A patient with hereditary antithrombin (AT) deficiency is at increased risk for which of the following complications?

Antithrombin (AT; also known as antithrombin III) is a serine protease inhibitor that strongly inhibits factors IXa, Xa and thrombin (IIa). AT deficiency is inherited in an autosomal dominant fashion and is relatively rare, having a prevalence of 0.02 percent to 0.2 percent. Most affected individuals have deficient amounts of AT rather than abnormal AT function. AT deficiency can also be acquired. Clinical states associated with decreased levels of antithrombin include:

Individuals with hereditary AT deficiency are approximately 20 times more likely to experience venous thromboembolism (VTE) than the general population.

Heparin exerts an anticoagulant effect by binding to antithrombin (Figure 1), and enhances its inhibitory effects (by more than 1,000-fold). AT deficiency is associated with relative heparin resistance and can complicate anticoagulation for the treatment of VTE, or prior...

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