Proceeds from website advertising help sustain Lab Tests Online. AACC is a not-for-profit organization and does not endorse non-AACC products and services.


Print this article
Share this page:
Also known as: Functional Antithrombin III; AT III; AT 3
Formal name: Antithrombin (Activity and Antigen)

Board approvedAll content on Lab Tests Online has been reviewed and approved by our Editorial Review Board.

At a Glance

Why Get Tested?

To help investigate the cause of recurrent inappropriate blood clotting; to help diagnose an antithrombin deficiency

When to Get Tested?

A couple of months after getting a blood clot (thrombotic episode) or when you are not responding as expected to heparin anticoagulation therapy

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?