The International Normalized Ratio of Prothrombin Time in the Model for End-Stage Liver Disease Score: A Reliable Measure
Section snippets
Generic causes of variation in the international normalized ratio
Generic causes of variation in the INR are related to incorrect determination of the MNPT, using an arithmetic mean rather than a geometric mean. Differences may also exist between the sodium citrate concentration used to determine the ISI by the manufacturer and the sodium citrate concentration used in the local laboratory. The ISI is typically calibrated for each reagent/instrument combination. Because different instruments use different end points (manual, photo-optical, or mechanical
Liver-specific causes of variation in the international normalized ratio
The liver-specific causes of an incorrect INR are related to the fact that the ISI has been standardized for patients on warfarin therapy (see also the article by Tripodi in this issue). Because patients who have liver disease have more complex coagulopathy, the ISI as calibrated for warfarin therapy may not be applicable to liver disease.
The Model for End-stage Liver Disease as a predictor of survival
On the other hand, virtually every study that has looked at the MELD score as a predictor of survival has demonstrated that the MELD score, using the INR with ISI calibrated for patients on warfarin, has a ‘c’ statistic of approximately 0.8, indicating excellent discrimination. Although the current method of measuring INR has drawbacks, the INR is the only standardized method currently available in the United States for expressing the prothrombin time. Therefore, the INR, a reliable measure of
Summary
Currently, the ISI used in clinical laboratories to calculate the INR is not calibrated based on prolongation of prothrombin time in patients who have chronic liver disease. As a result, variation in the conventionally measured INR (using warfarin-derived standards to determine the thromboplastin ISI) has been well documented in patients who have liver disease. Despite this limitation, virtually every study that has looked at the MELD score as a predictor of survival has demonstrated that the
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