Blood Clotting (PT/INR) Test

What it is

Clotting is a chain reaction, and prothrombin time measures one branch of it. A reagent is added to the plasma and the laboratory records the seconds until a clot forms. Because different reagents give different second counts, the result is converted into the INR, a ratio that lets a value from one lab be read against a value from another. The INR is the number patients recognize, and it exists precisely so that a person on warfarin can be monitored anywhere without the target shifting. It is not a general measure of bleeding risk, and it does not track the newer direct oral anticoagulants.

Why it is done

• To monitor warfarin and keep the dose inside the range chosen for that person's condition
• To investigate unusual bruising or bleeding, as one piece of a clotting workup
• To assess liver function, since the liver makes several of the clotting factors this test depends on
• To check clotting before surgery or another procedure where blood loss is a consideration

What to expect

Either a venous draw into a tube with a set amount of anticoagulant, or a fingerstick run on a small meter in an anticoagulation clinic or at home. The fingerstick gives a number in about a minute; the lab version comes back the same day. Nothing needs to be done to prepare, but diet, alcohol and other medications all move the INR, so clinicians usually ask what has changed since the last reading. People on stable warfarin are typically rechecked at intervals rather than once.

Questions worth asking

• Is this being billed as a lab test each visit, or under an anticoagulation management arrangement? The two can total very differently across a year of monitoring.
• Does the charge for a fingerstick in clinic differ from a venous draw sent to the lab, and is there a choice?
• If a home INR meter is an option, what does the device and its test strips cost against the running cost of clinic visits?
• Is a clinician visit being billed on top of the test on days when the only purpose was the INR check?