28/12/2023
QUESTIONS
1. What is the INR?
2. Why Do We Monitor it?
ANSWER
1. INR
The international normalised ratio (INR) is a laboratory measurement of how long it takes blood to form a clot.
It is used to determine the effects of oral anticoagulants on the clotting system.
Devised in 1983, the INR provides a standardised method of reporting the effects of an oral anticoagulant such as warfarin on blood clotting.
PROTHROMBIN TIME (PT)
Prior to this, the effects of warfarin on blood clotting were determined by a laboratory measure known as PT prothrombin time.
ADVANTAGES
- cheap
- simple to perform.
DISADVANTAGES
- large differences in the values obtained, depending on which laboratory the value was obtained from different commercial measuring systems can produce different results.
- it is impossible to compare results that come from different laboratories.
In an attempt to overcome the variability between laboratories, prothrombin times were initially expressed as a ratio of the prothrombin time of a control value.
Control value - the average of prothrombin times from 20 or more healthy subjects.
While expressing prothrombin times compared to a control value was seen as an improvement, it was still insufficient!
THE INTERNATIONAL NORMALIZED RATIO
So in 1983, the international sensitivity index (ISI), was applied to this ratio to derive the INR:
INR = (prothrombin test / prothrombin control) ISI
The ISI is a numerical value representing the responsiveness of any given commercial system relative to the international standard.
It takes into account the variability in results obtained using different commercial systems in calculating the result.
In this way, results from different laboratories and countries can be compared more readily.
Why do we monitor the INR?
We regularly monitor the INR of people using warfarin in order to
- Balance the risk of excessive bleeding (when the INR is too high, meaning that the blood is too thin)
- against the risk of clotting or thrombosis (when the INR is too low or the blood is too thick). INR values over 4.5 increase the risk of major haemorrhage (bleeding),
- when theINR less than 2 increases the risk of thromboembolism (formation of blood clots within the blood vessels) and associated conditions such as heart attack and stroke.
Regular monitoring is important with a drug such as warfarin because:
1. There is a very small difference between the lowest dose that gives a good effect and the highest dose before side effects (which may be serious) are experienced;
2. The doses used by different individuals to achieve the same effect can be highly variable;
3. Several drugs interact with the medication to either increase or decrease its effectiveness; and
4. What a person eats can also affect the action of warfarin and either increase or decrease its effectiveness.
Evidence suggests that more frequent testing will result in more time within the desired INR target range. For example, studies suggest that the desired results are achieved 50% of the time with monthly monitoring, compared to 85% of the time with weekly monitoring.
Anesthesia
