Widening the PsA diagnostic net
What’s elusive is some other parameter to identify the 3% of psoriasis patients who will develop PsA during the next year – or at least a better way to find patients as soon as they have what is identifiably PsA.
The diagnostic definition for PsA that many experts now accept is actually a set of classification criteria, CASPAR (Classification Criteria for Psoriatic Arthritis) (Arthritis Rheum. 2006 Aug;54[8]:2665-73). But in actual practice, diagnosing PsA relies heavily on clinical skill, case recognition, and ruling out mimickers.
Dr. Alexis R. Ogdie
“The CASPAR criteria were developed as classification criteria to use in studies, but they are also quite helpful in diagnosing PsA. It is the agreed on definition of PsA at the moment,” said Alexis Ogdie, MD, director of the psoriatic arthritis clinic at the University of Pennsylvania.
“CASPAR fills the role at present for clinical trials, but the diagnosis remains clinical, based on history, physical findings, and supported by lab and radiology findings,” Dr. Merola said. “A clinical prediction tool and a proper biomarker would be of great value.”
“We certainly use CASPAR criteria in the clinic, but they do not include all PsA patients; sometimes we diagnose PsA in patients who don’t meet the CASPAR criteria,” Dr. Coates said.
“In practice, rheumatologists mostly use their clinical judgment: a patient with history and findings typical of PsA after ruling out other causes. But distinguishing inflammatory from noninflammatory arthritis – like osteoarthritis or fibromyalgia – can be quite difficult; that’s the main challenge,” Dr. Eder said. “Rheumatologists rely on their clinical skill and experience. PsA can be difficult to diagnose. There is no biomarker for it. PsA is complex [to diagnose], and that’s why it’s diagnosed later. A primary care physician might get a negative result on a rheumatoid factor test and think that rules out PsA, but it doesn’t.”
Dr. Eder and others suggest that ultrasound may be a useful tool for earlier diagnosis of PsA. Ultrasound is more sensitive than a physical examination and can detect inflammation in joints and entheses, she noted. Another effective method may be more systematic use of screening questionnaires, Dr. Coates said, or simply more systematic questioning of patients.
“Questionnaires are not a high priority for a primary care physician who may have only a few patients with psoriasis. Even some dermatologists may not use the questionnaires because they take time to administer and assess. But just asking a psoriasis patient whether they have joint symptoms is enough,” Dr. Eder said. All clinicians who encounter patients with psoriasis should ask about musculoskeletal symptoms and refer when appropriate to a rheumatologist, Dr. Reddy said.
The earliest indicators of PsA
Evidence supporting ultrasound’s value for early PsA diagnosis came in a report at the most recent annual meeting of the American College of Rheumatology in Chicago in October 2018. Researchers from the University of Rochester (New York) used ultrasound to examine 78 patients with psoriasis but without PsA or musculoskeletal symptoms and 25 healthy controls. They found ultrasound abnormalities in almost half of the patients with psoriasis, significantly more than in the controls (Thiele R et al. Arthritis Rheumatol. 2018;70[suppl 10]: Abstract 2904).