There is no conclusive test for psoriatic arthritis, and it can therefore be a difficult condition to diagnose.
Psoriatic arthritis is an inflammatory joint disease closely associated with psoriasis, which affects the joints and tendons. ‘Inflammatory arthritis’ means that there is inflammation present in the affected joints, rather than just wear and tear. It can be difficult to diagnose, as symptoms can be similar to other types of arthritis, and a patient does not necessarily have to have psoriasis to develop it. It is also possible for people with psoriasis to develop a different type of arthritis (such as rheumatoid arthritis or osteoarthritis); the fact that they have psoriasis does not alone mean their arthritis is psoriatic.
How is it diagnosed?
At present there are no definitive guidelines for diagnosing psoriatic arthritis; a doctor will make a diagnosis based on symptoms and medical history, and by ruling out other conditions. Usually, a blood test will be carried out to test for rheumatoid factor (the antibody found in rheumatoid arthritis). This is usually negative in people with psoriatic arthritis, although a positive result can be due to causes other than rheumatoid arthritis. A doctor may also use X Rays, ultrasounds or other scans, such as an MRI to look at the patient’s joints. These scans often show inflammation or areas of new bone growth with poorly-defined edges in people with psoriatic arthritis.
Traditionally, the Moll and Wright (1973) criteria have been used to diagnose psoriatic arthritis. The criteria are: an inflammatory arthritis, the presence of psoriasis, and a blood test negative for rheumatoid factor. Although this criteria set is still used, it does have limitations, for example, psoriatic arthritis can occur without there being current psoriasis on the skin.
For more information on diagnosing psoriatic arthritis, have a look at our information sheet.