Continuing Diagnostic Challenges
In their 1973 paper, Moll and Wright described many diagnostic problems that still challenge physicians, because no clear biomarker of the disease has yet been found. For example, they wrote, “A point that has been insufficiently stressed is the frequency with which patients are unaware that they have psoriasis and also the frequency with which minimal psoriasis in ‘hidden’ areas (scalp, natal cleft, perineum, and umbilicus) remains undetected by the clinician. Often, the only manifestation of the rash is a tiny patch either in one of these hidden areas or in one of the classical sites (back of the elbow or front of the knee).” They also noted that it is important for clinicians to realize nail involvement may be the only apparent manifestation of psoriasis.1
The pair discussed some of the challenges in distinguishing psoriatic arthritis from osteoarthritis, Reiter’s disease, osteoarthritis, traumatic arthritis and rheumatoid arthritis. The latter, in particular, remains a challenge to this day. Dr. Espinoza notes, “In patients with polyarticular involvement and disease duration longer than five to 10 years, it is often difficult to distinguish the disease from rheumatoid arthritis.”
Dr. Ritchlin acknowledges that diagnosis is sometimes fluid. “Sometimes you’ll diagnose a patient with seronegative rheumatoid arthritis, and then, over time, it becomes apparent they have psoriatic arthritis, after they develop psoriasis or other features. Right now, the diagnosis is totally based on clinical characteristics, which can be very complicated and subtle.”
Yet it is important to get the correct diagnosis as soon as possible, Dr. Ritchlin notes, because some therapies are more effective than others (see “Rheumatology Drugs at a Glance, Part 1: Psoriatic Arthritis,” The Rheumatologist, April). “The crossover is anti-TNF agents and methotrexate. But in terms of the biologics, there are some very big differences, particularly with drugs like rituximab, IL-6 receptor antagonists like [tocilizumab], which are more effective for RA, and the anti-IL-17 drugs, which work for psoriatic arthritis but not for RA.”
The hunt is on for a serum biomarker that may be used to more easily diagnose psoriatic arthritis. New biomarkers are under development, but this process is time consuming and challenging. Notes Dr. Ritchlin, “The National Psoriasis Foundation is entertaining proposals for investigators to propose how to find a biomarker for this disease, and they are giving out a lot of money to do that.”
Until such a biomarker is found, clinicians will have to continue to rely on their clinical acumen and detailed knowledge of possible disease presentations.