Hemostasis Testing

Question 1

Tests for Fibrin Formation

Answer 1

• Prothrombin time / INR
• Activated partial thromboplastin time (aPTT or PTT)
• Thrombin time
• Fibrinogen level
• Mixing study (screen for inhibitors)

Question 2

Tests for Fibrinolysis

Answer 2

• D-dimer: higher levels = more lysis
• Alpha-2 antiplasmin: protease inhibitor that inhibits plasmin so that as plasmin is generated it is used up; Cannot distinguish increased consumption due to rapid plasmin generation from decreased production due to liver disease – not sure if lack of making or more breaking

Question 3

PT/INR

Answer 3

Prothrombin Time / International Normalized Ratio

PT: thromboplastin (tissue factor, phospholipid, calcium) added to plasma; time to clot is measured; longer clotting time when deficit in VII, X, V, II, and fibrinogen (not sensitive to XI, IX, VIII, or contact factor levels; also insensitive to heparin)

INR: way to make PT tests more comparable between labs; INR = (patient PT/mean normal PT)^ISI

• ISI = international sensitivity index; specific to the type of thromboplastin used; higher INR means slower clotting

Question 4

aPTT (PTT)

Answer 4

Activated Partial Thromboplastin Time (aPTT)

• Incubate citrated plasma with contact system activator
• Sensitive to levels of contact factors XI, IX, VII and all of the common pathway factors
• More sensitive to heparin than PT; can be used to monitor heparin

Question 5

What causes a long PT/INR and a normal PTT?

Answer 5

• Factor VII deficiency
• Mild deficiency of “common pathway” factors
  
  • Warfarin
  • Vitamin K deficiency
  • Liver disease

Question 6

What causes a long PTT and a normal INR?

Answer 6

• Deficiency of VIII, IX, XI
• Deficiency of a contact factor (usually XII) – does not cause bleeding
• Heparin
• Factor VIII inhibitor

Question 7

What if both PT/INR and PTT are long?

Answer 7

• Liver disease
• Vitamin k deficiency
• Warfarin
• DIC
• High level of heparin
• Other inhibitor affecting common pathway (i.e. direct thrombin inhibitor)
• Isolated deficiency of X, V, II, fibrinogen (rare)

Question 8

Thrombin Time

Answer 8

• Put thrombin in plasma and see how long it takes to clot
• Very sensitive to heparin and other thrombin inhibitors
• Also prolonged by low fibrinogen, dysfibrinogenemia, high levels of fibrin degradation products
• note: too sensitive to use to monitor heparin; aPTT used to monitor heparin instead

Question 9

Mixing Study

Answer 9

• to determine whether long aPTT or PT is due to clotting factor deficiency or circulating inhibitor (i.e. fact or VIII inhibitor, heparin, lupus-type inhibitor)
• mix patient plasma 1:1 with normal plasma, measure aPTT or PTT
• Failure to correct prolonged clotting time by mixing with normal plasma implies presence of a circulating inhibitor
• Note: warfarin does not act as a circulating inhibitor; it inhibits synthesis of clotting factors by the liver

Question 10

Platelet function analysis (PFA-100)

Answer 10

• Citrated whole blood passed through a capillary tube coated with collagen plus either ADP or epinephrine
• Platelets adhere to collagen (requires VWF from patient plasma) and are activated
• Platelet plug forms and occludes tube
• Time to occlusion is measured
• Moderately sensitive to defects in platelet function

Question 11

Ristocetin

Answer 11

Ristocetin assay looks at GP1 receptors and VWFs - force them to clump together; in the absence of either vWF or GP1 receptor there won’t be clumping – no clumping indicates that one of these is deficient