Laboratory Tests
Other Autoantibodies
In those individuals with a positive ANA, additional tests can be done for certain particular antibodies that may better establish a diagnosis of SLE. The knowledge of which particular antibody is responsible for the positive ANA test can sometimes be helpful in determining which autoimmune disease is present.
- Antibodies to DNA (the protein that makes up the body's genetic code) are found primarily in SLE.
- Antibodies to histones (DNA packaging proteins) are usually found in people with drug-induced lupus (DIL), but may also be found in those with SLE.
- Antibodies to the Sm antigen are found almost exclusively in lupus, and often help to confirm the diagnosis of SLE.
- Antibodies to RNP (ribonucleoprotein) are found in a number of connective tissue diseases. When present in very high levels, RNP antibodies are suggestive of mixed connective tissue disease (MCTD), a condition with symptoms like those of SLE, polymyositis, and scleroderma.
- Antibodies to Ro/SS-A are found in people with either lupus or Sjogren's syndrome, and are almost always found in babies who are born with neonatal lupus.
- Antibodies to Jo-1 are associated with polymyositis.
- Antibodies to PM-Scl are associated with certain cases of polymyositis that also have features of scleroderma.
- Antibodies to Scl-70 are found in people with a generalized form of scleroderma.
- Antibodies to the centromere (a structure involved in cell division) are found in people with a limited form of scleroderma which tends to have a chronic course.
Complement
Laboratory tests which measure complement levels in the blood may also be helpful to the physician in making a diagnosis of SLE.
- Complement is a blood protein that destroys bacteria and also influences inflammation.
- Complement proteins are identified by the letter "C" and a number.
- The most common complement tests are C3, C4, and CH50.
If the total blood complement level is low, or the C3 or C4 complement values are low and the person also has a positive ANA, some weight is added to the diagnosis of lupus. Low C3 and C4 complement levels in individuals with a positive ANA may signify the presence of active disease, especially kidney disease.
Biopsy
Sometimes examination of a tissue sample (biopsy) can be helpful in making a diagnosis. The biopsy is one of the best ways to evaluate an organ or tissue. The procedure involves removal of a small sliver of tissue, which is then examined under a microscope.
- The doctor can use the biopsy to identify the amount of inflammation and damage to the tissue.
- Further tests can be performed on the specimen to determine whether the problem is due to lupus or is caused by some other factor such as infection or medication.
- Almost any tissue can be biopsied. The most common sites biopsied in lupus are the skin and kidney.
- The results of the biopsy, like any other laboratory test, should be examined in combination with the individual's medical history and clinical findings.