Coag screen interpretation

Question 1

Background knowledge

Answer 1

Natural anticoagulants: proteins C and S (inactivate factors V and VIII); antithrombin (inactivates many clotting factors)

Vitamin K dependent clotting factors: II, VII, IX, X (+ proteins C and S)

Question 2

PT and INR = extrinsic

Answer 2

Only factor VII involved in extrinsic pathway
Isolated factor VII deficiencies are rare so PT / INR is only affected by globally reduced clotting factor synthesis or increased consumption:

Warfarin / vit K deficiency
Liver disease
DIC

Question 3

APTT = intrinsic

Answer 3

Involves similar clotting factors to extrinsic pathway + VIII, IX and XI so is affected by:

Warfarin / vit K deficiency
Liver disease
DIC
Plus anything that affects factor VIII (haemophilia A, vWB), factor IX (haemophilia B) or factor XI (haemophilia C)

Question 4

Bleeding time = platelet function

Answer 4

Measures platelet plug formation

Rarely used in practice

Question 5

Fibrinogen

Answer 5

Clotting factor involved in final stages of coagulation
Reflects blood’s clotting ability (low = increased bleeding risk; high = may increase clotting)
Low levels due to increased consumption (DIC) or decreased production (liver disease, malnutrition)
Acute phase protein so may be high due to inflammation, malignancy, trauma, infection

Question 6

Factor synthesis problems

Answer 6

Vitamin K deficiency - can be due to fat malabsorption or a simple dietary deficiency. Affects clotting factors 2,7,9 and 10 therefor both pathways.

Liver failure - global deficiency affecting both extrinsic and intrinsic pathways. Management is mainly supportive.

Question 7

Consumption

Answer 7

DIC - in severe systemic illness dying cells release procoagulants resulting in fibrin generation which can occlude small vessels. This consumes platelets and clotting factors resulting in bleeding elsewhere. Thrombocytopaenia, increased PT and APTT, raised d-dimer and fibrin degredation products. TX CAUSE.

Question 8

Drugs

Answer 8

Warfarin (vitamin K antagonist) - factors 2, 7, 9 and 10. Monitored using INR (extrinsic pathway).

Heparin (increases antithrombin activity):
SC LMWH (e.g. enoxaparin) - activity mediated by inhibition of factor Xa. Effects more predictable therefor monitoring not required. Anti-factor Xa assay.
Unfractionated heparin - use in renal failure. Only works for 4 hours. Monitor using APTT ratio.

Question 9

Deficiencies

Answer 9

Haemophilia A (VIII), B (IX), C (XI) - haemarthroses and muscle haematomas

VWd - platelet disorder of bleeding (petechiae, menorrhagia, contact bleeding)

Question 10

Autoimmune

Answer 10

Anti-phospholipid syndrome - arterial and venous thromboembolism. Prolonged APTT.