Antithrombin III Activity in Plasma (Heparin Resistance Screening)
This test measures how well your blood naturally prevents clots from forming. Antithrombin III is a protein that helps regulate blood clotting, and low levels can increase your risk of abnormal clots.
["Check your blood’s natural clot-prevention ability to help prevent dangerous clots."]
Key facts at a glance
- LOINC
- 50731-9
- Short code
- SRA 100IU/mL LMW heparin Ser-aCnc
- Synonyms
- SRA 100IU/mL LMW heparin Ser-aCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 0
- Ref high
- 17
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- 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 This Test Matters
Antithrombin III plays a key role in keeping blood flowing smoothly by inhibiting certain clotting factors. If your body doesn’t produce enough of it, you may be at higher risk for conditions like deep vein thrombosis (DVT) or pulmonary embolism. This test helps doctors assess whether you need additional treatments, such as heparin, to prevent clots.
What to Expect Before the Test
No special preparation is needed for this test. You’ll have a small needle inserted into your arm to draw a blood sample. The process is quick and usually causes only mild discomfort. If you’re on heparin or other blood thinners, let your healthcare provider know before the test.
How the Test Works
Blood is collected and analyzed to measure the activity of antithrombin III. The test checks how effectively this protein can stop clotting in the lab. Results help determine if your natural clot-prevention system is working as expected.
Possible Results and Next Steps
If your results are low, your doctor may recommend further testing or adjustments to your treatment plan, such as monitoring heparin levels or switching to a different anticoagulant. Normal results suggest your body’s natural clot-prevention mechanisms are functioning well.
Reference range
NCBI: NSE 0-17 ng/mLN/A
No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Low antithrombin III levels can signal an increased risk of clotting disorders, especially in people with conditions like liver disease, kidney disease, or those taking certain medications. Doctors use these results to guide treatment decisions, such as adjusting heparin doses or identifying underlying causes of low levels (e.g., liver dysfunction).
Short name
Antithrombin III Activity
Frequently asked questions
What does a low result mean for my health?
Will I need to take any special steps if my result is low?
Can I eat or drink before this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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 17, 2026. · Read our editorial policy