Dr. S. Parthasarathy MD DA DNB

PhD (physio)Mahatma Gandhi medical

college and research institute , puducherry – India

Prothrombin time

Coagulation factors• I (fibrinogen) Forms clot (fibrin) • II (prothrombin) • III) Calcium • phospholipid (formerly known as factor IV) • V (proaccelerin, labile factor• VI Unassigned – old name of Factor Va • VII (stable factor, proconvertin) • VIII (Antihemophilic factor A) • IX (Antihemophilic factor B or Christmas factor) • X (Stuart-Prower factor) • XI (plasma thromboplastin antecedent) • XII (Hageman factor) • XIII (fibrin-stabilizing factor) Crosslinks fibrin

INITIATION OF BLOOD COAGULATION

Extrinsic Pathway

Tissue trauma

Leakage of Tissue Factor

X Xa

Prothrombin activator

Ca+2, factor VII

Ca+2

Prothrombin Thrombin(factor II)

Ca+2

Intrinsic Pathway

Blood trauma/ contact with collagen

Activation of factor XII, IX, VIII

X Xa

Ca+2

Prothrombin activator

Prothrombin Thrombin(factor II)

Thrombin converts fibrinogen to fibrin

Prothrombin time tests extrinsic pathway

• Performed by adding a mixture of calcium and thromboplastin to citrated plasma

• As a control, a normal blood sample is tested continuously

• PT ratio (PTR) = Patient’s PT Control PT

Normal value

• 11- 14 seconds

• What is INR?

• Thromboplastins are extracts from brain, lung or placenta of animals

• Thromboplastins from various manufacturers differ in their sensitivity to prolong PT

• May result in erratic control of anticoagulant therapy

INR = [PTpt] ISI

[PTRef]

PTpt – prothrombin time of patient

PTRef – prothrombin time of normal pooled sample

ISI – International Sensitivity Index

ISI – International Sensitivity Index

• Thromboplastin may vary.• Each manufacturer gives ISI value• With reference to international

control.• If the manufacturer gives 1 , it

means ,the sensitivity is same• If 2, batch is half sensitive.• Usually between 1 to 1.4

Example

• Patient PT = 16 sec.• Control = 12 seconds.• 16/12 = 1.33.

• But if ISI is 2 • INR = (1.33)2

• = 1.768.

Prothrombin time

• Factor 7

• It is also prolonged in 1,2,5,10

• But not useful for factor 8 and 9

importance of Prothrombin time

• Liver disease• Warfarin therapy• Vit. K deficiency

Factors raise INR

• Chr. Alcohol• Anabolic steroids• Septran• Propranolol• Metrogyl• Fluroquinolones.• INH,omez

Factors decrease INR

• Barbiturates• Carbamazipine.• Griseofulvin• Sucralfate.• Rifampicin.

• Thank you all!!