ASO titer negative, streptozyme positive with appropriate controls—what can you conclude from these test results?

• A. The ASO is falsely negative.
• B. The patient has an antibody to a streptococcal exoenzyme other than streptolysin O.
• C. The patient has not had a previous streptococcal infection.
• D. The patient has scarlet fever.

Answer: (B)

Streptococcal serology looks at antibodies to different bacterial antigens. The ASO test measures antibodies to streptolysin O, while the streptozyme panel detects antibodies to several other streptococcal exoenzymes (like DNase B, streptokinase, hyaluronidase, etc.). If the ASO result is negative but the streptozyme test is positive, it means the patient has mounted antibodies to a streptococcal exoenzyme other than streptolysin O, indicating past exposure to Streptococcus pyogenes with an immune response to those other antigens. This pattern does not diagnose scarlet fever and can occur even when ASO is not elevated at that time.