Psoriatic arthritis (PsA) is a chronic inflammatory disease that affects the joints, often occurring in people who already have the skin condition psoriasis. PsA is a type of spondyloarthritis, involving inflammation in the joints and the entheses, where tendons and ligaments connect to bone. Diagnosing PsA is often complex because its symptoms frequently overlap with those of other inflammatory joint conditions. Understanding the role of laboratory tests is important for patients seeking clarity during this diagnostic process.
The Search for a Definitive Blood Test
No single blood test exists that can definitively confirm a diagnosis of psoriatic arthritis. Unlike some other autoimmune conditions, PsA currently lacks a distinct serological biomarker, meaning the diagnosis relies heavily on clinical observation and a process of elimination. The condition is considered “seronegative” because blood tests designed to identify specific antibodies often come back negative. Consequently, a PsA diagnosis is largely one of exclusion, reached after ruling out other conditions like rheumatoid arthritis.
Blood Tests That Aid Diagnosis
While no single blood test confirms psoriatic arthritis, several laboratory markers are used to determine the presence of inflammation and rule out other diseases. These tests provide supportive evidence, helping the rheumatologist build a clinical picture. The results of this initial blood work are integrated with a patient’s physical symptoms and medical history.
Tests to Rule Out Other Conditions
Two common tests ordered are the Rheumatoid Factor (RF) and the Anti-Cyclic Citrullinated Peptide (Anti-CCP) antibody tests. RF is an autoantibody frequently found in the blood of people with rheumatoid arthritis (RA), a condition PsA can closely mimic. A negative RF result is typical for a PsA patient and helps differentiate the two conditions. Similarly, Anti-CCP antibodies are highly specific for RA, and their absence further supports a diagnosis that is not RA. Testing for both RF and Anti-CCP is therefore used as a crucial step to exclude rheumatoid arthritis, especially when the PsA presentation is polyarticular, involving many joints.
Tests to Measure Inflammation
To measure systemic inflammation, clinicians often order the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP) tests. The ESR measures how quickly red blood cells settle, while CRP is a protein released by the liver in response to inflammation. Although elevated levels indicate systemic inflammation, they are non-specific and can be raised by various conditions, including infection or trauma. Furthermore, up to 60% of people with active PsA may have normal or only mildly elevated ESR and CRP levels, making these tests supportive rather than diagnostic. Their main utility is to gauge the severity of the inflammatory process and monitor treatment effectiveness over time.
Genetic Markers
Genetic testing for the HLA-B27 allele is sometimes performed, especially if the patient shows signs of spinal involvement (axial PsA). This gene is associated with spondyloarthropathies, including PsA and ankylosing spondylitis. However, the presence of HLA-B27 is neither necessary nor sufficient for a PsA diagnosis. Many people with the gene never develop the condition, and many people with PsA do not carry the gene, meaning the clinical presentation remains far more important than the genetic result.
How Psoriatic Arthritis is Clinically Diagnosed
Since blood work is often inconclusive, PsA diagnosis relies on a thorough clinical assessment by a rheumatologist, combining a detailed patient history and physical examination. The clinician looks for specific symptoms characteristic of PsA but less common in other forms of arthritis. These include dactylitis (sausage-like swelling of an entire digit) and enthesitis (inflammation where tendons or ligaments attach to bone, often at the heel). The physical exam also focuses on skin and nail changes, such as pitting or thickening of the nail. A detailed medical history includes asking about a personal or family history of psoriasis, as the condition often follows the skin disease.
CASPAR Criteria
To standardize the diagnosis, rheumatologists use the Classification Criteria for Psoriatic Arthritis (CASPAR), a validated, point-based system. A patient must have an established inflammatory joint, spine, or entheseal disease and score at least three points from seven possible features to meet the criteria.
The components considered include:
- Evidence of current psoriasis (worth 2 points).
- A history of dactylitis.
- Characteristic psoriatic nail changes.
- The presence of juxta-articular new bone formation on X-ray.
- A negative Rheumatoid Factor test (worth 1 point).
This comprehensive approach integrates blood test results into the clinical criteria, ensuring multiple aspects of the disease are considered.
Imaging Studies
Imaging studies are a primary pillar of the diagnostic process, providing objective evidence of joint and tissue changes. Standard X-rays identify signs of bone damage typical of PsA, such as new bone formation near joints, which distinguishes it from the bone loss seen in RA. X-rays can also reveal the “pencil-in-cup” deformity in severe cases. More sensitive imaging, such as magnetic resonance imaging (MRI) and ultrasound, can detect soft-tissue inflammation not visible on X-ray. Ultrasound is highly effective at visualizing early enthesitis and synovitis (inflammation of the joint lining), providing necessary structural and inflammatory detail to confirm the diagnosis.