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Serum Anti-Streptolysin O (ASO) Antibody, Quantitative

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

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

Quick summary

["Helps diagnose past or recent strep infections that may have caused symptoms like sore throat, fever, or rash."]

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

Key facts at a glance

LOINC
6557-3
Short code
S pyo Ag Throat Ql IF
Synonyms
S pyo Ag Throat Ql IF
Category
coagulation
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
if
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

What Is the ASO Test Used For?

The ASO test helps doctors determine if you’ve had a recent infection caused by Group A Streptococcus bacteria. These bacteria can lead to strep throat, scarlet fever, or other complications if untreated. It’s often used alongside throat cultures or rapid strep tests to confirm infection.

How Is the ASO Test Performed?

A blood sample is taken from a vein in your arm. The blood is then sent to a lab where antibodies against strep bacteria are measured using an immunoassay method.

What Does an ASO Test Result Mean?

Elevated levels of ASO antibodies suggest recent strep infection. However, results can take time to rise after infection and may remain elevated for months. A positive result doesn’t always mean you’re contagious, but it confirms past or recent exposure.

When Might You Need This Test?

Your doctor may recommend an ASO test if you have symptoms like persistent sore throat, fever, swollen lymph nodes, or rash, especially if other tests (like rapid strep) were negative. It’s also used in cases where strep is suspected but not confirmed by culture.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

ASO Antibody Test

Frequently asked questions

Why did my doctor order an ASO test?
Your doctor may have ordered it to check for strep infection if symptoms like sore throat, fever, or rash were present, especially if other tests (like rapid strep) didn’t confirm the infection. It can also help monitor long-term effects of strep exposure.
What does a high ASO result mean?
A high ASO result suggests you’ve had a recent or past Group A Streptococcus infection. It doesn’t always mean you’re contagious now, but it confirms the bacteria were in your system at some point. Follow-up with your doctor to discuss treatment or next steps.
How long does it take for ASO levels to return to normal?
ASO levels can remain elevated for weeks or months after infection. They don’t always return to normal quickly, so your doctor may monitor you over time if needed. Treatment with antibiotics can help reduce symptoms and lower the risk of complications.
  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