The INR reference value for a patient not taking a vitamin K antagonist is 1.0. The Geometric Mean Normal Prothrombin Time (GMNPT) for a group of 20 normal donors not on anticoagulants was 12.1s. The table below illustrates a number of patients with varying PTs and in whom the INR was calculated using three thromboplastins with varying ISIs. So for a patient on Warfarin with a PT of 23 seconds and a mean normal PT of 12 seconds using a Thromboplastin with an ISI of 1.2, the INR is 2.18: It is the value for the Prothrombin Time Ratio which has been obtained using the first WHO Reference Thromboplastin with an ISI of 1.0. The International Normalised Ratio (INR) is the PT ratio of a test sample compared to a normal PT (derived from the log mean normal prothrombin time (LMNPT) of 20 normal donors) corrected for the sensitivity of the Thromboplastin used in the test. This involves testing a set of plasma samples with known INRs with the a laboratory-specific Thromboplastin and on the coagulometers which will be used to derive the PT.Ģ. For these reasons, a local calibration of thromboplastins is recommended. Whilst the development of an International Reference Thromboplastin resulted in significant improvements in the standardisation of anticoagulant control, the use of different coagulometers for the PT and the differing methods of end-point detection can lead to significant variations in PT. In contrast if the ISI of the Thromboplastin is 2.0 an increase in the PT from 15s to 30s results in an INR outwith the therapeutic range. Using a Thromboplastin with an ISI of 1.0 the PT can range from 15s - 35s but the INR is still in the therapeutic range. The table below under INR illustrates this. Thromboplastins with high ISIs are less sensitive to small changes in the PT. Thromboplastins should be chosen with an ISI close to 1.0. So if the ISI of the Reference Thromboplastin is 1.1 and the ISI derived from the slope is 1.14, the ISI of the Test Thromboplastin is 1.1 x 1.14 = 1.25. So the ISI of the test Thromboplastin is calculated from the formula: The slope of the line is calculated and this represents the ISI.ĭistance A to C = 125mm and distance B to C = 110mm For more information relating to the derivation of the ISI - see References.Ī best fit line is drawn with points above the highest recorded PT and the lowest PT (see figure above). The PTs of all the plasma samples are converted to the corresponding logarithms, an orthogonal regression line is calculated and from which the ISI can be derived. The ISI can be calculated in (at least) two ways: Deriving the ISI A line of best fit is drawn and the slope of this line is the ISI. The use of the double-log paper removes the necessity to derive the log for each PT result. The mean of each pair of PT results are plotted on double-log paper with the reference sample on the Y axis and the test plasma on the X-axis. If there is more than a 10% difference in the clotting times between duplicate samples, the tests on that plasma sample should be repeated. Tests are historically performed on 20 normal donors not on anticoagulants and 60 patients who have been on oral anticoagulant treatment for at least 6 weeks. Prothrombin Times are performed in duplicate for each Thromboplastin and the mean for each pair of tests derived. human against human, rabbit against rabbit etc. The calibration of a test Thromboplastin must be against a reference Thromboplastin of the same species e.g. Subsequent WHO IRPs contain no adsorbed bovine plasma. The first WHO IRP was a human brain extract to which adsorbed bovine plasma was added to optimise the content of the non-vitamin K dependent coagulation factors - adsorption with Al(OH) 3 removes all the vitamin K dependent proteins]. The first WHO reference Thromboplastin was assigned an ISI of 1.0 and it is against this (and subsequent reference preparations) that all other Thromboplastin are calibrated. Individual Thromboplastins can be calibrated against an International WHO reference Thromboplastin (International Reference Preparation or IRP) to assign them an International Sensitivity Index or ISI. This difference in sensitivities is known as the sensitivity index. rabbit, human, bovine etc) used in the assay. The Prothrombin Time (PT) in an individual with one or more deficiencies of a clotting factor will vary with the type of Thromboplastin (e.g. International Sensitivity Index & The International Normalised Ratio
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