Thrombophilia and Inherited Hypercoagulable States

Thrombophilia is a tendency, often inherited, toward inappropriate clot formation, arising when the natural brakes on coagulation are weakened or procoagulant activity is increased. This topic describes the major inherited hypercoagulable states — including factor V Leiden, the prothrombin G20210A variant, and deficiencies of antithrombin, protein C, and protein S — and the laboratory approach used to characterize them.

Definition

Thrombophilia is an inherited or acquired predisposition to venous (and sometimes arterial) thrombosis caused by an imbalance favoring coagulation — typically reduced activity of natural anticoagulants or gain-of-function procoagulant variants.

Scope

Coverage includes the mechanisms by which loss of anticoagulant control predisposes to venous thrombosis, the principal inherited defects, and the laboratory and genetic tests used to identify them, framed within the broader understanding that thrombosis is usually multifactorial. It is a reference and laboratory-pattern overview; testing indications, anticoagulation, and management decisions are outside its scope and require specialist judgment.

Core questions

• How do natural anticoagulants normally restrain clot propagation, and what happens when they fail?
• What are the major inherited thrombophilic defects and how common are they?
• How does activated protein C resistance link factor V Leiden to thrombosis risk?
• Why is venous thrombosis best understood as a multifactorial, gene-environment outcome?

Key concepts

• Natural anticoagulants: antithrombin, protein C, protein S
• Factor V Leiden and activated protein C resistance
• Prothrombin G20210A variant
• Loss-of-function versus gain-of-function defects
• Venous thromboembolism risk
• Multifactorial, gene-environment causation
• Inherited versus acquired thrombophilia

Key theories

Multicausal (gene-environment) model of thrombosis

Venous thrombosis typically results from the interaction of inherited predispositions with acquired and transient risk factors rather than a single cause, a framework that explains why many carriers of thrombophilic variants never develop thrombosis.

Mechanisms

Coagulation is normally restrained by natural anticoagulant systems: antithrombin inhibits thrombin and other proteases, and the protein C pathway, with its cofactor protein S, inactivates the activated factors V and VIII. Inherited deficiencies of antithrombin, protein C, or protein S are loss-of-function defects that weaken these brakes. The most common inherited thrombophilias are gain-of-function variants: factor V Leiden renders activated factor V resistant to cleavage by activated protein C (activated protein C resistance), and the prothrombin G20210A variant raises prothrombin levels. Each shifts the hemostatic balance toward thrombin generation and clot formation, predominantly in the venous system. Because any single defect usually confers only a modest absolute risk, thrombosis generally emerges when such a predisposition combines with acquired triggers such as immobilization, surgery, or pregnancy.

Clinical relevance

Thrombophilia testing and its interpretation are part of hematopathology because they help characterize a thrombotic tendency, but a positive result does not by itself determine outcomes or treatment. This entry describes the biology and laboratory categories as reference material; it provides no guidance on whom to test, anticoagulation, or duration of therapy, all of which require individualized specialist assessment.

Epidemiology

Factor V Leiden is the most common inherited thrombophilia in populations of European ancestry and is more prevalent than the prothrombin G20210A variant, while antithrombin, protein C, and protein S deficiencies are individually rarer but tend to confer higher relative risk. The clinical effect of these variants on recurrent venous thromboembolism is generally modest compared with major clinical risk factors.

History

The natural anticoagulant deficiencies (antithrombin, then protein C and protein S) were identified across the 1960s to 1980s as causes of familial thrombosis. The field was transformed in 1993-1994 when activated protein C resistance was described and traced by Bertina and colleagues to a single point mutation in factor V (factor V Leiden), soon followed by the prothrombin G20210A variant, establishing thrombophilia as a common, often multifactorial trait.

Debates

How much does inherited thrombophilia affect recurrence risk?

Prospective cohort data indicate that common inherited thrombophilias contribute less to the risk of recurrent venous thromboembolism than major clinical factors, which has tempered enthusiasm for routine thrombophilia testing.

Key figures

• Björn Dahlbäck
• Rogier Bertina
• Frits Rosendaal
• Trevor Baglin

Seminal works

• bertina-1994
• dahlback-2008
• baglin-2003

Frequently asked questions

What is factor V Leiden?

Factor V Leiden is a common inherited variant of coagulation factor V that resists inactivation by activated protein C, a phenomenon called activated protein C resistance. This shifts the hemostatic balance toward clotting and is the most frequent inherited thrombophilia in people of European ancestry.

Does having a thrombophilia mean a clot is inevitable?

No. Many carriers of inherited thrombophilic variants never develop thrombosis. Venous thrombosis is usually multifactorial, emerging when a predisposition combines with acquired triggers such as surgery, immobilization, or pregnancy.