Rheumatic fever anti-streptolysin O is generally above the upper limit of the normal range. Since each hospital uses different testing methods, there are differences. You need to determine based on the reference range of your local hospital laboratory what is normal and what exceeds the normal range. Anti-streptolysin O, also known as anti-O, is typically abnormal in cases of rheumatic fever. Rheumatic fever is a systemic inflammatory disease caused by an infection with Group A beta-hemolytic streptococcus. Clinically, it is primarily characterized by fever, erythema marginatum, joint pain, subcutaneous nodules, and it can also include carditis. Some patients may exhibit symptoms of chorea.