They are used to determine the clotting tendency of blood, in conditions such as the measure of warfarin dosage, liver damage (cirrhosis), and vitamin K status.

[1] Prothrombin time is typically analyzed by a laboratory technologist on an automated instrument at 37 °C (as a nominal approximation of normal human body temperature).

[5][6][7] The INR is the ratio of a patient's prothrombin time to a normal (control) sample, raised to the power of the ISI value for the analytical system being used.

In addition, poor factor VII synthesis (due to liver disease) or increased consumption (in disseminated intravascular coagulation) may prolong the PT.

[13] In addition to the laboratory method outlined above, near-patient testing (NPT) or home INR monitoring is becoming increasingly common in some countries.

In the United Kingdom, for example, near-patient testing is used both by patients at home and by some anticoagulation clinics (often hospital-based) as a fast and convenient alternative to the lab method.

A similar form of testing is used by people with diabetes for monitoring blood sugar levels, which is easily taught and routinely practiced.

In Germany and Austria, patients may adjust the medication dose themselves,[citation needed] while in the UK and the US this remains in the hands of a health care professional.

A meta analysis which reviewed 14 trials showed that home testing led to a reduced incidence of complications (bleeding and thrombosis) and improved the time in the therapeutic range, which is an indirect measure of anticoagulant control.

Other advantages of the NPT approach are that it is fast and convenient, usually less painful, and offers, in home use, the ability for patients to measure their own INRs when required.

Among its problems are that quite a steady hand is needed to deliver the blood to the exact spot, that some patients find the finger-pricking difficult, and that the cost of the test strips must also be taken into account.

The release on 19 March 2008 said, "[t]he Centers for Medicare & Medicaid Services (CMS) expanded Medicare coverage for home blood testing of prothrombin time (PT) International Normalized Ratio (INR) to include beneficiaries who are using the drug warfarin, an anticoagulant (blood thinner) medication, for chronic atrial fibrillation or venous thromboembolism."

In addition, "those Medicare beneficiaries and their physicians managing conditions related to chronic atrial fibrillation or venous thromboembolism will benefit greatly through the use of the home test.

The INR was invented in the early 1980s by Tom Kirkwood working at the UK National Institute for Biological Standards and Control (and subsequently at the UK National Institute for Medical Research) to provide a consistent way of expressing the prothrombin time ratio, which had previously suffered from a large degree of variation between centres using different reagents.