International Sensitivity Index (ISI)
Quick values measured with different thromboplastins cannot be directly compared with each other as, for example, sensitivity to clotting factors may differ. To make clotting times as comparable as possible, the WHO approved a reference standard thromboplastin in 1983. All thromboplastin manufacturers must calibrate their reagent against the WHO standard (there are two references: one for recombinant human thromboplastins and one for rabbit brain thromboplastins). The value obtained is known as the “international sensitivity index” (ISI). This allows the different sensitivities of thromboplastins to be known and is used to calculate the INR.
WHO recommends that the ISI be less than 1.7. In many countries, a low ISI (1.0) is preferred.
International normalized ratio (INR)
The INR is a standardization method whose objective is to reduce differences between thromboplastin reagents through a calibration process in which all commercial thromboplastins are compared with an international reference preparation (IRP) preserved by the WHO.
The INR method is not perfect at correcting for differences between different laboratories using different thromboplastin reagents, although it does reduce the variation between different laboratories and provides clinically useful results.
The INR should only be used in patients receiving stable anticoagulant therapy.
INR Calculation
Calculation
TPNM (mean average TP) is the geometric mean of the TP of a minimum of 20 healthy individuals in a defined laboratory population.
For example, the PT of a patient receiving oral anticoagulant treatment is 64 seconds (= 18% Quick). The prothrombin time of a normal plasma is 22 seconds (= 100% Quick). The ISI of the thromboplastin used is 0.93. Substituting this value in the formula above, the following INR is obtained:
(64) / (22) 0.93 = 2.7 INR
This means a clotting time that is 2.7 times longer than the standard.
The longer the patient’s clotting time, the higher the INR.
To understand what this blood test consists of, it is necessary to know what prothrombin is. It is a protein that is produced in the liver, converting fibrinogen into fibrin. This, together with the platelets, has the function of preventing bleeding. For this reason, it is also known as Coagulation Factor II, as it is the primary determination used in the control of anticoagulant treatment. Therefore, the prothrombin time (PT) is the time it took for the blood to clot, preventing hemorrhage or bleeding. The usual thing is that it oscillates between 10 and 14 seconds.
In the body, the clotting process contains chemical reactions that lead to the final formation of a blood clot. But for that to happen, all the clotting factors need to function normally. If this does not happen, two unwanted cases can occur:
Excessive clotting.
Inadequate bleeding.This test, which is carried out through a blood test, is used in different scenarios: in diagnosing liver problems, so that the doctor knows if the anticoagulant medicine that the patient takes is effective and to know how the blood of a person about to undergo surgery.
It is also requested when there is a bleeding disorder in which there are symptoms such as frequent bruising, heavy nosebleeds, frequent bleeding from the gums, blood in the urine, heavy bleeding during menstruation, or increased joint pain.
High prothrombin time
A prolonged high PT indicates that it takes too long for the blood to form a blood clot. This parameter is often interpreted in conjunction with the partial thromboplastin time (TTP).
We can mention coagulation pathologies among the causes that prolong this time and, therefore, favour bleeding. Some of them are:
Hemophilia.
Vitamin K deficiency.
Use of anticoagulants.
Gastrointestinal diseases.Low prothrombin time
When the prothrombin time is low, we must interpret that the clotting time occurs too fast, which increases the risk of the formation of clots, which can cause diseases such as:
Thromboembolism
Acute myocardial infarction.
Cerebrovascular disease.Among the causes, we can mention the increase in vitamin K through an incorrect diet or supplements containing this vitamin. Another major cause is the use of estrogens (birth control pills).
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Prothrombin time or PT is a blood test that evaluates the blood’s ability to clot, that is, the time it takes to stop bleeding, for example.
So the prothrombin time test is used whenever frequent bleeding or bruising occurs to detect the cause of the problem and when liver problems are suspected, and liver enzyme measurements are also requested like TGO, TGP and GGT, for example. Learn more about liver function tests.
In the case of people who use oral anticoagulants such as Warfarin or Aspirin, the doctor will periodically request the INR, which is a more specific measure than the PT, to evaluate the effect of the drugs since the PT is usually found high under those conditions.
Prothrombin, also known as coagulation factor II, is a protein produced by the liver, which, when activated, promotes the conversion of fibrinogen to fibrin, which together with platelets, forms a layer that prevents bleeding. So prothrombin is an essential factor for blood clotting to happen.
Prothrombin Time: What It Is and Why It May Be High
Normal values
The standard value of prothrombin time for a healthy person should range from 10 to 14 seconds. In the case of INR, the value of a healthy person must vary between 0.8 and 1.
However, if you are using oral anticoagulants, the value should be between 2 and 3, depending on the disease you need to treat with this type of medication.
