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Anti-Streptol Olysin O Titer (ASOT) is a quantitative or semi-quantitative serological test (latex agglutination, turbidimetry, nephelometry, or ELISA) for detecting antibodies against streptolysin O, an exotoxin produced by Group A Streptococcus. Elevated or rising titers indicate recent streptococcal infection and are essential for diagnosing post-streptococcal sequelae including acute rheumatic fever (Jones criteria) and post-streptococcal glomerulonephritis. The test requires serum samples; acute and convalescent (2-4 weeks apart) with fourfold rise confirms recent infection. Reference range typically <200-250 Todd units/mL (adults), varies by age and population. Primary clinical applications include diagnosis of Group A streptococcal infections, acute rheumatic fever evaluation, post-streptococcal glomerulonephritis diagnosis, differentiation of acute vs. past infection, evaluation of unexplained arthritis or carditis, pediatric inflammatory conditions (PANDAS), and monitoring disease activity in rheumatic fever. Critical safety precautions include proper timing of acute and convalescent samples, awareness of false negatives/positives, clinical correlation for diagnosis, and standard biohazard precautions. Essential test for rheumatology, nephrology, cardiology, and infectious disease practice.