Antistreptolysin O Ab (ASO)

What This Test Measures

The ASO (Antistreptolysin O) test measures antibodies produced by the immune system in response to Group A Streptococcus (strep) infection. These antibodies target streptolysin O, a toxin released by the bacteria.

Why This Test Is Important

This test helps determine whether the body has mounted a recent immune response to strep, even when the infection is no longer detectable on a throat swab.

This is especially important in patients with:

• Sudden onset anxiety, OCD, or behavioral changes
• Suspected PANDAS/PANS
• Persistent symptoms following a recent illness
• Recurrent or unclear strep exposure

Clinical Relevance

An elevated ASO level supports the presence of a systemic immune response to strep, which may contribute to inflammation affecting the brain, nervous system, or other tissues.

This test is often paired with:

• Anti-DNase B (to improve detection accuracy)
• Throat culture (to assess current infection)

How It Works

After a strep infection, ASO antibodies:

• Begin rising within 1–3 weeks
• Peak around 3–5 weeks
• May remain elevated for several months

Because of this, the ASO test is useful for identifying recent past infections, not just active ones.

Who Should Consider This Test

This test may be appropriate if there are:

• Behavioral or neurological changes following illness
• Chronic or recurring symptoms without clear cause
• Suspicion of immune-triggered inflammation
• History of strep infection with ongoing symptoms