Psoriatic Arthritis (PsA) is a chronic inflammatory autoimmune disease that primarily affects the joints and skin, developing in up to 30% of people who have psoriasis. It causes pain, stiffness, and swelling, which can lead to progressive joint damage if not managed effectively. There is no single, definitive test that confirms a diagnosis of PsA. Instead, diagnosis relies on a comprehensive evaluation of the patient’s clinical picture.
Why a Single Diagnostic Test Does Not Exist
Psoriatic Arthritis is considered a diagnosis of exclusion, requiring the ruling out of other conditions that cause similar joint symptoms. The disease is highly heterogeneous, presenting in different patterns and affecting various parts of the body, including peripheral joints, the spine, and the sites where tendons attach to bone. This wide variation prevents the development of a single, universal biological marker. Therefore, the diagnostic approach relies on a specific pattern of symptoms and findings that differentiate PsA from other inflammatory arthritides, such as Rheumatoid Arthritis or Gout.
Clinical Criteria and Physical Examination
Establishing a Psoriatic Arthritis diagnosis begins with a detailed physical examination and patient history taken by a rheumatologist. The physician looks for specific clinical signs characteristic of PsA, which provides context for subsequent testing. Diagnosis often requires the presence of inflammatory arthritis along with features from the Classification Criteria for Psoriatic Arthritis (CASPAR).
The presence of psoriasis, current or historical, is a heavily weighted factor in the clinical assessment, often contributing two points to the CASPAR criteria.
The doctor also looks for unique physical manifestations, including:
- Dactylitis, which is the uniform, “sausage-like” swelling of an entire finger or toe due to inflammation in the joints and tendons.
- Enthesitis, which is inflammation where ligaments or tendons insert into the bone, especially around the heel or Achilles tendon.
- Psoriatic nail dystrophy, such as pitting or onycholysis (nail separation from the nail bed).
The Role of Laboratory Testing
Blood tests are utilized primarily to rule out other inflammatory conditions and confirm systemic inflammation. Tests for inflammatory markers, such as Erythrocyte Sedimentation Rate (ESR) and C-reactive protein (CRP), are frequently ordered. While elevated levels confirm active inflammation, these markers are nonspecific and can be elevated in numerous conditions. Notably, up to 40% of people with PsA may have normal ESR and CRP levels.
To distinguish PsA from Rheumatoid Arthritis (RA), the rheumatologist tests for Rheumatoid Factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) antibodies. PsA is considered “seronegative,” meaning these antibodies are absent, and a negative result supports the PsA diagnosis over RA. Joint fluid analysis may also be performed on fluid drawn from a swollen joint to check for uric acid crystals, which would suggest Gout. Genetic testing for the human leukocyte antigen B27 (HLA-B27) is sometimes utilized, as its presence is associated with an increased risk of spinal involvement, but it is not definitive for PsA.
Visual Confirmation Through Imaging Studies
Imaging studies provide visual evidence of the characteristic damage PsA inflicts on the joints and surrounding structures. Conventional X-rays look for signs of advanced disease, such as bone erosions, joint space narrowing, and new bone formation near the joints. A unique feature sometimes seen is the “pencil-in-cup” deformity, where the end of one bone fits into a cup-shaped erosion in the adjacent bone.
More sensitive imaging techniques, such as Magnetic Resonance Imaging (MRI) and ultrasound, detect early signs of the disease before they are visible on standard X-rays. MRI is valuable for assessing soft tissues and can identify active inflammation within the bone (bone marrow edema), tendons (tenosynovitis), and at the entheses. Ultrasound, often with power Doppler, can directly visualize inflammation in the lining of the joints (synovitis) and the tendon attachments, helping to stage the severity of the disease. These findings contribute to the CASPAR criteria if juxta-articular new bone formation is visible on a plain radiograph.