The ANA test is the gold-standard screening tool for autoimmune disorders. If you're experiencing unexplained fatigue, joint pain, or skin rashes, this test is the essential first step. It detects "antinuclear antibodies"—proteins that mistakenly attack your body's own tissues. A positive result is a key indicator for conditions like Lupus (SLE), Sjögren's syndrome, and scleroderma. This simple blood test helps your doctor "see" if your immune system is the cause, putting you on the fast track to a clear diagnosis.