Laboratory Tests Used to Diagnose and Evaluate SLE

Lupus is characterized by abnormalities in many laboratory test results. These abnormalities are different for every patient and vary significantly during the course of a patient's disease. The serial evaluation of an individual's tests along with the physician's observations and the patient's history determine the diagnosis of SLE, its course, and the treatment regimen. All laboratory values must be interpreted in light of the patient's present status, other correlating laboratory test results, and coexisting illnesses. This article briefly describes the major tests used to diagnose and evaluate SLE and provides information on their rationale and clinical usefulness. Nurses and other health professionals should consult rheumatologists, manuals of laboratory and diagnostic tests, or hospital clinical laboratory departments for further information on possible interpretations of results from these tests and their implications for SLE.

Tests for Blood Cell Abnormalities

Anemia

Tests for anemia include those for hemoglobin, hematocrit, and red blood cell (RBC) count. In addition, the levels of iron, total iron-binding capacity, and ferritin may be tested. At any time during the course of the disease, about 40% of patients with SLE will be anemic. The anemia may be caused by iron deficiency, GI bleeding, medications, or autoantibody formation to RBCs. When first diagnosed, about 50% of patients have a form of anemia in which the concentration of hemoglobin and the size of the RBCs are normal. This is called normochromic-normocytic anemia, or "anemia of chronic disease." Autoimmune hemolytic anemia, with a positive Coombs test, is much less common.

Leukopenia and Thrombocytopenia

Abnormalities in the white blood cell (WBC) and platelet counts are an important indicator of SLE. Leukopenia, a decrease in the number of WBCs, is very common in active SLE and is found in 15-20% of patients. Thrombocytopenia, or a low platelet count, occurs in 25-35% of patients with SLE.

Measurements of Autoimmunity

The presence of certain autoantibodies have diagnostic value for SLE. The most specific tests are those that detect high levels of these autoantibodies. The most common and specific tests for autoantibodies and other elements of the immune system are listed first.

Antinuclear Antibody (ANA)

A screening test for ANA is standard in assessing SLE because it is positive in close to 100% of patients with active SLE. However, it is also positive in 95% of patients with mixed connective tissue disease, in more than 90% of patients with systemic sclerosis, in 70% of patients with primary Sjögren's syndrome, in 40-50% of patients with rheumatoid arthritis, and in 5-10% of patients with no systemic rheumatic disease. Patients with SLE tend to have high titers of ANA. False-positive results are found during chronic infectious diseases, such as subacute bacterial endocarditis, tuberculosis, hepatitis, and malaria. The sensitivity and specificity of ANA determinations depend on the technique used.