Blood Tests to Diagnose Arthritis

Blood Tests for RA and Other Autoimmune Conditions
Blood Tests to Diagnose Arthritis
Your doctor will use several blood tests to help diagnose you with rheumatoid arthritis (RA) and other inflammatory conditions.
Blood tests are usually fast. The doctor sends you to a lab where a worker puts a needle into one of your veins. They take, or "draw," blood into several test tubes. The tests take a few days, and the doctor will call you to go over the results. Common blood tests for rheumatoid arthritis include:
What it measures: Rheumatoid factor is a group of proteins your body makes when your immune system attacks healthy tissue.
Whatâs normal: 0-20 u/mL (units per milliliter of blood)
Whatâs high: 20 u/mL or higher
What it means: About 70% to 90% of people with a high reading have RA. But people without RA can still have rheumatoid factor. In general, if you have RA but donât have high RF, your disease will be less severe. RF levels may stay high even if you go into remission.
What it measures: Proteins your body makes when there is inflammation. Youâll probably have it done along with the RF test.
Whatâs normal: 20 u/mL or less
What it means: This test offers a way to catch RA in its early stages. Levels are high in people who have RA or those who are about to get it. A positive test means thereâs a 97% chance you have RA. If you have anti-CCP antibodies, your rheumatoid arthritis might be more severe.
Other conditions you might have: None. This test is used only to look for RA.
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Erythrocyte sedimentation rate (ESR)
What it measures: The speed at which your red blood cells clump and fall together to the bottom of a glass tube within an hour. Your doctor might call it a sed rate.
Whatâs normal:
Men younger than 50: 0-15 mm/h (millimeters per hour)
Men older than 50: 0-20 mm/h
Women younger than 50: 0-20 mm/h
Women older than 50: 0-30 mm/h
What it means: In healthy people, the ESR is low. Inflammation makes cells heavier, so they fall faster. Higher levels tend to happen along with active disease, though not exactly.
Other conditions you might have: A high ESR rate doesn't point to any particular disease, but it's a general sign of how much inflammation is in your body. It could be tied to disease activity if you have:
What it measures: A protein your liver makes when inflammation is present
Whatâs normal: Generally, less than 10 milligrams per liter, but results vary from person to person and from lab to lab
What it means: CRP levels often go up before you have symptoms, so this test helps doctors find the disease early. A high level suggests significant inflammation or injury in your body. Doctors also use this test after youâre diagnosed to monitor disease activity and to understand how well your treatment is working.
What it measures: This series of tests measures the presence of certain unusual antibodies in your blood.
Whatâs normal: These tests are measured in titer, a ratio for the lowest mix of a solution and a substance at which a reaction takes place. A value of 1:40 dilution (or 1 part antibodies to 40 parts solution) is negative.
If the ANA is positive, you may have an autoimmune disorder, but the test alone can't make a reliable diagnosis. If the ANA is negative, it is likely that you don't have one.
Other conditions you might have: The profile helps your doctor look for diseases such as:
It is not an abnormal finding: 8%-10% of white people may have it, though most do not have a disease.
What it means: HLA-B27 is a gene thatâs linked to a group of conditions (you might hear it called a genetic marker) known as spondyloarthropathies. They involve joints and the places where ligaments and tendons attach to your bones.
What it means: You might have an inflammatory muscle disease. Higher levels of CPK can also show up after trauma, injections into a muscle, muscle disease due to an underactive thyroid, and while taking certain medications such as cholesterol-lowering drugs called statins.
What it measures: More than 30 blood proteins that work together in your immune system during an inflammatory response. Complement proteins can get used up during this process.
Whatâs normal:
Serum CH50: 30-75 u/mL (units per milliliter)
Serum C3:
Men: 88-252 mg/dL (milligrams per deciliter)
Women: 88-206 mg/dL
Serum C4:
Men: 12-72 mg/dL
Women: 13-75 mg/dL
What it means: Lower levels of all three components may signal lupus and vasculitis, or inflamed blood vessels. If you have lupus with kidney disease, your doctor may continue to give you this test because levels rise and fall along with disease activity.
Testing for Other Autoimmune Conditions
Whatâs normal: A negative result (no antibodies in your blood), or a titer of less than 1:20
What it means: You have a form of vasculitis, or inflamed blood vessels. You may get this test after youâre diagnosed, too. It helps your doctor see how your disease is progressing, though the link to disease activity isnât perfect.
Other conditions you might have:
Granulomatosis with polyangiitis
Microscopic polyangiitis
Churg-Strauss syndrome
Show Sources
SOURCES:
American Association for Clinical Chemistry: "Rheumatoid Factor."
National Institute of Arthritis and Musculoskeletal and Skin Diseases: "How is Rheumatoid Arthritis Diagnosed?"