Thrombotic Disorders



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Answer: D

Explanation: Although the mutations of factor V Leiden and prothrombin gene are common, in the heterozygous state they are not strong risk factors for thrombosis at a young age. Combined heterozygous prothrombin and factor V Leiden as well as homozygous factor V Leiden may increase this risk substantially. The strength of homozygous prothrombin mutation is not known. The polymorphisms of MTHFR and PAI-1 are of debatable significance. Deficiencies of the natural anticoagulants are very strongly associated with thrombotic events prior to age 40. See table below for additional details.

Inherited thrombophilia

Frequency in population

Risk of thrombosis

Factor V Leiden

Heterozygous state:

3%–8% of Caucasians

1.2% African American

Rare in Asian



-Heterozygous state 3–5 X increase over a lifetime

-Minimal increased risk of recurrent events

-Homozygous 18 X increased risk

-Combined with prothrombin gene 30–50 X increased risk



Prothrombin gene mutation

Heterozygous state: 2%–3% of U.S. Caucasians; 0.5% African Americans

-Heterozygous state 3 X increased risk over lifetime

-Possible increased risk of recurrent events in children, but not adults

-Homozygous increased but unclear

-Combined with factor V Leiden 30–50 X increased risk



Antithrombin deficiency

Heterozygous 1 in 500 to 5,000

-50% risk of event prior to age 40

-Increased risk of recurrent events (10%–17% per year)

-Homozygous state not compatible with life


Protein S deficiency

Heterozygous state 1 in 800 to 3,000


-Heterozygous 31 X increased risk prior to age 55

-Increased risk of recurrent events (44% in 5 years)

-Homozygous state associated with purpura fulminansVTE


Protein C deficiency

Heterozygous state 1 in 500 to 600

Homozygous state 1 in 1,000,000



-Heterozygous 24 X increased risk prior to age 55

-Increased risk of recurrent events (37% in 5 years)

-Homozygous state associated with purpura fulminans as neonate


MTHFR polymorphisms:

C677T (thermolabile) and

A1298C


Heterozygous state 35% of population

Homozygous state 12%–20% of population



Debatable significance unless leads to elevated plasma homocysteine

PAI-1 polymorphism

Heterozygous state 12% of population

Debatable significance unless PAI-1 activity also elevated


Question 12


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