Antithrombin III Activity (AT III)
A natural blood thinner whose deficiency raises clotting risk and can blunt heparin's effect.
Consider this test if:
- A venous blood clot or pulmonary embolism with no clear cause, especially before age 50
- Recurrent clots or a family history of inherited antithrombin deficiency or thrombophilia
- Appearing resistant to heparin and needing unusually high doses
- Building out a thrombophilia workup alongside other clotting markers
- HSA/FSA eligible
- Typical results in about 4 days · Reviewed by a real clinician
- Drawn at a CLIA/CAP-accredited lab near you ·
Pre-test considerations
No fasting required. Timing matters more than diet: antithrombin can read falsely low during an active clot or while on heparin or other anticoagulants, so testing is best done away from an acute event and, where possible, off interfering therapy. The sample is citrated plasma that must be separated and frozen promptly, so collection is handled at a lab.
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What this test is for
Antithrombin III is a protein the liver makes to keep clotting in check, neutralizing thrombin and other clotting factors so blood does not coagulate uncontrollably. This activity assay measures how well your antithrombin actually works, not just how much is present, which is the relevant question for clotting risk. Low activity points to a hypercoagulable state and is investigated after unexplained venous thrombosis, recurrent clots, clots at a young age, or a family history of inherited antithrombin deficiency. It is also useful when heparin is not working as expected, since heparin relies on antithrombin to do its job. Levels can drop temporarily during an active clot, with heparin therapy, in liver disease, or with certain conditions, so abnormal results are usually confirmed on a repeat sample away from those influences.
What to expect
- 1 Book instantly
Click, book, done. Choose a convenient lab location near you. Transparent, up-front pricing.
- 2 Quick lab visit
Testing to fit your busy schedule, usually 15 minutes or less. Walk-in and appointments available.
- 3 Typical results in about 4 days
Your results post straight to your dashboard as soon as the lab completes them.
- 4 Expert guidance
Included with Mito membership. A clinician reviews your results and your personalized action plan follows, with clear next steps.
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Every test at our cost
Members pay our cost on every test, with lab fees passed straight through. The full receipt is itemized, never padded.
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Year-round clinician support
Text anytime and get clinician-reviewed answers. When you want to go deeper, 1:1 consultations are available at affordable rates.
All for $9/month
Order any test or consult without joining. For $9/month, members unlock member prices, trend tracking, and year-round clinician guidance.
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Frequently asked questions
View all FAQsHow does pricing work?
Every test shows the member price next to the standard non-member price, so you can see what membership saves you. The member price is our cost — covering the lab and what it takes to run the service — never a profit on the test itself; Mito makes its money on the $9 membership, not on marking up your tests. Membership is $9/mo, and you still pay the lab’s order fee. Prices are itemized before you pay, with no hidden fees.
Where do I get tested?
Choose a partner lab (Quest, Labcorp, or BioReference) at checkout. If your cart spans multiple tests, we consolidate the whole order onto a single lab so you only make one visit.
Is this eligible for HSA/FSA?
Yes. This test is HSA/FSA eligible, and you can pay with your HSA/FSA card at checkout.
When will I get my results?
Your results post to your dashboard once your lab completes them, then a clinician reviews them and your full analysis and personalized action plan (with clear next steps) follow. Turnaround varies by test: specialty assays and at-home kits take longer, and each test shows its expected turnaround before you buy.
Do I need a doctor’s order?
No. Mito provides the lab order for you, so you can book and get tested without a separate doctor visit.
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