Hematologic Laboratory Testing: Coagulopathies

What are the three steps of hemostasis?

1. Vascular spasm/vasoconstriction

2. Formation of platelet plug

3. Coagulation cascade

What is Vascular spasm/vasoconstriction?

Vasoconstriction immediately following vascular injury to help stem blood flow

How is the platelet plug formed?

activated at the site of injury, become "sticky" and attempt to plug the area of bleeding by triggering the coagulation cascade

What is an important part of primary hemostasis?

Von Willebrand Factor (vWF)

What is "secondary hemostasis"?

End result is the formation of a fibrin-linked clot that strengthens the platelet plug

How is the intrinsic pathway activated?

internal vascular/endothelial injury that damages inside of blood vessel and exposes collagen and other molecules that trigger pathway

How is the extrinsic pathway activated?

external trauma

How is the common pathway activated?

convergence of intrinsic and extrinsic pathways

What are the four coagulation cascade factors dependent on?

vitamin-K dependent, II, VII, IX, and X, as well as Proteins C and S

What are the characteristics of the extrinsic pathway?

very rapid and provides the initial burst of clotting

What are the steps of the extrinsic pathway?

1. External trauma to the tissue activates tissue factor (III)

2. Tissue factor (III) binds factor VII + calcium forming VIIa

3. Factor VIIa activates factor X

What are the steps of the intrinsic pathway?

1. Internal vessel trauma/ endothelial damage

2. Factor XII → XIIa

3. Factor XIIa: Factor XI → XIa

4. Factor XIa: IX → IXa

5. Factor IXa binds VIIIa and Ca and activates X → Xa

6. Factor VIII activated by Thrombin (IIa) which cleaves it from its carrier proteinvWF

What is the clot formation process?

Factor II (Prothrombin) → Factor IIa (Thrombin) → Factor I (Fibrinogen) → Fibrin monomers

What labs are used for hemostasis?

CBC, peripheral smear, PT, INR, PTT< aPTT, platelet function

What labs are used for venous thromboembolism?

renal & liver function tests, UA, D-dimer, fibrin degradation products (FDP)

What platelet count is considered thrombocytopenia?

< 100,000

What platelet count may cause prolonged bleeding after injury or trauma?

< 40,000

What platelet count may allow for spontaneous bleeding?

< 20,000

What conditions cause thrombocytosis?

Malignancy and Polycythemia vera

What conditions cause thrombocytopenia?

Malignancy, DIC< thrombotic/idiopathic thrombocytopenia

What conditions cause a prolonged platelet function assay?

thrombocytopenia, von Willebrand's disease, bone marrow failure, DIC< collagen vascular disease, malignancy

What is prothrombin time (PT)?

Measures how long it takes for a plasma to form a fibrin clot when exposed to tissue factor (III)

What is prothrombin time (PT) used for?

Used to evaluate the extrinsic and common pathways

What is International Normalized Ratio (INR)?

Ratio of patient's PT vs. reference PT to help eliminate potential variance based on lab or processing

What is the normal PT/INR value?

11-13 seconds (1.0) therefor most conditions are treated to an INR of 2.0-3.0 (target of 2.5)

When is PT/INR prolonged?

- Vitamin K deficiency

- Coumadin (Warfarin) – test of choice to monitor coumadin therapy

When is PT the only thing prolonged?

Deficiency of Factor VII

What is partial thromboplastin time (PTT)?

Measures the time it takes plasma to form a fibrin clot when exposed to specific reagents that activate the

intrinsic pathway of the coagulation cascade

What is PTT used for?

Used to assesses the intrinsic and common pathways

What is activated partial thromboplastin time (aPTT)?

uses an activator to speed up clotting time which is more sensitive than PTT

What is a normal aPTT value?

30-45 seconds

What is PTT and aPTT prolonged?

Hemophilia A – factor VIII deficiency

Hemophilia B – factor IX deficiency

Hemophilia C – factor XI deficiency

* all intrinsic factor deficiencies *

When is PTT and aPTT considered the test of choice?

monitoring therapeutic levels of heparin!

What will affect the common pathway which in turn affect both PT and aPTT?

Liver disease, DIC, vitamin K deficiency, anticoagulants

Which anticoagulants affect both PT and aPTT?

Direct thrombin inhibitors

Dual coumadin and heparin therapy

Direct factor Xa inhibitors (variable)

What are mixing studies used for?

differentiate between the causes

of abnormal PT & PTT that is either factor deficiency or the presence of inhibitors

What are examples of inhibitors that impact mixing studies?

heparin, lupus anticoagulant, other autoantibodies

What is the mixing study result if a patient's sample is missing normal clotting factors?

NORMAL clotting factors in the control blood will correct the PT/aPTT

What stops the coagulation cascade?

anticoagulation, antithrombin III, protein C and S

What is antithrombin III?

Made by liver and endothelial cells inactivates thrombin and factors IXa & Xa

What is Protein C and S?

Vitamin K dependent and activates factor Va and VIIIa

What does a deficiency of antithrombin III and

protein C & S lead to?

Hypercoagulable states

When does platelet contraction normally occur?

~60 minutes after hemostasis, clots contract, reduce in size pulling edges of wound together for stabilization

What is the process of fibrinolysis?

~ 2 days or so after normal clotting, clots degrade, plasminogen forms plasmin to degrade/digest fibrin strands for clot breakdown

When are fibrin degradation products (FDP) present in the blood?

present in the blood when the

thrombolytic enzyme plasmin cleaves fibrin or fibrinogen

What is the FDP level used for?

Provides direct indication of

the activity of the fibrinolytic

system

When is FDP gonna be increased?

DIC and other fibrinolytic disorders

What is D-dimer?

type of FDP produced when fibrin is

cleaved by plasmin

What causes an increased D-dimer?

DVT, PE, DIC, Sickle cell anemia, pregnancy, malignancy, surgery, tPA