Data interpretation: Coagulation screen

Question 1

What are the 2 main roles of a coagulation screen?

Answer 1

Assesses function of the clotting cascade (extrinsic and intrinsic)

Screens for congenital and acquired clotting disorders

Question 2

What triggers the intrinsic clotting cascade, and which substances initiate it?

Answer 2

Spontaneous internal damage of vascular endothelium

Clotting factors circulating in blood vessels: 12, 11, 9, 8

Question 3

What triggers the extrinsic clotting cascade, and which substances initiate it?

Answer 3

External trauma

Tissue factor from outside of blood vessels, 7

Question 4

Explain the intrinsic clotting cascade and the common pathway?

Answer 4

1. Factor 12 is activated to become 12a (a denotes activated form)
2. 12a amplifies activation of factor 11 to 11a
3. 11a amplifies activation of factor 9 to 9a
4. 9a amplifies activation of factor 8 to 8a
5. 9a and 8a together activate factor 10 to 10a, now becomes common pathway

Question 5

Explain the extrinsic clotting cascade and the common pathway?

Answer 5

1. Tissue factor amplifies activation of factor 7 to 7a
2. 7a amplifies activation of factor 10 to 10a, now becomes common pathway

Question 6

Explain the common pathway of the clotting cascade?

Answer 6

1. 10a converts prothrombin (factor 2) into thrombin (factor 2a)
2. Thrombin converts fibrinogen (factor 1) into fibrin

Question 7

Explain how blood clots are broken down?

Answer 7

Plasminogen produces plasmin, which breaks down the fibrin into Fibrin Degradation Products (FDPs)/D-dimers

Question 8

Explain platelet plug formation in primary haemostasis?

Answer 8

1. Inactive platelets are activated to aggregate (stick together) and form platelet mass, this plug is also formed by activated clotting factors which anchor neighbouring platelets together, and platelets to the subendothelium. Adhesive interactions cause these other clotting factors and platelets to be recruited to the site of injury, and phospholipids on the outside activate the clotting cascades.
2. Von Willebrand clotting factor becomes bound to exposed collagen, so that platelet surface receptors can bind to von Willebrand factors
3. Platelet-von Willebrand binding stimulates platelets to change from inactive discoid shape to activated shape with higher SA:V ratio which promotes aggregation to other platelets and other von Willebrand clotting factors.
4. Aggregation occurs until wound is completely filled

Question 9

Which 3 lab tests are in the coagulation screen?

Answer 9

Prothrombin time (PT)

Activated Partial Thromboplastin time (APTT)

Thrombin time (TT)

Question 10

What is the prothrombin time, and which clotting factors is it testing?

Answer 10

Time taken to form a blood clot through the extrinsic clotting cascade

Factor 7

Question 11

What is activated partial thromboplastin test, and what clotting factors is it testing?

Answer 11

Time taken to form a blood clot through the intrinsic clotting cascade

Factors 12, 11, 9, 8

Question 12

What is thrombin time?

Answer 12

Time taken for the conversion of fibrinogen to fibrin after the addition of thrombin to plasma

Common pathway (factor 10a, 2, 1)

Question 13

What causes a raised PT and all other coagulation tests are normal?

Answer 13

Factor 7 deficiency

Question 14

What causes a raised APTT and all other coagulation tests are normal?

Answer 14

Factor 8,9,11 deficiencies

Haemophilia A: Hereditary bleeding disorder due to congenital deficiency/total absence of clotting factor 8

Haemophilia B: Hereditary bleeding disorder due to congenital deficiency/total absence of clotting factor 9

Haemophilia C: Hereditary bleeding disorder due to congenital deficiency/total absence of clotting factor 11

Question 15

Which condition can increase the APTT, but this is a false reading and why?

Answer 15

Antiphospholipid syndrome

Antiphospholipids in blood can react with phospholipid activator, so there is less to react in the clotting cascade

Question 16

Which conditions can increase both PT and APTT?

Answer 16

Liver disease: Most clotting factors are made in the liver

DIC

Vitamin K deficiency: This is used to produce factors 2,7,9,10