In diagnosing systemic lupus erythematosus (SLE), which laboratory test is essential to confirm the disease?

Antinuclear antibody (ANA) testing is essential in diagnosing systemic lupus erythematosus (SLE) because it is one of the hallmarks of autoimmune disorders and is commonly found in individuals with SLE. The presence of ANAs indicates an autoimmune response where the body's immune system produces antibodies against its own nuclear components. While a positive ANA test is not exclusive to lupus, it is often the first step in the diagnostic process, as a significant majority of patients with SLE will test positive for these antibodies.

In contrast, the other tests have different roles in the examination of SLE. The anti-ssDNA antibody test can be used to support the diagnosis, but it is less commonly performed and not considered essential compared to ANA testing. A complete blood count can identify associated cytopenias often seen in SLE but does not confirm the disease itself. Complement levels can provide insight into disease activity, particularly in cases of active lupus, but cannot be relied upon alone to confirm a diagnosis. Thus, ANA testing serves as the critical initial laboratory test in the diagnostic workflow for SLE.