Glossary of lupus blood tests

This test is used to detect antinuclear antibodies in your blood. ANA antibodies attack healthy cells, mistaking them for foreign invaders. If you have a positive ANA test, it may mean you have an autoimmune disease. Autoimmune diseases cause your immune system to attack your healthy cells, tissues, or organs. Most people with lupus have ANA antibodies in their blood. But a positive ANA test doesn’t automatically mean you have lupus. The test can be positive in people with other autoimmune illnesses and healthy people. 

These antibodies attack the DNA, the genetic material inside the cell nucleus. Anti-dsDNA antibodies are found in half of the people with lupus, but lupus can still be present even if these antibodies are not detected.

Histone is a protein that surrounds the DNA molecule. These antibodies are sometimes found in people with systemic lupus erythematosus, but are more often seen in people with drug-induced lupus. This form of lupus is caused by certain medications, and usually goes away after the medication is stopped.

These antibodies can cause narrowing of blood vessels, leading to blood clots in the legs or lungs, stroke, heart attack or miscarriage. The most commonly measured aPLs are lupus anticoagulant, anticardiolipin antibody, and anti-beta2 glycoprotein I. Nearly 30 percent of people with lupus will test positive for antiphospholipid antibodies. Phospholipids found in lupus are also found in syphilis, and the blood test cannot always tell the difference between the two diseases. A positive result to a syphilis test does not mean that you have or have ever had syphilis. Approximately 20 percent of people with lupus will have a false-positive syphilis test result.

These antibodies target ribonucleoproteins, which help control chemical activities of the cells. Anti-RNPs are present in many autoimmune conditions and will be at very high levels in people whose symptoms combine features of several diseases, including lupus.

These antibodies are often found in people with Sjögren’s syndrome. Anti-Ro antibodies in particular will be found in people with a form of cutaneous (skin) lupus that causes a sun-sensitive rash. It is especially important for your doctor to look for the Ro and La antibodies if you are pregnant, as both autoantibodies can cross the placenta and cause neonatal lupus in the infant.

Neonatal lupus is rare and not usually dangerous, but it can be serious in some cases.

These antibodies target Sm proteins in the cell nucleus. Found in 30-40 percent of people with lupus, the presence of this antibody almost always means that you have lupus.

Complements are a group of proteins that protect the body against infections. They work by strengthening the body’s immune reactions. Complement proteins are used up by the inflammation lupus causes, which is why people with inflammation due to active lupus often have low complement levels. There are nine protein groups of complement, so complement is identified by the letter C and the numbers 1 through 9. The most common complement tests for lupus are CH50, C3 and C4. CH50 measures the overall function of complement in the blood. Low levels of C3 or C4 may indicate active lupus. A new combination blood test is using a subset of the C4, called C4d, to help physicians “rule in” lupus and “rule out” other diseases and conditions.

C-reactive protein is produced by the liver. High levels of CRP in your blood may mean you have inflammation due to lupus.

Erythrocyte sedimentation rate is another test for inflammation. It measures the amount of a protein that makes the red blood cells clump together. This “sed” rate is usually high in people with active lupus, but can also be high due to other reasons such as an infection.

This test measures blood clotting and can show whether you may be at risk for not clotting quickly enough at the site of a wound.

This test also measures how long it takes your blood to begin to clot.

These are three other, more sensitive blood-clotting time tests.