The Anti-Smooth Muscle Antibody (ASMA) test is a blood test detecting autoantibodies that mistakenly attack smooth muscle cells, primarily to diagnose autoimmune hepatitis (AIH), a liver disease where the immune system attacks the liver, but can also signal other autoimmune issues or infections like mono. A positive result, especially at high levels (titer > 1:40), points towards AIH but requires correlation with liver function tests, imaging, and clinical symptoms for a definitive diagnosis, as other conditions like viral hepatitis, scleroderma, or cancer can also cause ASMA production