UHS and UTHSCSA
Department of Pathology


Typical Relationship of the PT (seconds) to the INR

Assume: GeoMean of Controls = 11.9 Seconds; ISI of Reagent = 1.19
INR PT Ratio PT   INR PT Ratio PT
0.7 0.74 9   5.4 4.12 49
0.8 0.83 10   5.5 4.19 50
0.9 0.92 11   5.6 4.25 51
1 1 12   5.7 4.31 51
1.1 1.08 13   5.8 4.38 52
1.2 1.17 14   5.9 4.44 53
1.3 1.25 15   6 4.5 54
1.4 1.33 16   6.1 4.57 54
1.5 1.41 17   6.2 4.63 55
1.6 1.48 18   6.3 4.69 56
1.7 1.56 19   6.4 4.76 57
1.8 1.64 19   6.5 4.82 57
1.9 1.71 20   6.6 4.88 58
2 1.79 21   6.7 4.94 59
2.1 1.86 22   6.8 5 60
2.2 1.94 23   6.9 5.07 60
2.3 2.01 24   7 5.13 61
2.4 2.09 25   7.1 5.19 62
2.5 2.16 26   7.2 5.25 62
2.6 2.23 27   7.3 5.31 63
2.7 2.3 27   7.4 5.37 64
2.8 2.37 28   7.5 5.43 65
2.9 2.45 29   7.6 5.49 65
3 2.52 30   7.7 5.55 66
3.1 2.59 31   7.8 5.62 67
3.2 2.66 32   7.9 5.68 68
3.3 2.73 32   8 5.74 68
3.4 2.8 33   8.1 5.8 69
3.5 2.86 34   8.2 5.86 70
3.6 2.93 35   8.3 5.92 70
3.7 3 36   8.4 5.98 71
3.8 3.07 37   8.5 6.04 72
3.9 3.14 37   8.6 6.1 73
4 3.2 38   8.7 6.15 73
4.1 3.27 39   8.8 6.21 74
4.2 3.34 40   8.9 6.27 75
4.3 3.4 41   9 6.33 75
4.4 3.47 41   9.1 6.39 76
4.5 3.54 42   9.2 6.45 77
4.6 3.6 43   9.3 6.51 77
4.7 3.67 44   9.4 6.57 78
4.8 3.73 44   9.5 6.63 79
4.9 3.8 45   9.6 6.69 80
5 3.86 46   9.7 6.74 80
5.1 3.93 47   9.8 6.8 81
5.2 3.99 48   9.9 6.86 82
5.3 4.06 48   10 6.92 82
Upper limit of the reference range for children and adults is an INR of 1.2
Upper limit of the reference range for infants (0-6 months) is an INR of 1.6
Shaded this color, the therapeutic interval for oral anticoagulant treatment of most cases of venous thromboembolism is INR=2.0-3.0

Calculation of the INR

The International Normalized Ratio (INR) was developed to compensate for the wide variation among reagent and instrument systems. The sensitivity of a reagent used with an instrument is compared to an international standard. The relationship of the test system to the standard, the International Sensitivity Index (ISI), is determined. The INR of the patient can then be calculated by the formula:

INR = (PTpat/PTcont)ISI

 

Where PTpat is the protime of the patient and PTcont is the gometric mean of >20 normal patient prothrombin times.


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Last updated on 16 July, 2004 by John D. Olson, M.D., Ph.D.