Systemic Lupus Erythematosus - Diagnosis

Tests for Autoantibodies

Methods for measuring the antibodies involved with SLE vary and the range of results can be bewildering. Repeat tests may be needed.

Antinuclear Antibodies (ANAs). A primary test for SLE checks for antinuclear antibodies (ANA), which attack the cell nucleus.

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High levels of ANA are found in more than 98% of patients with SLE. A number of other conditions, however, also cause high levels of ANA, so a positive test is not a definite diagnosis for SLE:

Antinuclear antibodies may be strongly present in other autoimmune diseases (such as scleroderma, Sjgren's syndrome, or rheumatoid arthritis).
They also may be weakly present in about 20 -40% of healthy women.
Some drugs can also produce positive antibody tests, including hydralazine, procainamide, isoniazid, and chlorpromazine.

A negative ANA test makes a diagnosis of SLE unlikely but not impossible. High or low concentrations of ANA also do not necessarily indicate the severity of the disease, since antibodies tend to come and go in patients with SLE.

In general, the ANA test is considered a screening test:

If SLE symptoms are present and the ANA test is positive, other tests for SLE will be administered.
If SLE symptoms are not present and the test is positive, the doctor will look for other causes, or the results will be ignored if the patient is feeling healthy.

ANA Subtypes. In some cases, doctors may test for specific ANA subtypes.

Anti-double stranded DNA (Anti-ds DNA) is usually found only in patients with SLE. It may play an important role in injury to blood vessels found in SLE and high levels often indicate kidney involvement. Anti-ds DNA levels tend to fluctuate over time and may even disappear.
Anti-Sm antibodies are also usually found only with SLE. They are more constant and are more likely to be detected in African-American patients.
When the ANA is negative but the diagnosis is still strongly suspected a test for anti-Ro anti-La antibodies may identify patients with a rare condition called ANA negative, Ro lupus.

Antibodies to SR Proteins. An advance in diagnosing SLE has been the detection of antibodies to molecules called SR proteins, which are carried by most patients. The test accurately detects lupus in 50% to 70% of patients who test positive for these antibodies.

Antiphospholipid Antibodies. In patients with SLE in whom blood abnormalities are suspected, tests will be administered to detect the presence of the two major antiphospholipid antibodies:

The test for the lupus anticoagulant antibody measures the time it takes blood to clot. A longer than normal blood clotting time indicates a higher chance for clotting in the body and, therefore, the presence of lupus anticoagulant.
An ELISA test (enzyme-linked immunosorbent assay) is performed to detect the anticardiolipin antibody.