What is INR?

International normalized ratio (INR) is a standardization calculation and measurement that minimizes differences between thromboplastin reagents through a calibration process in which all commercial thromboplastins are compared with an International Reference Preparation (IRP) maintained by the World Health Organization (WHO). 

The INR method is not perfect in correcting for differences between laboratories using different thromboplastin reagents, but it does reduce the variation between different laboratories, especially in the therapeutic range.


What are the advantages for patients who self-test?

There are a number of advantages, including the following:

  • Self-monitoring helps patients participate in their treatment, leading to greater patient satisfaction and a higher quality of life.
  • Studies indicate that patient self testing leads to greater time in therapeutic range and fewer thromboembolic events.1
  • Patients need to make fewer trips to the hospital lab or your office for INR testing, enabling them to enjoy a greater quality of life.
  • Venous punctures are not necessary; only a drop of blood from the fingertip is required. 

Are the CoaguChek meters and supplies covered by insurance for patient self-testing?

If a patient has a mechanical heart valve, chronic atrial fibrillation or venous thromboembolism and is on long-term warfarin therapy, Medicare Part B may cover meter training, services, equipment and supplies for monitoring a patient’s INR at home with a portable handheld meter.2 Most private insurance companies also cover INR self-testing.3 (Coverage varies based on patient’s condition and other factors.)


How can I make it easier to get a drop of blood sample?

Try these tips:

  • Have the patient gently shake their arm down at their side for about 30 seconds.
  • Massage the finger to be pricked.
  • Have the patient warm their hand by holding it under their arm or using a hand warmer.


How is the INR value calculated?

The first step in calculating an INR is to measure the patient’s prothrombin time and normalize it to the population mean. INR value is also calculated from the patient’s coagulation time. However, the International Sensitivity Index (ISI) value is included in the calculation. Roche calculates the ISI value by comparison to an international standard thromboplastin. This way, the different sensitivities of reagents available in the market are compensated.


How are INRs from different manufacturers comparable? What does ISI mean?

By using the International Sensitivity Index (ISI), the different international normalized ratio (INR) can be compared. The results are then harmonized. Each manufacturer matches its reagent to a standard reagent, the reference thromboplastin, proposed by the World Health Organization (WHO). Each manufacturer determines the ISI value of its reagent lot by matching its thromboplastin production lot to a master lot, which is measured against the WHO reference thromboplastin. The ISI value determined this way enters the calculation of INR as an exponent value depending on lot and method.


What does an INR value of 1 or 2 mean?

A person who does not take any oral anticoagulants typically has an international normalized ratio (INR) value of 1. A measured INR value of 2 means that blood coagulation takes twice as long. Consequently, with an INR of 3, coagulation takes three times as long.

So, the higher the INR value (i.e., prolonged coagulation time), the more coagulation is inhibited.


What is the advantage of using the INR value?

International normalized ratio (INR) is the international unit, recommended worldwide, for prothrombin time (PT) determination. The World Health Organization (WHO) recommended the use of the INR instead of the Quick value as the unit for the measurement of coagulation in the early 1980s.

Reasons for using INR include the following:

  • INR is comparable (i.e., coagulation measuring values can be compared despite different thromboplastins).
  • INR allows a standardization of coagulation intensity for certain indication groups, independent of the thromboplastin and instrument used.


Is clotting time enhanced or inhibited by other medications?
Certain medications can increase the effect of warfarin or inhibit the effect of warfarin. Be sure to check the label of each medication you are prescribed and consult with your provider regarding any potential medication interactions.



  1. Heneghan CJ, Garcia-Alamino JM, Spencer EA, et al. Self-monitoring and self-management of oral anticoagulation. Cochrane Database of Systematic Reviews 2016, Issue 7. 
  2. Centers for Medicare and Medicaid Services, National Coverage Determination (NCD) for Home Prothrombin Time/International Normalized Ratio (PT/INR) Monitoring for Anticoagulation Management (190.11)
  3. Coverage varies by plan
  4. Matcher, et al. “Effect of Home Testing of International Normalized Ratio on Clinical Events.” New England Journal of Medicine. 363.17 (Oct. 2010) : 1608-1620.