An Anti-Histone Antibody IgG test detects autoantibodies against histones, crucial proteins in cell nuclei, primarily to diagnose Drug-Induced Lupus (DIL), showing high positivity (around 95%) in DIL patients, distinguishing it from other autoimmune issues like systemic lupus (SLE) where they’re less common but present, and helping monitor treatment response when a suspect drug is stopped. The test uses a blood sample, often following a positive ANA, and helps pinpoint medications (like hydralazine, procainamide) causing lupus-like symptoms, with results interpreted semi-quantitatively (negative, weak, moderate, strong positive)