Diagnosing psoriatic arthritis can be challenging because there’s no single test, says Magdalena Cadet, MD, a clinical rheumatologist and adjunct assistant professor in the department of medicine at New York University’s Grossman School of Medicine in New York City. Along with a physical exam, you’ll likely need a series of both imaging procedures and blood tests in order to make a psoriatic arthritis diagnosis, as well as rule out other forms of arthritis, such as rheumatoid arthritis or gout. “Many types of arthritis are associated with inflammation, and it’s important to distinguish between them in order to initiate a treatment plan,” Dr. Cadet explains. The tests can help detect conditions associated with psoriatic arthritis, too, such as heart disease. “A physician may also closely monitor your blood pressure or cholesterol levels to assess cardiac risk factors,” Cadet says. Below are 12 key medical tests that can help detect and monitor psoriatic arthritis. “X-rays allow the doctor to see changes to the bone,” says Elyse Rubenstein, MD, a rheumatologist in Santa Monica, California. In people with psoriatic arthritis, X-rays may show bone erosion, new bone formation, bone fusion, or a phenomenon called “pencil in a cup,” in which the ends of the bone have been eroded to a pencillike point. Any of these changes indicate that the disease is getting worse, Dr. Rubenstein says. Frequency of Testing A doctor may take an initial X-ray to help diagnose psoriatic arthritis and rule out other forms of arthritis, such as rheumatoid arthritis, which have different patterns of joint involvement, says Rubenstein. After that, how often you have X-rays depends on your physician and the state of your disease. Some doctors take X-rays just once a year for routine monitoring, while others may take them only when a patient’s condition changes. During an MRI, you lie inside a machine (typically a large tube-shaped magnet) and remain very still while the device moves a strong magnetic field, then radio waves, through your body to excite protons (subatomic particles) found in the water that makes up human tissue, according to the National Institute of Biomedical Imaging and Bioengineering. The procedure is painless and, unlike X-ray imaging, does not emit radiation. A radiologist analyzes the MRI, then reports back to the rheumatologist. Inflammation, swelling, and bone erosion all indicate that psoriatic arthritis is active, notes Rubenstein. Frequency of Testing A doctor may order an MRI during initial testing to help with making a psoriatic arthritis diagnosis, as well as later to monitor the disease or look for any changes in a patient’s psoriatic arthritis symptoms. Tests may be done several times a year, says Cadet.
3. Psoriatic Arthritis Blood Test: Erythrocyte Sedimentation Rate
Erythrocyte sedimentation rate, or ESR or sed rate, is a blood test that measures inflammation in the body, which helps determine a psoriatic arthritis diagnosis, explains Elaine Husni, MD, MPH, vice chair of rheumatology and director of the Arthritis and Musculoskeletal Center at the Cleveland Clinic. The test measures how many milliliters of red blood cells settle per hour in a vial of blood. When swelling and inflammation are present, the blood’s proteins clump together and become heavier; as a result, they will fall and settle faster at the bottom of the test tube, according to Johns Hopkins Medicine. As with many blood tests, labs each have their own, slightly different reading of what ESR numbers mean, which they interpret based on past results, explains Cadet. Age is also a factor. “ESR can be elevated slightly in elderly patients and still be normal for that person,” she says. Frequency of Testing In addition to diagnosis, “Testing may be done several times a year to determine if there’s ongoing inflammation,” says Cadet. “Your doctor may use the test if your ESR is normal, since CRP is more accurate at detecting inflammation in some people,” adds Cadet. Again, different labs may have slightly different interpretations of readings. Frequency of Testing CRP analysis may be done for diagnosis and then several times a year to assess whether inflammation has responded to treatment, notes Cadet.
5. Psoriatic Arthritis Blood Test: Rheumatoid Factor
Rheumatoid factor (RF), a protein produced by the immune system that attacks healthy tissue, is an indication of systemic inflammation. Although RF is mostly associated with rheumatoid arthritis, it can also occur in a small percentage of people with psoriatic arthritis, says Rubenstein. To distinguish the two conditions, doctors will look at RF levels in the context of other factors, such as a certain pattern of joint involvement and symptoms of psoriasis, which can accompany psoriatic arthritis. Frequency of Testing This is usually done only at the initial diagnostic appointment, says Rubenstein.
6. Psoriatic Arthritis Blood Test: Anti-Cyclic Citrullinated Peptide Test
Blood tests that look for the presence of anti-cylic citrullinated peptide antibodies (anti-CCPs), which are inflammatory, are commonly used to diagnose rheumatoid arthritis, but anti-CCPs can also indicate psoriatic arthritis. Roughly 8 to 16 percent of people with psoriatic arthritis will test positive for anti-CCPs, says Rubenstein. Frequency of Testing “Some physicians will perform the test yearly,” says Cadet.
7. Psoriatic Arthritis Blood Test: HLA-B27
HLA-B27 is a blood test that looks for a genetic marker for psoriatic arthritis — a protein called human leukocyte antigen B27 (HLA-B27), which is located on the surface of white blood cells. About 20 percent of people with psoriatic arthritis are positive for HBL-B27, according to CreakyJoints. HLA-B27 is associated with a larger group of autoimmune diseases, called spondyloarthropathies, which includes psoriatic arthritis, Cadet says. These conditions can cause inflammation in the enthesis (the area where bone and tendons meet) anywhere in the body but mainly in the spine. If untreated over a long period, this inflammation may cause the destruction of cartilage, muscle spasms, and a decrease in bone mineral density that may lead to osteopenia or osteoporosis. Frequency of Testing “The HLA-B27 test is usually performed only at an initial visit to help establish a diagnosis,” says Cadet.
8. Psoriatic Arthritis Skin and Blood Tests: Tuberculosis Test
Tuberculosis (TB) is a bacterial infection that typically affects the lungs but can also reach bones, joints, and kidneys. Symptoms include fever, night sweats, chills, coughing, weight loss, and fatigue. People with psoriatic arthritis must have a negative TB test before they can take biologic medications, which are protein-based drugs given by injection or infusion. By suppressing the immune system, these medications may reactivate latent (inactive) tuberculosis. There are two kinds of TB tests: a skin test and a blood test. The skin test involves injecting a small amount of a protein called tuberculin into the skin of the lower arm, then checking the area around 48 to 72 hours later to see if there has been a reaction. The result depends on the size of the raised, hard area or swelling, according to the Centers for Disease Control and Prevention. A TB blood test assesses whether the body has launched an immune response to the presence of M. tuberculosis bacteria. The test is done in a lab after a blood sample is drawn. Frequency of Testing Doctors order a TB test before prescribing biologics and may repeat testing annually as long as a patient is taking the medication, says Cadet. She adds, “Any patient who exhibits symptoms or has been exposed to TB should have an immediate TB test.” Frequency of Testing As with the TB skin test, doctors may order a chest X-ray prior to prescribing biologics, repeating the test annually as long as the patient is taking the medication, says Cadet.
10. Psoriatic Arthritis Blood Test: Serum Uric Acid
Uric acid is a substance that forms when the body breaks down purines, which are found in human cells and many foods, according to the Arthritis Foundation. Elevated blood levels of uric acid are sometimes identified in people with psoriatic arthritis and can also be linked to gout, heart disease, and high blood pressure, according to Cadet. Frequency of Testing Testing may be done several times a year, says Cadet.
11. Psoriatic Arthritis Imaging Test: Bone Mineral Density
The most common bone mineral density test is called DXA (also abbreviated DEXA), for dual-energy X-ray absorptiometry. This test uses X-rays to measure how many grams of calcium and other bone minerals are packed into a segment of bone. The denser the bones, the stronger and healthier they are. Unfortunately, common psoriatic arthritis medications — such as prednisone, a corticosteroid — can weaken bones over time and increase the risk of osteoporosis. “And psoriatic arthritis itself is associated with a decrease in bone mineral density,” notes Rubenstein. If you’re diagnosed with osteopenia, a condition involving weakened bones that may lead to osteoporosis, your doctor will discuss medications that can slow or stop bone loss, and may recommend calcium and vitamin D supplements along with resistance exercise, says Rubenstein. Frequency of Testing “Bone density screening is done during menopause and every one to two years after that,” says Rubenstein. “If a patient is on prednisone or other medications that decrease bone mineral density, the test may be done earlier and repeated every one to two years.”
12. Psoriatic Arthritis Blood Test: Anemia
When you have psoriatic arthritis, ongoing inflammation may cause anemia, a decrease in healthy red blood cells that can lead to dizziness, shortness of breath, and exhaustion, says Cadet. By measuring your blood levels of hemoglobin (the pigmented, oxygen-carrying component of red blood cells), your doctor can determine if you have anemia. A normal reading for women is 12 to 16 grams of hemoglobin per deciliter of blood; 14 to 18 grams is normal for men, according to the Mayo Clinic. If blood work reveals anemia, your doctor will give you an exam and other blood tests to find the cause. In people with psoriatic arthritis, treatments that reduce inflammation also help with anemia, explains Cadet. Frequency of Testing Doctors may order tests to be done several times a year to see if the anemia has worsened or improved. Additional reporting by Julia Califano.
