Platelet function 8/23

Question 1

Physiologic Clotting: Five Fundamental Steps

Answer 1

Vessel Injury - Endothelial Damage

Primary Hemostasis – Platelet function

Secondary Hemostasis – Coagulation-Fibrin

Antithrombosis - Clotting Inhibition

Fibrinolysis – Breakdown - Removal of Clot

*Hemostasis: Maintenance of an intact vessel surface and blood in a fluid, clot-free condition to ensure tissue perfusion.

Question 2

3 sources of vWF

Answer 2

1. Endothelial vWF secreted onto subendothelial ECM and forms polymers
2. vWF produced by megakaryocytes circulates in blood
3. platelet alpha granules

Factor VIII attaches to vWF, and forms the circulating VIII-vWF complex which is used to connect platelets together.

Question 3

3 roles of vWF

Answer 3

1. Endothelial-derived vWF adheres to the ECM
2. vWF acts as a connective interface between ECM surface exposed by injury and the inital platelets that must adhere to form the “platelet plug”
3. Promotes platlet aggregation and increases the half life of factor VIII

Question 4

Distinguish serum from plasma and describe how plasma is obtained for hemostasis testing.

Answer 4

Serum: Whole blood is allowed to clot after phlebotomy. Then, blood sample is centrifuged to separate fluid supernatant (serum) from cellular elements and clot.

Plasma: Blood is drawn into a collection tube containing an anticoagulant that prevents clotting. Blood is then centrifuged and fluid supernatant (plasma) is removed and stored.

Anticoagulation to prepare plasma is accomplished by an agent that chelates Ca++- (a cofactor essential in multiple steps of the coagulation cascade-blue circles). The most common anticoagulant used to obtain plasma for hemostasis studies is sodium citrate 3.2%

Question 5

Blood collection for Lab hemostasis studies

Answer 5

Blue: has sodium citrate, which binds calcium and keeps blood not coagulated- used for PT, a-PTT, Fibrinogen, platlet aggregation, factor VIII Activity, individual factor assays

Purple: anticoagulant EDTA binds calcium and also inhibits coagulation – used for platelet quantitative analysis

Red: Has silica clot activator: For just serum chemistry study – does not have an anticoagulant – for chemistry studies

Question 6

Describe how the Bleeding Time test is performed and explain its use in detecting abnormalities in:
-Initiating a platelet hemostatic plug at the site of
vessel injury
-Primary hemostasis and
-Secondary Hemostasis

Answer 6

• good test for deficits in primary hemostasis (detects in vWF) but does not detect problems in secondary hemostasis
• Bleeding time: measures the time taken for a standardized skin incision to stop bleeding. It is time-consuming, difficult to perform and not a good predictor of bleeding during hemostatic stresses such as surgery.
• less than 8 minutes, with 5 minutes optimal
• bleeding time is increased with a drop in platelet count

Question 7

Describe the relationship between quantitative platelet count and the corresponding effect on the bleeding time result.

Answer 7

• decreased platelet count results in a linear increased time in bleeding when a certain minimal amount of platelets is reached

Question 8

Von Willebrand’s disease

Answer 8

• Endothelial cells do not make vWF, thus platelets are not recruited to the endothelial membrane
• results in not enough factor VIII, so coagulation does not occur

Question 9

Binding domains of vWF

Answer 9

vWF A3 domain binds to collage of the ECM

vWF A1 domain links with platelet receptor GPIb on filapodia

Question 10

Glanzmann Thrombasthenia

Answer 10

• mutation in GpIIb-IIIa results in deficiency in platelet aggregation
• Platelet count and platelet morphology – normal
• Aggregation studies – no response to epinephrine, ADP, collagen, arachidonic acid, or thrombin
• Platelet flow cytometry-decreased expression of GPIIb/IIIa
• Abnormal clot retraction (testing not widely available)

Question 11

Filapodia and drugs that counteract its function….

Answer 11

“link” via fibrogen bridges bound to activated platelet GPIIb-IIIa protein receptors

“Reopro” (Abciximab)= a monoclonal antibody to GPIIb-IIIa that blocks platelet plug formation by inhibiting binding.

Question 12

Testing Secondary Hemostasis Coagulation Testing

Answer 12

1. Prothrombin Time (PT) - used to monitor defects in extrinsic pathway - used to monitor patients who are given cumidin
2. activated Partial Thromboplastin Time (aPTT) - monitors defects in intrinsic pathway - used to monitor heparin treatment

Question 13

Mean Platelet Volume (MPV)

Answer 13

Platelets, like red cells, decrease progressively in size in peripheral blood over time. Young platelets are larger and more functional.

Any increase in platelet production, particularly if coupled with decreased survival in blood, will cause a significant increase in MPV.