Medical Laboratory Science Review Harr 2.4 Coagulation: Inhibitors, Thrombotic Disorders, and Anticoagulant Drugs

C. It is a cofactor of heparin

C Antithrombin is heparin cofactor and it is the most important naturally occurring physiological inhibitor of blood coagulation. It represents about 75% of antithrombotic activity and is an α2-globulin made by the liver.

Which characteristic describes antithrombin (AT)?

A. It is synthesized in megakaryocytes

B. It is activated by protein C

C. It is a cofactor of heparin

D. It is a pathological inhibitor of coagulation

A. Thrombin time

A Heparin is an antithrombin drug, and therefore increases the thrombin time test along with the APTT and PT. Heparin therapy has no effect on fibrinogen, protein C, or protein S assays. APTT is the test of choice for monitoring heparin therapy

Which laboratory test is affected by heparin therapy?

A. Thrombin time

B. Fibrinogen assay

C. Protein C assay

D. Protein S assay

D. Not corrected with normal plasma

D In the presence of a pathological circulating anticoagulant, a mixing test using normal plasma does not correct the abnormal APTT. These anticoagulants are pathological substances and are endogenously produced. They are either directed against a specific clotting factor or against a group of factors. A prolonged APTT due to a factor deficiency is corrected when mixed with a normal plasma. Factors VIII and IX deficient plasmas are used for assaying factor VIII and IX activities, respectively.

An abnormal APTT caused by a pathological circulating anticoagulant is:

A. Corrected with factor VIII-deficient plasma

B. Corrected with factor IX-deficient plasma

C. Corrected with normal plasma

D. Not corrected with normal plasma

D. Phospholipid-dependent assays

D The lupus anticoagulant interferes with phospholipid-dependent coagulation assays such as the PT and APTT tests. The lupus anticoagulant does not inhibit clotting factor assays, and does not inhibit in vivo coagulation.

The lupus anticoagulant affects which of the following tests?

A. Factor VIII assay

B. Factor IX assay

C. VWF assay

D. Phospholipid-dependent assays

B. It is not recommended for pregnant and lactating women

B Coumadin (warfarin) crosses the placenta and is present in human milk; it is not recommended for pregnant and lactating women. Warfarin is a vitamin K antagonist drug that retards synthesis of the active form of vitamin K-dependent factors (II, VII, IX, and X). Antithrombin is a heparin (not warfarin) cofactor. The International Normalized Ratio (INR) is used to monitor warfarin dosage.

Which statement about Coumadin (warfarin) is accurate?

A. It is a vitamin B antagonist

B. It is not recommended for pregnant and lactating women

C. It needs antithrombin as a cofactor

D. APTT test is used to monitor its dosage

D. Its activity is enhanced by protein S

D Protein S functions as a cofactor of protein C and as such enhances its activity. Activated protein C inactivates factors Va and VIIIa

Which statement regarding protein C is correct?

A. It is a vitamin K-independent zymogen

B. It is activated by fibrinogen

C. It activates cofactors V and VIII

D. Its activity is enhanced by protein S

B. Diluted Russell's viper venom test (DRVVT)

B Russell's viper venom (RVV) reagent contains factors X and V, activating enzymes that are strongly phospholipid dependent. The reagent also contains RVV, calcium ions, and phospholipid. In the presence of phospholipid autoantibodies such as lupus anticoagulant, the reagent phospholipid is partially neutralized causing prolongation of the clotting time. Thrombin time evaluates fibrinogen. FDP and D-dimer tests evaluate fibrinogen and fibrin degradation products.

Which of the following is an appropriate screening test for the diagnosis of lupus anticoagulant?

A. Thrombin time test

B. Diluted Russell's viper venom test (DRVVT)

C. D-dimer test

D. FDP test

B. Thrombosis

B Activated protein C resistance is the single most common cause of inherited thrombosis. In 90% of individuals, the cause is gene mutation of factor V (factor V Leiden). Affected individuals are predisposed to thrombosis, mainly after age 40. Heterozygous individuals may not manifest thrombosis unless other clinical conditions coexist

Which of the following is most commonly associated with activated protein C resistance (APCR)?

A. Bleeding

B. Thrombosis

C. Epistaxis

D. Menorrhagia

D. PT, APTT, TT

D Heparin is a therapeutic anticoagulant with an antithrombin activity. Heparin also inhibits factors XIIa, XIa, Xa, and IXa. In patients receiving heparin therapy, the PT, APTT, and TT are all prolonged. Quantitative fibrinogen assay, however, is not affected by heparin therapy

A 50-year-old man has been on heparin for the past 7 days. Which combination of the tests is expected to be abnormal?

A. PT and APTT only

B. APTT, TT only

C. APTT, TT, fibrinogen assay

D. PT, APTT, TT

D. Prasugrel

D Prasugrel (Effient) is an antiplatelet drug that reduces platelet aggregation by irreversibly blocking the P2Y12 receptors on the platelet surface membrane, thereby inhibiting platelet aggregation to ADP. Aspirin is another antiplatelet drug that inhibits platelet aggregation by blocking the action of the enzyme cyclo-oxygenase. Warfarin and heparin are anticoagulant drugs that act against clotting factors

Which of the following drugs inhibits ADP mediated platelet aggregation?

A. Heparin

B. Warfarin

C. Aspirin

D. Prasugrel

A. Protein C

A Protein C is activated by thrombin-thrombomodulin complex. Thrombomodulin (TM) is a transmembrane protein that accelerates protein C activation 1,000-fold by forming a complex with thrombin. When thrombin binds to TM, it loses its clotting function, including activation of factors V and VIII. Activated protein C deactivates factors Va and VIIIa. Protein S is a cofactor necessary for the activation of protein C.

Thrombin-thrombomodulin complex is necessary for activation of:

A. Protein C

B. Antithrombin

C. Protein S

D. Factors V and VIII

B. APTT

B Heparin dosage may be monitored by the APTT test. Heparin dose is adjusted to an APTT of 1.5-2.5 times the mean of the laboratory reference ranges. This level of APTT is equivalent to plasma heparin levels of 0.3-0.7 U/mL. The PT would be prolonged in heparin therapy, but the test is not as sensitive as the APTT. Heparin inhibits thrombin, and therefore, causes a prolonged TT. The TT test, however, is not used to monitor heparin therapy.

What test is used to monitor heparin therapy?

A. INR

B. APTT

C. TT

D. PT

A. INR

A Warfarin is a vitamin K antagonist drug. It inhibits vitamin K-dependent factors (II, VII, IX, and X) and other vitamin K-dependent proteins such as proteins C and S. Warfarin therapy is monitored by the INR. An INR of 2.0-3.0 is used as the target when monitoring warfarin therapy for prophylaxis and treatment of DVT. A higher dose of warfarin (giving an INR of 2.5- 3.5) is required for patients with mechanical heart valves.

What test is commonly used to monitor warfarin therapy?

A. INR

B. APTT

C. TT

D. Ecarin time

B. Va and VIIIa

B Factors Va and VIIIa are deactivated by protein S and activated protein C.

What clotting factors (cofactors) are inhibited by protein S?

A. V and X

B. Va and VIIIa

C. VIII and IX

D. VIII and X

C. Urokinase

C Urokinase is a thrombolytic drug commonly used to treat acute arterial thrombosis. Urokinase can also be used for the treatment of venous thromboembolism, myocardial infarction, and clotted catheters. Warfarin and heparin are anticoagulant drugs, whereas aspirin prevents platelet aggregation by inhibiting cyclo-oxygenase.

Which drug promotes fibrinolysis?

A. Warfarin

B. Heparin

C. Urokinase

D. Aspirin

C. Neutralization of the antibody by high concentration of platelets

C The International Society of Hemostasis and Thrombosis has recommended four criteria for the diagnosis of lupus anticoagulant: (1) a prolongation of one or more of the phospholipid-dependent clotting tests such as APTT or DRVVT; (2) the presence of an inhibitor confirmed by mixing studies (not corrected); (3) evidence that the inhibitor is directed against phospholipids by neutralizing the antibodies with a high concentration of platelets (platelet neutralization test or DRVVT with platelet-rich plasma); (4) lack of any other causes for thrombosis. Lupus inhibitor is not commonly time or temperature dependent.

Diagnosis of lupus anticoagulant is confirmed by which of the following criteria?

A. Decreased APTT

B. Correction of the APPT by mixing studies

C. Neutralization of the antibody by high concentration of platelets

D. Confirmation that abnormal coagulation tests are time and temperature dependent

B. Prolonged APTT/thrombosis

B Lupus anticoagulant interferes with phospholipids in the APTT reagent, resulting in prolongation of APTT. However, in vivo, lupus anticoagulant decreases fibrinolytic activity, causing an increased risk of thrombosis. Lupus anticoagulant does not result in a bleeding tendency unless there is a coexisting thrombocytopenia or other coagulation abnormality.

Which of the following abnormalities is consistent with the presence of lupus anticoagulant?

A. Decreased APTT/bleeding complications

B. Prolonged APTT/thrombosis

C. Prolonged APTT/thrombocytosis

D. Thrombocytosis/thrombosis

C. Has a longer half-life than unfractionated heparin

C Low molecular weight heparin (LMWH) is a small glycosaminoglycan that is derived from unfractionated heparin (UFH). The LMWH has a low affinity for plasma proteins and endothelial cells and therefore has a longer half-life. The half-life of the drug does not depend on the dosage. LMWH has an inhibitory effect on factors Xa and IIa. It does not require routine monitoring except in patients with renal failure, obese patients, pediatric patients, and pregnant patients.

Which of the following is a characteristic of low molecular weight heparin (LMWH)?

A. Generally requires monitoring

B. Specifically acts on factor Va

C. Has a longer half-life than unfractionated heparin

D. Can be used as a fibrinolytic agent

C. Bleeding time

C Aspirin is an antiplatelet drug. It prevents platelet aggregation by inhibition of cyclo-oxygenase. Aspirin has no effect on the platelet count, platelet morphology, or prothrombin time

Which of the following tests is most likely to be abnormal in patients taking aspirin?

A. Platelet morphology

B. Platelet count

C. Bleeding time

D. Prothrombin time

B. Thrombosis

B Antithrombin is a physiological anticoagulant. It inhibits factors IIa, Xa, IXa, XIa, and XIIa. Deficiency of antithrombin is associated with thrombosis. Thrombotic events may be primary (in the absence of a triggering factor) or may be associated with another risk factor such as pregnancy or surgery.

Which of the following is associated with antithrombin deficiency?

A. Thrombocytosis

B. Thrombosis

C. Thrombocytopenia

D. Bleeding

A. Decreased protein C

A Protein C is a physiological inhibitor of coagulation. It is activated by thrombin thrombomodulin complex. Activated protein C inhibits cofactors Va and VIIIa. The deficiency of protein C is associated with thrombosis. Increased fibrinolysis, afibrinogenemia, and ITP are associated with bleeding

Which of the following may be associated with thrombotic events?

A. Decreased protein C

B. Increased fibrinolysis

C. Afibrinogenemia

D. ITP

C. Thrombosis

C Up to 22% of patients taking aspirin become resistant to aspirin's antiplatelet effect. Patients who are aspirin resistant have a higher risk of thrombosis (heart attacks and strokes).

Aspirin resistance may be associated with:

A. Bleeding

B. Factor VIII deficiency

C. Thrombosis

D. Thrombocytosis

D. Heparin

D Heparin is an antithrombin drug causing prolonged TT in patients who are on heparin therapy. Prasugrel, clopidogrel, and aspirin are antiplatelet drugs causing inhibition of platelet aggregation.

A prolonged thrombin time is indicative of which of the following antithrombotic therapies?

A. Prasugrel

B. Clopidogrel

C. Aspirin

D. Heparin

C. Patients with thrombotic events occurring at a young age

C Laboratory tests for evaluation of thrombophilia are justified in young patients with thrombotic events, in patients with a positive family history after a single thrombotic event, in those with recurrent spontaneous thrombosis, and in pregnancies associated with thrombosis.

Screening tests for thrombophilia should be performed on:

A. All pregnant women because of the thrombotic risk

B. Patients with a negative family history

C. Patients with thrombotic events occurring at a young age

D. Patients who are receiving anticoagulant therapy

B. Single mutation of prothrombin molecule/ thrombosis

B Prothrombin G20210A is defined as a single-point mutation of the prothrombin gene, resulting in increased concentration of plasma prothrombin and thereby a risk factor for thrombosis. Prothrombin G20210A is the second most common cause of inherited hypercoagulability (behind factor V Leiden). It has the highest incidence in whites from southern Europe. The thrombotic episodes generally occur before age 40.

Prothrombin G20210A is characterized by which of the following causes and conditions?

A. Single mutation of prothrombin molecule/ bleeding

B. Single mutation of prothrombin molecule/ thrombosis

C. Decreased levels of prothrombin in plasma/ thrombosis

D. Increased levels of prothrombin in plasma/ bleeding

A. Deactivation of factor Va

A Factor V Leiden is a single-point mutation in the factor V gene that inhibits factor Va inactivation by protein C. Activated protein C enhances deactivation of factors Va and VIIIa

Factor V Leiden promotes thrombosis by preventing:

A. Deactivation of factor Va

B. Activation of factor V

C. Activation of protein C

D. Activation of protein S

B. 2%

B The incidence of antiphospholipid antibodies in the general population is about 2%.

What is the approximate incidence of antiphospholipid antibodies in the general population?

A. <1%

B. 2%

C. 10%

D. 20%

D. Platelet aggregation

D Currently, the platelet aggregation test is considered the gold standard for evaluation of aspirin resistance. In aspirin resistance, platelet aggregation is not inhibited by aspirin ingestion. Aspirin resistance has no effect on platelet count and morphology

Which of the following laboratory tests is helpful in the diagnosis of aspirin resistance?

A. APTT

B. PT

C. Platelet count and morphology

D. Platelet aggregation

B. Increased thrombotic risk

B Tissue factor pathway inhibitor (TFPI) is released from the vasculature and is the most important inhibitor of the extrinsic pathway. TFPI inhibits factors Xa and VIIa-TF complex. Therefore, the deficiency of TFPI is associated with thrombosis.

Which of the following complications may occur as a result of decreased tissue factor pathway inhibitor (TFPI)?

A. Increased hemorrhagic episodes

B. Increased thrombotic risk

C. Impaired platelet plug formation

D. Immune thrombocytopenia

D. 10%-20%

D Factor VIII inhibitors (antibodies) occur in 10%-20% of patients with factor VIII deficiency receiving factor VIII replacement.

Factor VIII inhibitors occur in ____________ of patients with factor VIII deficiency?

A. 40%-50%

B. 30%-40%

C. 25%-30%

D. 10%-20%

B. Recombinant factor VIIa (rVIIa) to activate factor X

B Recombinant factor VII (rVIIa) is effective for the treatment of a high titer factor VIII inhibitor. Factor VIIa can directly activate factor X to Xa in the absence of factors VIII and IX. Recombinant factor VIIa does not stimulate anamnestic responses in patients with factor VIII inhibitor. Factor VIII concentrate is used for a low titer factor VIII inhibitor. Factor X concentrate and FFP are not the treatments of choice for factor VIII inhibitor.

Which therapy and resulting mode of action are appropriate for the treatment of a patient with a high titer of factor VIII inhibitors?

A. Factor VIII concentrate to neutralize the antibodies

B. Recombinant factor VIIa (rVIIa) to activate factor X

C. Factor X concentrate to activate the common pathway

D. Fresh frozen plasma to replace factor VIII

B. Factor VIII inhibitor titer

B The Bethesda assay is a quantitative assay for factor VIII inhibitor. In this assay, normal plasma is incubated with different dilutions of the patient's plasma or a normal control. The inhibitor inactivates factor VIII present in normal plasma following incubation for 2 hours at 37°C. The residual activities in the sample are determined, and the inhibitor titer is calculated.

The Bethesda assay is used for which determination?

A. Lupus anticoagulant titer

B. Factor VIII inhibitor titer

C. Factor V Leiden titer

D. Protein S deficiency

C. Thrombosis

C Elevated plasma homocysteine is a risk factor for the development of venous thrombosis. Homocystinemia may be inherited or acquired. Acquired homocystinemia is caused by the dietary deficiencies of vitamins B6, B12, and folic acid.

Hyperhomocysteinemia may be a risk factor for:

A. Bleeding

B. Thrombocythemia

C. Thrombosis

D. Thrombocytopenia

C. Oral contraceptives

C Oral contraceptive drugs are acquired risk factors for thrombosis. Aspirin and Plavix are antiplatelet drugs and tPA is a fibrinolytic drug used for the treatment of thrombosis.

Which drug may be associated with deep venous thrombosis (DVT)?

A. Aspirin

B. tPA

C. Oral contraceptives

D. Plavix (clopidogrel)

A. DVT

A Argatroban is a thrombin inhibitor drug and may be used as an anticoagulant in patients with heparin-induced thrombocytopenia (HIT) to prevent thrombosis. Argatroban is a small synthetic molecule that binds to free and clot-bound thrombin. Argatroban affects TT, PT, APTT, and ACT tests. The APTT test is recommended for monitoring the dosage with the target therapeutic range of 1.5 to 3.0 times the mean of the laboratory reference range. In patients with lupus anticoagulant or factor deficiencies, the baseline APTT is prolonged; in these conditions, the Ecarin time can be used as an alternative assay.

Argatroban may be used as an anticoagulant drug in patients with:

A. DVT

B. Hemorrhage

C. TTP

D. Trombocytosis

D. Antibodies to heparin-PF4 complex

D Heparin-induced thrombocytopenia is an immune process caused by the production of antibodies to heparin-PF4 complex. This immune complex binds to platelet Fc receptors, causing platelet activation and formation of platelet microparticles that in turn induce hypercoagulability and thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) results from:

A. Antibodies to heparin

B. Antibodies to platelets

C. Antibodies to PF4

D. Antibodies to heparin-PF4 complex

C. Modified APTT with and without activated protein C

C Activated protein C resistance can be evaluated by a two-part APTT test. The APTT is measured on the patient's plasma with and without the addition of activated protein C (APC). The result is expressed as the ratio of the APTT with APC to the APTT without APC. The normal ratio is 2:5. Patients with APCR have a lower ratio than the reference range. A positive screening test should be followed by a confirmatory test such as polymerase chain reaction (PCR) for factor V Leiden.

Which laboratory test is used to screen for activated protein C resistance?

A. Mixing studies with normal plasma

B. Mixing studies with factor-deficient plasma

C. Modified APTT with and without activated protein C

D. Modified PT with and without activated protein C

D. Hirudin therapy

D Ecarin clotting time, a snake venom-based clotting assay, may be used to monitor hirudin therapy in instances when the baseline APTT is prolonged due to lupus anticoagulant or factor deficiencies. The APTT is insensitive to hirudin levels above 0.6 mg/L, and this insensitivity may result in a drug overdose despite a monitoring protocol. Heparin therapy is monitored by the APTT; warfarin therapy is monitored by the INR. Fibrinolytic therapy may be monitored by D-dimer

Ecarin clotting time may be used to monitor:

A. Heparin therapy

B. Warfarin therapy

C. Fibrinolytic therapy

D. Hirudin therapy

A. Lupus anticoagulant

A The lupus anticoagulant interferes with the APCR screening assay based on the APTT ratio with and without APC addition. Persons with the lupus anticoagulant have a prolonged APTT that renders the test invalid for APCR screening.

Which of the following may interfere with the activated protein C resistance (APCR) screening test?

A. Lupus anticoagulant

B. Protein C deficiency

C. Antithrombin deficiency

D. Protein S deficiency

D. Hyperfibrinogenemia

D Hyperfibrinogenemia is a risk factor for thrombophilia. Fibrinogen is an acute phase reactant and may be increased in inflammation, stress, obesity, smoking, and medications such as oral contraceptives. Hypofibrinogenemia, afibrinogenemia, and factor VIII inhibitors are associated with bleeding.

Thrombophilia may be associated with which of the following disorders?

A. Afibrinogenemia

B. Hypofibrinogenemia

C. Factor VIII inhibitor

D. Hyperfibrinogenemia

D. Lepirudin

D Lepirudin is a recombinant analogue of hirudin. It is an alternative anticoagulant drug used in patients with HIT who cannot tolerate heparin or LMWH therapy. Warfarin should not be used to anticoagulate persons with HIT because it causes a fall in protein C concentration prior to inducing a decrease in coagulation factors derived from vitamin K. The lower protein C predisposes HIT patients to leg thrombosis.

Which of the following anticoagulant drugs can be used in patients with HIT?

A. Warfarin

B. Heparin

C. Aspirin

D. Lepirudin

B. Activated clotting time test (ACT)

B The activated clotting time (ACT) is a point-of-care coagulation test used to monitor high-dose heparin therapy during cardiac surgery, cardiac angioplasty, hemodialysis, and other major surgeries. It is the preferred method to determine if sufficient heparin was administered to prevent clotting during surgery because it is more rapid than the APTT test. The test uses a clot activator such as Kaolin or Celite to stimulate coagulation, and the time in seconds is linearly related to the dose of heparin administered. The ACT test is available in different formats, and the reference range varies depending on the method used. At low to moderate heparin doses, the ACT test does not correlate well with the APTT or the antifactor Xa assay

Which of the following is the preferred method to monitor heparin therapy at the point of care during cardiac surgery?

A. APTT

B. Activated clotting time test (ACT)

C. PT

D. TT

C. Lupus anticoagulant

C Mixing studies differentiate factor deficiencies from factor inhibitors. Lupus anticoagulant is associated with thrombosis, and it is directed against phospholipid-dependent coagulation tests such as the APTT. In patients with lupus anticoagulant, the APTT after mixing patient's plasma with normal plasma remains prolonged immediately after mixing and following 2-hours incubation. Factor VIII deficiency and factor VIII inhibitor are associated with bleeding. Factor VIII inhibitor is time and temperature dependent. The prolonged APTT may be corrected immediately after mixing, and becomes prolonged following incubation. In factor VIII deficiency, the prolonged APTT would be corrected after mixing the patient's plasma with normal plasma.

Mrs. Smith has the following laboratory results, and no bleeding history:

APTT: prolonged

APTT results on a 1:1 mixture of the patient's plasma with normal plasma:

Preincubation: prolonged APTT

2-hour incubation: prolonged APTT

These results are consistent with:

A. Factor VIII deficiency

B. Factor VIII inhibitor

C. Lupus anticoagulant

D. Protein C deficiency

C. Anti-Xa heparin assay

C The anti-factor Xa heparin assay is used to monitor LMWH therapy when required because the APTT test is insensitive to LMWH. The assay can be performed by chromogenic end-point detection used on automated analyzers. The principle of the test is to measure the inhibition of Xa by heparin. The reagent is a mixture of a fixed concentration of factor Xa, a substrate which is specific for factor Xa, and a fixed concentration of antithrombin (AT). Some kits rely on the antithrombin in patient's plasma. Heparin forms a complex with AT and factor Xa (AT-heparin-Xa). Excess free factor Xa cleaves the chromogenic substrate and releases a yellow product. The color intensity of the product is inversely proportional to plasma heparin concentration, and is measured by a photodetector at 405 nm. LMWH therapy does not usually require monitoring; however, exceptions include pediatric, obese, and pregnant patients and those with renal

failure

Which test may be used to monitor LMWH therapy?

A. APTT

B. INR

C. Anti-Xa heparin assay

D. Activated clotting time