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Serum or Plasma Immunoassay for Antistreptolysin O (ASO) Antibody

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

This test measures the level of antibodies your body produces in response to certain bacterial infections, such as strep throat caused by Group A Streptococcus.

Quick summary

["Helps diagnose past or recent strep infections and guides treatment decisions."]

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

Key facts at a glance

LOINC
14022-8
Short code
HTLV I+II p26 Ab Ser Ql IB
Synonyms
HTLV I+II p26 Ab Ser Ql IB
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ib
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 detects antibodies (ASO) your immune system creates after exposure to Streptococcus bacteria. It’s often used to identify strep throat or other strep-related infections, even if symptoms aren’t severe or if tests like throat cultures are unavailable. This test can also help monitor chronic conditions like rheumatic fever, which may occur after untreated strep infections.

How Is the ASO Test Performed?

A blood sample is drawn from a vein in your arm. The sample is then tested in a lab using an immunoassay method to measure ASO antibody levels. Results are typically reported in international units per milliliter (IU/mL) or a comparable unit.

What Are Common Results and Their Meanings?

Higher-than-normal ASO levels often suggest a recent strep infection. However, results can vary depending on when after infection testing is done—antibodies may take days to weeks to build up. Low or normal levels might mean no recent strep exposure or an unrelated condition.

When Might You Need This Test?

Your healthcare provider may recommend an ASO test if you have symptoms like sore throat, fever, rash, or joint pain, especially if strep throat is suspected. It’s also used in people with a history of rheumatic fever or those with unexplained joint or heart issues.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

ASO Antibody Test

Frequently asked questions

Why might my doctor order an ASO test if I have strep throat symptoms?
While throat cultures are the gold standard for diagnosing strep throat, ASO tests can provide useful information—especially if cultures aren’t available or if symptoms are mild. Elevated ASO levels indicate past or recent strep exposure, helping guide treatment, such as antibiotics, to prevent complications like rheumatic fever.
Can ASO levels be affected by other conditions?
Yes, ASO levels can be influenced by other infections (like other bacterial or viral illnesses) or conditions like lupus or certain autoimmune diseases. If your results are abnormal, your doctor may order additional tests to rule out these possibilities.
How long do ASO levels stay elevated after a strep infection?
ASO antibodies can remain in your system for weeks to months after infection. They typically peak around 2–4 weeks post-infection and may linger for several months, even after symptoms resolve. This is why ASO testing is most useful shortly after exposure but can still be informative later on.
  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