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INR from Prothrombin Times and Reagent ISI

Use matching time units and the calibrated reagent/instrument ISI, rather than assuming the preset 1.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

Patient PT 24 seconds, mean normal PT 12 seconds and ISI 1 give INR 2.

INR = (patient PT / control PT) raised to the reagent ISI.

Worked example

Enter these known values and leave the other values blank.

Patient PT (seconds)
24 sec
Mean normal PT (seconds)
12 sec
Calibrated international sensitivity index
1
International normalized ratio
2

Assumptions and limitations

  • Use matching time units and the calibrated reagent/instrument ISI, rather than assuming the preset 1.3 applies to a laboratory. Therapeutic targets depend on indication and clinician assessment; this calculation does not recommend a dose.
  • Arithmetic inverses use independent observations. Clear the unknown control before entering the remaining measurements.
  • This measurement calculation does not determine a diagnosis or treatment.

Common questions

How is this result calculated?

INR = (patient PT / control PT) raised to the reagent ISI.

Which measurements and units should I use?

Use matching time units and the calibrated reagent/instrument ISI, rather than assuming the preset 1.3 applies to a laboratory. Therapeutic targets depend on indication and clinician assessment; this calculation does not recommend a dose.

References

Bookify adds independently reviewed bounds for inr: ISI > 0.

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