AssessmentsThrombophilia: Clinical practice
USMLE® Step 2 style questions USMLE
A 40-year-old man comes to the emergency department because of cough, shortness of breath, and pleuritic chest pain for the past 10 days. He has had a cerebrovascular accident in the past year. He believes he is allergic to contrast dye. His temperature is 37.2°C (99°F), pulse is 80/min, respirations are 16/min, and blood pressure is 120/80 mm Hg. Physical examination shows residual hemiparesis. A complete blood count shows:
His prothrombin time is 12s and partial thromboplastin time is 40s. An ECG is within normal limits. A chest X-ray shows left-sided pleural effusion. Which of the following is the most appropriate next step in management?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Before going into the individual thrombophilia disorders, let’s go through the normal coagulation system. The coagulation pathway is divided into an extrinsic and intrinsic pathways, which join into a common pathway that ultimately forms a clot made of strong fibrin mesh.
Let's start with the extrinsic pathway. It starts when trauma damages a blood vessel, and exposes the cells under the endothelial layer, which have tissue factor in their membrane.
Activated factor VII binds to tissue factor, forming a complex that then binds to and activates factor X.
The intrinsic pathway starts when a circulating factor XII, activates factor XI, which then activates factor IX.
Finally, factor IX forms a complex with factor VIII, and this complex binds to and activates factor X.
In the common pathway, activated factor X activates factor V, which converts prothrombin to thrombin, or factor II.
Thrombin then converts fibrinogen into fibrin, which cross-links to form a fibrin clot.
In order to regulate coagulation, the liver makes protein S, which activates protein C.
Protein C inactivates factors V and VIII.
Finally, a protein called antithrombin III inhibits factor X and factor II.
Disorders that cause thrombophilia can be genetically inherited, like factor V Leiden, prothrombin gene mutation, and protein C and S deficiency, or acquired like antiphospholipid syndrome - which is an autoimmune condition.
And the key is knowing when to screen for these rare thrombophilias in a person with venous thromboembolism.
Finally, the risk of having a venous thromboembolism in some of the thrombophilias can be quite low, but that risk can increase when there’s an additional risk factor, like surgery, pregnancy, prolonged immobility, or oral contraceptive use.
It results from an autosomal dominant point mutation in the gene encoding for factor V, causing it to become resistant to inactivation by protein C.
The best initial screening test is a clotting assay, in which the patient’s plasma is extracted, and activated protein C is added, but it fails to inactivate factor V.
If this test is positive, factor V Leiden is confirmed by looking for the mutation in the factor V gene using PCR.
So venous thromboembolism prophylaxis with anticoagulants is not recommended in asymptomatic patients, unless the patient is in a high-risk situation, like surgery or pregnancy.
If, on the other hand, a patient with Factor V Leiden has already had a venous thromboembolism, then the management includes anticoagulation with unfractionated or low-molecular weight heparin, followed by warfarin for 3 to 6 months.
This mutation involves the substitution of adenine for guanine at position 20210 in the noncoding region of the prothrombin gene, causing an increase in the circulating levels of prothrombin.
The diagnosis is straightforward, using PCR to detect the mutation.
Alright, now onto protein C and S deficiency. Proteins C and S are vitamin K-dependent anticoagulant proteins that are made in the liver.
In the presence of protein S as a cofactor, protein C acts by enzymatically inactivating factors V and VIII.