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.