Which laboratory test is primarily used to diagnose systemic lupus erythematosus?

The primary laboratory test used to diagnose systemic lupus erythematosus (SLE) is the antinuclear antibody (ANA) test. This test is crucial because antibodies that target the nucleus of cells are often present in individuals with SLE. A positive ANA test is typically one of the first steps in the diagnostic process for lupus, as it can indicate the potential presence of various autoimmune disorders, including SLE. However, it is important to note that while a positive ANA test can support the diagnosis, it is not specific to lupus alone, as many healthy individuals may also test positive.

In the context of diagnosing SLE, the ANA test can be followed by more specific tests to confirm the diagnosis and assess the disease's activity. Therefore, the significance of the ANA test lies in its high sensitivity for systemic lupus erythematosus, making it a cornerstone of the diagnostic criteria.

Other tests, such as a complete blood count, liver function tests, and thyroid function tests, may provide supporting information about a patient's overall health and assist in monitoring certain complications of lupus or identifying manifestations of the disease, but they are not definitive for diagnosing SLE itself.