PT and INR
Also written as: PT, INR, prothrombin time, international normalized ratio
Times one clotting pathway; the INR standardizes it, mainly for monitoring warfarin.
What it measures
The prothrombin time (PT) is how many seconds a plasma sample takes to clot when one arm of the clotting system is triggered in the lab. The INR is the same result converted onto a standard scale, so values are comparable between different labs.
Why a doctor orders it
Ordered to monitor the blood thinner warfarin, which is what the INR scale was designed for. It is also used before surgery, to investigate unusual bleeding or bruising, and to gauge how well the liver is making clotting proteins.
What is generally associated with a higher result
Broadly associated with slower-than-average clotting. Recognized causes include warfarin and some other blood thinners, low vitamin K, liver disease, and shortages of specific clotting factors. In people taking warfarin, diet changes and antibiotics commonly move the INR.
What is generally associated with a lower result
Broadly associated with faster-than-average clotting. Mildly low values on their own are generally not pursued.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- A difficult blood draw or an underfilled collection tube
- Vitamin K intake from leafy green vegetables
- Antibiotics and many other common medicines
- Alcohol intake
- Delay between drawing the sample and running the test
Questions to take to your appointment
- Could a change in my diet or a newly started medicine have shifted this?
- Could the way the sample was collected have affected the result?
- If I am not on a blood thinner, what was this test checking for?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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