Skip to content

Fluorescent Immunoassay for Antistreptolysin O Antibodies (ASO) in Serum

Published: August 9, 2026Updated: August 16, 2026

This test measures the presence of antibodies your immune system produces in response to certain bacterial infections, specifically Group A Streptococcus (strep throat or scarlet fever).

Quick summary

ASO helps diagnose past infections and assesses immune response to strep bacteria.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
50700-4
Short code
FLUAV ADM resis RNA Islt Ql NAA+probe
Synonyms
FLUAV ADM resis RNA Islt Ql NAA+probe
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
probe-amp-tar
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why Is This Test Done?

Your healthcare provider may order this test if you’ve had symptoms like sore throat, fever, or rash that could indicate a strep infection, or if you’ve been exposed to Group A Streptococcus. It can also help determine if you’ve recovered fully from such an infection.

How Is the Test Performed?

A blood sample is drawn from a vein in your arm. The blood is then processed to separate the serum, which is tested using a fluorescent immunoassay to detect ASO antibodies.

What Do the Results Mean?

Elevated levels of ASO antibodies suggest a recent or ongoing Group A Streptococcus infection. However, results alone don’t confirm the diagnosis—your provider will consider symptoms, physical exam, and other tests.

When Might Results Be Normal?

If you’ve fully recovered from a strep infection, ASO levels may return to normal within weeks or months. In some cases, they can remain elevated for years, even after the infection has cleared.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

ASO testing is primarily used to diagnose or confirm a history of Group A Streptococcus infections, such as strep throat or scarlet fever. While it doesn’t replace other diagnostic tools like rapid strep tests, it can help monitor immune response or guide treatment decisions in certain cases. Persistently high levels may indicate recurrent infections or autoimmune conditions, but further evaluation is needed.

Short name

ASO Antibody Test

Frequently asked questions

Could ASO levels be affected by other infections or conditions?
Yes, ASO levels can sometimes be elevated with other bacterial infections (like pneumonia or skin infections) or viral illnesses (like measles or rubella). However, they are most specific to Group A Streptococcus.
What if my ASO test is positive but I don’t have symptoms of strep?
A positive ASO test suggests past exposure to Group A Streptococcus, but symptoms may not always appear. Your provider may recommend further testing or monitoring if needed, especially if you have concerns about complications like rheumatic fever.
How long does it take to get results from this test?
Results are typically available within 24 to 48 hours, depending on lab processing times. Your healthcare provider will contact you once they’re ready to discuss the findings.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy