transformed by the rapid development and validation of specific outcome measures. Tools required to dramatically advance clinical research in this disease are now available. The emergence of new composite measures and definitive treatment targets lay the groundwork for future trials that focus on targeted strategies that hold great promise in PsA based on the experience in RA and the recent Tight Control of Psoriatic Arthritis