International Normalized Ratio
The international normalized ratio ( INR ) is a medical laboratory parameters of the functional performance of the extrinsic system of blood clotting . The INR is determined using the thromboplastin time (TPZ; English prothrombin time , PT ). INR is a standardization of the Quick value .
| INR | Thromboplastin time | Quickwert | consequences | causes |
|---|---|---|---|---|
| INR << 1 | short clotting time | high | Thrombosis risk | |
| normal (0.85 <INR <1.27) | normal | normal (70% <Quick value <125%) | ||
| INR >> 1 | long clotting time | low | high tendency to bleed | anticoagulant drugs, vitamin K deficiency, liver damage |
An INR close to 1 is normal. An increased INR value (without taking anticoagulant medication beforehand) can result in eg. B. be caused by vitamin K deficiency or severe liver disease. If blood coagulation has to be reduced for therapeutic reasons, the INR target range is determined based on the risk. The following applies: the higher the INR, the higher the anticoagulation and thus the protection against embolism . At the same time, a higher INR also increases the risk of bleeding (e.g. due to bleeding from the gastrointestinal tract). In most cases (e.g. thrombosis , pulmonary embolism, atrial fibrillation ) the target range is between 2 and 3. With mechanical heart valves, however, the target range is higher, between 2.5 and 3.5 depending on the risk profile . With a high risk profile, in rare cases even with a target range of up to 4.5. With newer generations of mechanical valves, the target range after three months is between 1.5 and 2.0.
The measurement of the INR is especially important when the blood clotting time is prolonged by medication with coumarin derivatives as oral anticoagulation . In a training course, those treated in this way can learn to determine their INR value themselves and, if necessary, to vary the drug dose without the intervention of a doctor (self- coagulation management ).
Calculation formula from the prothrombin time PT:
where “test” is the prothrombin time of the sample to be tested, “normal” is the standard value and ISI is the international sensitivity index. This is determined for each manufacturer or each prothrombinase in comparison to an internationally standardized sample (normally between 1.0 and 1.4).
Conversion PT / INR / Quick
The conversion of individual laboratory values can only be carried out by the determining laboratory, as the parameters depend heavily on the thromboplastin preparation used and usually even have to be recalibrated for each series.
The following conversions can be used to compare a hypothetical therapeutic range expressed in Quick or INR:
