There is no single test to diagnose psoriatic arthritis (PsA). Reaching a diagnosis of PsA is a process that includes a physical exam, patient history, imaging scans, biopsies, and blood tests. PsA can look similar to other inflammatory conditions, so doctors will often use blood tests to get a closer look at what might be causing your symptoms.1
Some symptoms of PsA look very similar to rheumatoid arthritis (RA). Your doctor may test for antibodies to cyclic citrullinated peptides (anti-CCPs), also known as rheumatoid factor. The presence of rheumatoid factor in the blood means that the patient’s immune system is attacking the body, which can lead to inflammatory symptoms that are common in RA.2,3 Still, patients with PsA may also test positive for anti-CCP.4
C-reactive protein and erythrocyte sedimentation rate
Your doctor may also test your blood to measure the level of c-reactive protein (CRP) or erythrocyte sedimentation rate (ESR)—the higher the level of CRP or ESR, the more inflammation in the body. CRP and ESR are only higher in about half of patients with PsA. These tests are also useful to predict the progression of disease.2,3
Another test used to mainly rule out another condition similar to PsA is a uric acid test. Uric acid tests are often used to diagnose gout. Patients with gout typically have a build-up of uric acid in the joints and surrounding tissues, which can form crystals that cause swelling and pain. Some doctors may continue to repeat uric acid tests after you have been diagnosed with PsA. There is some evidence that there could be a connection between gout and PsA when it comes to uric acid. Studies have shown that the inflammation experienced in PsA can cause uric acid to build up, which can increase the risk of gout down the road.5
Liver enzyme levels
Certain treatments available for PsA also require ongoing blood test monitoring. Your doctor may prescribe a disease-modifying antirheumatic drug (DMARD) to treat PsA. DMARDs work by blocking chemicals that cause inflammation in the body, therefore preventing joint damage and reducing pain and discomfort. DMARDs include medications such as methotrexate, sulfasalazine, cyclosporine, and leflunomide, as well as a newer class of DMARDs known as biologics. DMARDs can increase your risk for liver damage, so your doctor may choose to monitor your liver enzyme levels if you are prescribed any of these medications.6,7
Biologics blood monitoring
While biologic DMARDs can be very effective at slowing joint damage progression and treating the symptoms of PsA, they also suppress the immune system, which can increase the likelihood of infection. If you are prescribed a biologic to treat your PsA, in addition to liver enzyme testing, your doctor will likely monitor you for tuberculosis and take frequent complete blood count (CBC) panels to assess your risk of infection. Depending on which biologic you are prescribed, additional monitoring may be required.8
What if your bloodwork is normal?
Often patients with PsA will have normal CRP and ESR levels. Similarly, while anti-CCP is typically an RA indicator, anti-CCP can be both negative or positive in PsA. While bloodwork is certainly part of the diagnostic process, patient history and a physical examination are the most important factors to correctly diagnose PsA.4,9
National Psoriasis Foundation. About psoriatic arthritis. https://www.psoriasis.org/about-psoriatic-arthritis. Accessed May 10, 2018.
Punzi L, Podswiadek M, Oliviero F, et al. Laboratory findings in psoriatic arthritis. Reumatismo.2007;59(Suppl 1):52-5.
Rheumatoid Arthritis Support Network. Anti-CCP Test (Anti-cyclic Citrullinated Peptide). https://www.rheumatoidarthritis.org/ra/diagnosis/anti-ccp/. Accessed May 10, 2018.
Inanc, N., Dalkilic, E., Kamali, S. et al. Clin Rheumatol (2007) 26: 17. https://link.springer.com/article/10.1007%2Fs10067-006-0214-5. Accessed May 10, 2018.
Arthritis Foundation. Psoriasis and psoriatic arthritis linked to increased gout risk. https://www.arthritis.org/about-arthritis/types/psoriatic-arthritis/articles/psoriatic-arthritis-increases-gout-risk.php. Accessed May 8, 1018.
Arthritis Foundation. Psoriatic arthritis treatment. https://www.arthritis.org/about-arthritis/types/psoriatic-arthritis/treatment.php. Accessed May 10, 2018.
Curtis JR, Beukelman T, Onofrei A, et al. Elevated liver enzyme tests among patients with rheumatoid arthritis or psoriatic arthritis treated with methotrexate and/or leflunomide. Ann Rheum Dis. 2910;69(1):43-7.
Arthritis Foundation. Biologics: Monitoring. https://www.arthritis.org/living-with-arthritis/treatments/medication/drug-types/biologics/tests.php. Accessed May 10, 2018.
Birtane M, Yavuz S, Tastkkin N. Laboratory evaluation in rheumatic diseases. World J Methodol. 2017;7(1):1-8.