Coagulation:- part 5 – INR, International Normalized Ratio (PT, PTT)
- This sample is just like for PTT or APTT.
- The blood sample is taken with ESR (sodium citrate) solution.
- Mix blood 1.8 ml with ESR solution 0.2 ml.
Definition of INR
- 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
- PT and PTT are advised to find the cause of unexplained bleeding or blood clot formation.
- INR is advised to monitor the blood thinning drug’s medication like warfarin therapy.
- This is done for routine health screenings.
- To evaluate whether the medical condition is improving or worsening.
- To assess to measure the success or failure of a medication or treatment plan.
- This measures the extrinsic pathway.
Pathophysiology of INR and blood coagulation process
- Prothrombin (Factor II) is produced in the liver and is Vit. K dependent.
- Vit. K deficiency leads to a decreased level of Prothrombin and gives rise to bleeding tendency.
- PT is used to evaluate the extrinsic pathway and common pathway.
- The first reaction of the body to active bleeding is blood vessel constriction.
- The first response to vascular injury:
- In a small vessel, the constriction may be enough to stop bleeding, while in large blood vessels hemostatic mechanism is involved.
- The primary phase of hemostasis is platelet aggregation and activation.
- The hemostatic system forms the clot which plugs the vessel.
- Secondary hemostasis:
- It will first activate the tissue factor and then coagulation factors.
- The extrinsic pathway is activated by tissue factor and factor VII which stimulates factor X and enters the common pathway.
- The intrinsic pathway occurs through XII, XI, IX, and VIII and enters the common pathway by activating factors X and Xa.
INR (International normalized ratio):
- INR is a simple ratio. There is no unit of INR.
- This is calculated by dividing the patient PT by the mean of the normal range PT.
- The international sensitivity index (ISI) is 1.3 to 1.5. The following is the formula.
Use of INR:
- The prophylaxis range of 2.0 to 3.0 is used to prevent:
- Venous thrombosis.
- Pulmonary embolism.
- Myocardial infarction.
- In patients with anticoagulant therapy, the INR is maintained between 2 and 3.
- Raised range from 2.5 to 3.5 is used in the patient with mechanical heart valves.
- The INR test result is given as a number.
- INR = 1 represents equal to normal clotting time.
- INR = 2 represents twice the normal clotting time.
- Normal value =0.8 to 1.1
The required range of INR in various diseases :
|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:
|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|
|Vit. K deficiency||Prolonged|
Warfarin therapy is given in:
- Deep vein thrombosis
- Atrial fibrillation.
- Pulmonary embolism.
- Replacement of the artificial valves.
- Some cases of heart failure.
- Critical value and be Careful when INR = >5.5
- Please, for more information, see PT and PTT.