Blood tests can help diagnose psoriatic arthritis (PsA) and rule out other possible causes of joint pain and stiffness, such as rheumatoid arthritis (RA). Examples include tests for c-reactive protein (CRP) and rheumatoid factor.
Doctors do not rely solely on blood tests to diagnose PsA. This is because the following blood tests do not specifically check for PsA. Instead, they can check for inflammation and other markers of autoimmune diseases.
Some blood tests can help rule out the presence of other inflammatory arthritis diseases, such as rheumatoid arthritis (RA).
This article outlines the blood tests a doctor may order to help diagnose PsA.
Erythrocyte sedimentation rate (ESR) tests the blood for signs of inflammation.
ESR tests how quickly certain red blood cells, known as erythrocytes, fall to the bottom of a test tube. A high sedimentation rate indicates that inflammation is present.
The test is nonspecific, which means that it cannot measure the activity of PsA or rule out its diagnosis. This is because about 60% of people with PsA have normal test results.
However, it can help determine if a person has an autoimmune condition.
The liver releases c-reactive protein (CRP) into the blood when inflammation is present in the body. A CRP test looks for the presence of the protein in the blood.
Similarly to an ESR blood test, CRP testing cannot specifically look for PsA since about 60% of people with PsA do not have elevated levels of CRP. However, the presence of CRP can help a doctor determine if a person has an autoimmune condition.
A doctor may order this test to help diagnose PsA, but not to monitor the condition.
An anti-cyclic citrullinated peptide test (anti-CCP) checks for the presence of proteins produced by the immune system that attacks healthy cells.
A doctor may order this test to rule out RA.
HLA-B27 is a type of protein that is present on white blood cells. Its purpose is to help the immune system determine what substances in the body may be harmful, and what substances it needs to attack.
A doctor may order the test to help check for the presence of the protein during diagnosis. An abnormal result can indicate the presence of an autoimmune disorder.
Like the other blood tests, it cannot fully confirm a person has PsA. Despite this, a doctor may repeat this test during routine monitoring to check the progression of the condition.
Rheumatoid factor is a type of protein that the immune system releases. The protein attacks healthy cells and is present in people with RA not PsA.
People with PsA do not have this protein, but people with RA often do. As a result, a doctor may use this test to rule out RA as a possible cause of a person’s symptoms.
Abnormal test results can indicate the presence of inflammation in the body. They are not specific to PsA, which means that a doctor will need to perform additional testing to determine the cause of a person’s joint pain.
In some cases, blood tests can help to rule out RA. These tests include the rheumatoid factor and anti-CCP test.
Other tests may not accurately reflect the presence of PsA. A person with PsA may have normal results from ESR and CRP testing.
In addition to blood tests, a person’s doctor will likely need to:
- Examine the joints for range of motion and pain levels.
- Check for the presence of psoriasis, which can look like scaly patches on the skin.
- Order imaging tests which can include x-rays, MRI, CT-scans, and ultrasounds.
- Order a biopsy to help rule out other skin conditions.
About 30% of people with psoriasis develop PsA. Its presence can help a doctor determine if the joint pain is related to PsA or another type of arthritis.
Before the blood tests, people should speak with a doctor to:
- find out if they need to fast
- review any medications a person may be taking
- ask any questions they may have about the tests
A person should not discontinue medications unless the doctor instructs otherwise.
Positive results do not necessarily mean a person has PsA, but negative results do not necessarily mean a person does not have the condition either.
A doctor will likely use the tests to help rule out other conditions, like RA, that can present similarly to PsA.
Some other symptoms a doctor may look for to help confirm a diagnosis include:
- swollen fingers and toes
- pain, tenderness, and swelling in tendons and ligaments
- tiredness and stiffness in the morning
- generalized fatigue
- changes in nails, such as pitting or separation from the nail bed
- pain, stiffness, swelling, throbbing, and tenderness in one or more joints
- reduced range of motion
- redness and pain eyes
When describing their symptoms, a person should try to be as specific as possible. This can include describing the level and extent of pain in a joint and describing any other symptoms that may be occurring.
People should also let the doctor know if they have a rash or already have a diagnosis of psoriasis. This is because approximately 85% of cases of PsA occur after skin involvement.
To help diagnose PsA, a doctor may order the following blood tests:
- rheumatoid factor
Blood tests for PsA are not specific. This means that they can help rule out other conditions or show the presence of an autoimmune disorder.
To determine if a person has PsA, a doctor will need to perform other diagnostic techniques, such as a physical exam or imaging tests.
Before undergoing blood tests, people should speak with the doctor to determine what they might need to do to prepare.