Acute Rheumatic Fever (ARF): This condition is characterized by a positive Antistreptolysin O (ASO) titer, which indicates a recent streptococcal infection. The negative ESR and CRP suggest that the acute phase response may not be significantly elevated, which can occur in the early stages of ARF or in cases where the inflammation is not severe.

Post-Streptococcal Glomerulonephritis (PSGN): Although PSGN typically presents with elevated inflammatory markers, some cases may have a mild or delayed inflammatory response, resulting in negative ESR and CRP. The positive ASO titer supports a recent streptococcal infection.
Streptococcal Infection without Complications: A positive ASO titer can indicate a recent streptococcal infection, such as strep throat, without any complications like ARF or PSGN. The negative ESR and CRP suggest a mild or resolving infection.

Endocarditis: Although less likely, endocarditis can present with a positive ASO titer if the infection is caused by a streptococcal species. The negative ESR and CRP do not rule out endocarditis, as the inflammatory response can be variable. Missing this diagnosis can be catastrophic.
Vasculitis (e.g., Kawasaki Disease): Some forms of vasculitis, like Kawasaki disease, can have a positive ASO titer and may not always present with elevated ESR and CRP. Although less common, missing this diagnosis can lead to serious complications.

Streptococcal Toxic Shock Syndrome (STSS): This rare but severe condition can present with a positive ASO titer. Although STSS typically causes a significant inflammatory response, some cases may have a blunted response, resulting in negative ESR and CRP.
Autoimmune Disorders (e.g., Lupus): In rare cases, autoimmune disorders can have a positive ASO titer due to cross-reacting antibodies. The negative ESR and CRP do not rule out these conditions, but they are less likely given the specific combination of test results.