The plasma is analyzed by a medical technologist on an automated instrument at 37°C, which takes a sample of the plasma. An excess of calcium is added (thereby reversing the effects of citrate), which enables the blood to clot again. For an accurate measurement the proportion of blood to citrate needs to be fixed; many laboratories will not perform the assay if the tube is underfilled and contains a relatively high concentration of citrate. This is because Vacutainer test tubes generally contain a powdered anticoagulant to prevent blood from clotting. For the prothrombin time test the appropriate sample is the blue top tube, or citrate tube, which is a liquid anticoagulant. Just as adding solvent to any solution will dilute it, adding liquid anticoagulant to blood will dilute it. This dilution will cause a falsely long prothrombin time. So, all analysis takes this dilution into account by multiplying the result by 1.1 to account for the dilution. If a tube is underfilled or overfilled with blood, the standardized dilution of 1.1 is no longer valid.
Tissue factor (also known as factor III or thromboplastin) is added, and the time the sample takes to clot is measured optically. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic and icteric samples.
The prothrombin ratio is the prothrombin time for a patient, divided by the result for control plasma.
Each manufacturer gives an ISI (International Sensitivity Index) for any tissue factor they make. The ISI value indicates how the particular batch of tissue factor compares to an internationally standardized sample. The ISI is usually between 1.0 and 1.4.
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 control sample used.
The speed of the extrinsic pathway is greatly affected by levels of factor VII in the body. Factor VII has a short half-life and its synthesis requires vitamin K. The prothrombin time can be prolonged as a result of deficiencies in vitamin K, which can be caused by warfarin, malabsorption, or lack of intestinal colonization by bacteria (such as in newborns). In addition, poor factor VII synthesis (due to liver disease) or increased consumption (in disseminated intravascular coagulation) may prolong the PT.
A high INR level such as INR=5 indicates that there is a high chance of bleeding, whereas if the INR=0.5 then there is a high chance of having a clot. Normal range for a healthy person is 0.9–1.3, and for people on warfarin therapy, 2.0–3.0, although the target INR may be higher in particular situations, such as mechanical heart valves.
In a typical NPT setup a small table-top device is used; for example the Roche Coaguchek S, or the more recently (2005) introduced HemoSense INRatio. A drop of capillary blood is obtained with an automated finger-prick, which is almost painless. This drop is placed on a disposable test strip with which the machine has been prepared. The resulting INR comes up on the display a few seconds later. Similar testing methods are used by diabetics on insulin, and are easily taught and practiced.
Local policy determines whether the patient or a coagulation specialist (nurse, general practitioner or hospital doctor) interprets the result and determines the dose of medication. In Germany, patients may adjust the medication dose themselves, while in the UK and the USA this remains in the hands of a health care professional.
The 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. In the UK these are available on prescription so that elderly and unwaged people will not pay for them and others will pay only a standard prescription charge, which at the moment represents only about 20% of the retail price of the strips. In the USA, NPT in the home is currently reimbursed by Medicare for patients with mechanical heart valves, while private insurers may cover for other indications.
There is some evidence to suggest that NPT may be less accurate for certain patients, for example those who have the lupus anticoagulant.
Medicare coverage for home testing of INR has been expanded in order to allow more people access to home testing of INR in the USA. The release on the 19th 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, “[t]hose 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 introduced in the early 1980s when it turned out that there was a large degree of variation between the various prothrombin time assays, a discrepancy mainly due to problems with the purity of the thromboplastin (tissue factor) concentrate. The INR became widely accepted worldwide, especially after endorsement by the World Health Organisation.