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Coagulation:- part 5 – INR, International Normalized Ratio (PT, PTT)

September 19, 2022HematologyLab Tests

Table of Contents

  • INR
        • Sample
        • Definition of INR
        • Indications for INR
        • Pathophysiology of INR and blood coagulation process
    • INR (International normalized ratio):
        • Use of INR:
        • Normal INR
      • The required range of INR in various diseases :
      • PT values in various diseases:
        • Warfarin therapy is given in:

INR

Sample

  1. This sample is just like for PTT or APTT.
  2. The blood sample is taken with ESR (sodium citrate) solution.
    • Mix blood 1.8 ml with ESR solution 0.2 ml.

Definition of INR

  1. This is the patient’s prothrombin time (PT) ratio and the normal mean PT time raised to the power of the international sensitivity index.

Indications for INR

  1. PT and PTT are advised to find the cause of unexplained bleeding or blood clot formation.
  2. INR is advised to monitor the blood thinning drug’s medication like warfarin therapy.
  3. This is done for routine health screenings.
  4. To evaluate whether the medical condition is improving or worsening.
  5. To assess to measure the success or failure of a medication or treatment plan.
  6. This measures the extrinsic pathway.

Pathophysiology of INR and blood coagulation process

  1. Prothrombin (Factor II) is produced in the liver and is Vit. K dependent.
  2. Vit. K deficiency leads to a decreased level of Prothrombin and gives rise to bleeding tendency.
  3. PT is used to evaluate the extrinsic pathway and common pathway.
Blood coagulation process: Coagulation pathways

Blood coagulation process: Coagulation pathways

Summary of the coagulation pathways

Summary of the coagulation pathways

Coagulation hemostasis mechanism

Coagulation hemostasis mechanism

Prothrombin leading to stable clot formation

Prothrombin leads to stable clot formation

  1. The first reaction of the body to active bleeding is blood vessel constriction.
  2. The first response to vascular injury:
    1. In a small vessel, the constriction may be enough to stop bleeding, while in large blood vessels hemostatic mechanism is involved.
  3. The primary phase of hemostasis is platelet aggregation and activation.
    1. The hemostatic system forms the clot which plugs the vessel.
  4. Secondary hemostasis: 
    1. It will first activate the tissue factor and then coagulation factors.
  5. The extrinsic pathway is activated by tissue factor and factor VII which stimulates factor X and enters the common pathway.
  6. The intrinsic pathway occurs through XII, XI, IX, and VIII and enters the common pathway by activating factors X and Xa.
Prothrombin facts

Prothrombin facts

INR (International normalized ratio):

  1. INR is a simple ratio. There is no unit of INR.
  2. This is calculated by dividing the patient PT by the mean of the normal range PT.
  3. The international sensitivity index (ISI) is 1.3 to 1.5. The following is the formula.
INR calculation

INR calculation

Use of INR:

  1. The prophylaxis  range of 2.0 to 3.0 is used to prevent:
    1. Embolism.
    2. Venous thrombosis.
    3. Pulmonary embolism.
    4. Myocardial infarction.
  2. In patients with anticoagulant therapy, the INR is maintained between 2 and 3.
  3. Raised range from 2.5 to 3.5 is used in the patient with mechanical heart valves.

Normal INR

  1. The INR test result is given as a number.
    1. INR = 1 represents equal to normal clotting time.
    2. INR = 2 represents twice the normal clotting time.
  2. Normal value =0.8 to 1.1

The required range of INR in various diseases :

Disease Required  range
DVT patient on warfarin treatment 2.0  to 3.0
DVT prophylaxis 1.5 to 2.0
Atrial fibrillation 2.0  to 3.0
Pulmonary embolism  2.5 to 3.5
Orthopedic surgery 2.0  to 3.0
Prosthetic valve prophylaxis 3.0 to 4.0
Prevention of embolus in a patient in atrial fibrillation 2 to 3

PT values in various diseases:

Clinical condition PT 
Factor VIII deficiency Normal
Factor XI deficiency Normal
Factor XII deficiency Normal
Factor II deficiency Prolonged
Factor V deficiency Prolonged
Factor VII deficiency Prolonged
Factor X deficiency Prolonged
Heparin therapy Prolonged
Vit. K deficiency Prolonged
Liver diseases Prolonged
Coumarin therapy Prolonged

Warfarin therapy is given in:

  1. Deep vein thrombosis
  2. Atrial fibrillation.
  3. Pulmonary embolism.
  4. Replacement of the artificial valves.
  5. Some cases of heart failure.
  • Critical value and be Careful when  INR = >5.5
    • Please, for more information, see PT and PTT.

Possible References Used
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