Lab Tests for Lupus
Lupus is a difficult disease to diagnose because its symptoms can be vague. And unlike some other diseases, it cannot be diagnosed with a single lab test. However, when certain clinical criteria are met, lab tests can help confirm a diagnosis of lupus. Blood work and other tests can also help monitor the disease and show the effects of treatment.
WebMD takes a look at the uses and limitations of the tests that are commonly used to diagnose and monitor lupus.
Blood Tests for Lupus
Antinuclear Antibody (ANA)
- What it is: ANA is a type of antibody directed against the cells' nuclei.
- Why the test is used: ANA is present in nearly everybody with active lupus. Doctors often use the ANA test as a screening tool. Plus, looking at patterns of the antibodies can sometimes help doctors determine the specific disease a person has. That, in turn, helps determine which treatment would be most appropriate.
- Limitations of the test: Although almost all people with lupus have the antibody, a positive result doesn't necessarily indicate lupus. Positive results are often seen with some other diseases and in a smaller percentage of people without lupus or other autoimmune disorders. So a positive ANA by itself is not enough for a lupus diagnosis. Doctors must consider the result of this test along with other criteria.
Antiphospholipid Antibodies (APLs)
- What it is: APLs are a type of antibody directed against phospholipids.
- Why the test is used: APLs are present in up to 60% of people with lupus. Their presence can help confirm a diagnosis. A positive test is also used to help identify women with lupus that have certain risks that require preventive treatment and monitoring. Those risks include blood clots, miscarriage, or preterm birth.
- Limitations of the test: APLs may also occur in people without lupus. Their presence alone is not enough for a lupus diagnosis.
- What it is: Anti-Sm is an antibody directed against Sm, a specific protein found in the cell nucleus.
- Why the test is used: The protein is found in up to 30% of people with lupus. It's rarely found in people without lupus. So a positive test can help confirm a lupus diagnosis.
- Limitations of the test: Up to only 30% of people with lupus have a positive anti-Sm test. So relying on an anti-Sm result alone would miss a large majority of people with lupus.
- What it is: Anti-dsDNA is a protein directed against double-stranded DNA. DNA is the material that makes up the body's genetic code.
- Why the test is used: Between 75% and 90% of people with lupus has a positive anti-dsDNA test. Also, the test is very specific for lupus. Therefore, a positive test can be useful in confirming a diagnosis. For many people, the titer, or level, of the antibodies rises as the disease becomes more active. So, doctors can also use it to help measure disease activity. Also, the presence of anti-dsDNA indicates a greater risk of lupus nephritis, a kidney inflammation that occurs with lupus. So a positive test can alert doctors to the need to monitor the kidneys.
- Limitations of the test: Up to 25% of people with lupus has a negative test. So, a negative test doesn't mean a person doesn't have lupus.