Laboratory Tests for Lupus
Many different laboratory tests are used to detect physical changes or conditions in your body that can occur with lupus. Each test result adds more information to the picture your doctor is forming of your illness.
Routine Blood Tests
Usually your doctor will first request a complete blood count (CBC). Your blood is made up of red blood cells, white blood cells, platelets, and serum. The complete blood count measures the levels of each. In cases of lupus, these blood tests may reveal low numbers.
- Red blood cells carry oxygen to all parts of the body.
- White blood cells (lymphocytes and others) help the immune system to protect the body against foreign invaders. The white blood cells called T cells and macrophages are directly involved in this protection. Other white blood cells called B cells are indirectly involved, by producing antibodies to the foreign substances.
- Platelets form in bone marrow; they go to the site of a wound to begin the blood-clotting process.
- Blood serum is the fluid portion of whole blood from which certain substances in the clotting of blood have been removed.
Because your body’s waste is processed by the kidneys, testing a sample of urine (called a 'spot urine' test) can reveal any problems with the way your kidneys are functioning. Lupus can attack the kidneys without any warning signs, so these tests are very important. The most common urine tests look for cell casts (bits of cells that normally would be removed when your blood is filtered through your kidneys), and proteinuria (protein being spilled into your body because your kidneys are not filtering the waste properly). A collection of your urine over a 24-hour period can also give important information.
The antibodies your body makes against its own normal cells and tissues play a large role in lupus. Many of these antibodies are found in a panel, or group, of tests that are ordered at the same time. The test you will hear most about is called the ANA test. This is not a specific test for lupus, however.
Antinuclear antibodies (ANA) are antibodies that connect, or bind, to the nucleus -- the "command center" -- of the cell. This process damages, and can destroy, the cells. The ANA blood test is a sensitive test for lupus, since these antibodies are found in 97 percent of people with the disease. When three or more typical features of lupus are present -- such as involvement of the skin, joints, kidneys, lungs, heart, blood, or nervous system -- a positive ANA test will confirm a diagnosis of lupus. However, a positive ANA test result does not always mean you have lupus. The ANA can be positive in people with other illnesses, or positive in people with no illness. The ANA can also change from positive to negative, or negative to positive, in the same person. Still, lupus is usually the diagnosis when these antinuclear antibodies are found in your blood.
In addition to the ANA, doctors trying to diagnose lupus often look for the following specific antibodies.
Antibodies to double-stranded DNA (anti-dsDNA) are antibodies that 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.
Antibodies to histone -- a protein that surrounds the DNA molecule -- are sometimes found in people with systemic lupus 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.
Antibodies to phospholipids (aPLs) 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 ever had syphilis. Approximately 20 percent of those with lupus will have a false-positive syphilis test result.
Antibodies to Ro/SS-A and La/SS-B (Ro and La are the names of proteins in the cell nucleus) 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 which causes a rash that is very sun-sensitive. 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 can cause neonatal lupus in the infant. Neonatal lupus is rare and not usually dangerous, but it can be serious in some cases.
Antibodies to Sm 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.
Antibodies to RNP target ribonucleoproteins, which help to control chemical activities of the cells. Anti-RNPs are found in many autoimmune conditions and will be at very high levels in people whose symptoms combine features of several diseases, including lupus.
Other Blood Tests
Some blood tests measure levels of proteins that are not antibodies. The levels of these proteins can alert your doctor that there is inflammation somewhere in your body.
Complement is the name of 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 caused by lupus, 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 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.
C-reactive protein (CRP) is a protein produced by the liver, and high levels of CRP in your blood may mean you have inflammation due to lupus.
Erythrocyte sedimentation rate (ESR or "sed" rate) is another test for inflammation. It measures the amount of a protein that makes the red blood cells clump together. The sed rate is usually high in people with active lupus, but it can also be high due to other reasons, such as an infection.
Blood Clotting Time Tests
The rate at which your blood begins to clot is important: if it clots too easily, a blood clot, called a thrombus, can break free and travel through the body. Blood clots can cause damage such as a stroke or miscarriage. If your blood does not clot quickly enough, you could be at risk for excessive bleeding if you are injured.
Prothrombin time (PT) measures blood clotting and can show whether you may be at risk for not clotting quickly enough at the site of a wound.
Partial thromboplastin time (PTT) also measures how long it takes for your blood to begin to clot.
Modified Russell viper venom time (RVVT), platelet neutralization procedure (PNP), and kaolin clotting time (KCT) are other, more sensitive blood-clotting time tests.
A biopsy procedure involves removal of a small bit of tissue which the doctor then examines under a microscope. Almost any tissue can be biopsied.
- The skin and kidney are the most common sites biopsied in someone who may have lupus.
- The results of the biopsy can show the amount of inflammation and any damage being done to the tissue.
- Further tests can be performed on the tissue sample to look for autoimmune antibodies and to determine whether the problem is due to lupus or is caused by some other factor, such as infection or medication.
Frequently Asked Questions
Why is systemic lupus so difficult to diagnosis?
It is difficult for a number of reasons:
- Systemic lupus is a multi-system disease, and before a multi-system disease can be diagnosed, there have to be symptoms in many parts of the body and lab work that supports the presence of a multi-system disease.
- Systemic lupus is also difficult to diagnose because it is a disease that does not typically develop rapidly, but rather develops slowly and evolves over time. Symptoms come and go and it generally takes time to gradually accumulate enough symptoms to indicate that a multi-system disease is present.
- Systemic lupus is known as a Great Imitator because it mimics so many other diseases and conditions.
- Systemic lupus is difficult to diagnose because there is no single diagnostic test for lupus. In fact, many people may have positive lupus tests-particularly the anti-nuclear antibody test-and yet NOT have the disease.
I was told my ANA was negative, and I don't have lupus. Is it possible to have lupus with a negative ANA?
Approximately 97% of people with systemic lupus have a positive ANA. Only a small percentage have a negative ANA, and many of those have other antibodies detected in their blood (antiphospholipid antibodies, anti-Ro, anti-SSA). Sometimes the ANA will convert from positive to negative following administration of steroids, cytotoxic medications or uremia (kidney failure).
How can an ANA come back positive one time and negative the next? The doctor said I do not have lupus because of this.
In general the diagnosis of lupus is based off of a combination of physical symptoms and laboratory results. It is usually a diagnosis that evolves over time either towards more certainty or to the conclusion that the person does not have lupus. People can have bouts of autoimmunity with in elevated ANA result that are brief (sometimes associated with a viral infection) and then after it goes away and does not come back for a while it is reasonable to conclude that the person is unlikely to have systemic lupus.
Some things to know about diagnosing lupus:
- First and foremost, lupus cannot be diagnosed solely on lab work.
- Secondly, positive lab tests for lupus can come and go over time and that is quite common. When they come and go it is less likely that the person will end up with a diagnosis of systemic lupus although that is still possible.
- Thirdly, it is very common to get somewhat different results at different labs. However if a person has a very strong case of lupus, it is likely that the ANA will be positive at most laboratories most of the time and that other autoantibodies will be present as well. 97% of those with lupus have a positive ANA test.
Antibodies to double-stranded DNA (anti-dsDNA) are antibodies that attack the DNA—the genetic material—inside the cell nucleus. Anti-dsDNA antibodies are found in half of the people with lupus and antibodies to the Sm antigen are found in about 30–40 percent of people with lupus, but lupus can still be present even if these antibodies are not detected. Erythrocyte sedimentation rate (ESR or "sed" rate) is another test for inflammation. It measures the amount of a protein that makes the red blood cells clump together. The sed rate is usually high in people with active lupus, but it can also be high due to other reasons, such as an infection.
Please remember that nothing is set in stone. If you continue to have worrisome symptoms, seek further medical evaluation so that a determination can be made as to what may be causing your health issues.