INR Draws (Prothrombin time blood draws)
Prothrombin time (PT) is a blood test that measures how long it takes blood to
clot. A prothrombin time test can be used to check for bleeding problems. PT
is also used to check whether medicine to prevent blood clots is working.
A PT test may also be called an INR test. INR (international normalized ratio)
stands for a way of standardizing the results of prothrombin time tests, no
matter the testing method.
Blood clotting factors are needed for blood to clot (coagulation).
Prothrombin, or factor II, is one of the clotting factors made by the liver.
Vitamin K is needed to make prothrombin and other clotting factors.
Prothrombin time is an important test because it checks to see if five
different blood clotting factors (factors I, II, V, VII, and X) are present.
The prothrombin time is made longer by:
Blood-thinning medicine, such as heparin. Another test, the activated
partial thromboplastin time (APTT) test, is a better test to find out if the
right dose of heparin is being used.
- Low levels of blood clotting factors.
- A change in the activity of any of the clotting factors.
- The absence of any of the clotting factors.
- Other substances, called inhibitors, affect the clotting factors.
- An increase in the use of the clotting factors.
An abnormal prothrombin time is often caused by liver disease or injury or by
treatment with blood thinners.
Another blood clotting test, called partial thromboplastin time (PTT),
measures other clotting factors. Partial thromboplastin time and prothrombin
time are often done at the same time to check for bleeding problems or the
chance of too much bleeding in surgery.
Why Is It Done?
Prothrombin time (PT) is measured to:
- Find a cause for abnormal bleeding or bruising.
Check to see if blood-thinning medicine, such as warfarin (Coumadin), is
working. If the test is done for this purpose, a PT may be done every day at
first. When the correct dose of medicine is found, you will not need so many
Check for low levels of blood clotting factors. The lack of some clotting
factors can cause bleeding disorders such as hemophilia, which is passed in
Check for a low level of vitamin K. Vitamin K is needed to make prothrombin
and other clotting factors.
Check how well the liver is working. Prothrombin levels are checked along
with other liver tests, such as aspartate aminotransferase and alanine
Check to see if the body is using up its clotting factors so quickly that
the blood can’t clot and bleeding does not stop. This may mean the
person has disseminated intravascular coagulation (DIC).
How Should I Prepare?
Many medicines can change the results of this test. Be sure to tell your
doctor about all the nonprescription and prescription medicines you take, as
well as any supplements or herbal remedies you use.
How Is It Done?
The health professional drawing blood will:
Wrap an elastic band around your upper arm to stop the flow of blood. This
makes the veins below the band larger so it is easier to put a needle into
- Clean the needle site with alcohol.
Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
Put a gauze pad or cotton ball over the needle site as the needle is
- Put pressure on the site and then a bandage.
What are the Risks?
There is very little chance of a problem from having a blood sample taken from
You may get a small bruise at the site. You can lower the chance of bruising
by keeping pressure on the site for several minutes.
In rare cases, the vein may become swollen after the blood sample is taken.
This problem is called phlebitis. A warm compress can be used several times
a day to treat this.
Ongoing bleeding can be a problem for people with bleeding disorders.
Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make
bleeding more likely. If you have bleeding or clotting problems, or if you
take blood-thinning medicine, tell your doctor before your blood sample is
What Affects the Test?
Reasons you may not be able to have the test or why the results may not be
Taking medicines that can affect the action of blood thinners (such as
warfarin) and vitamin K. These include antibiotics, aspirin, cimetidine
(Tagamet), barbiturates, birth control pills, hormone replacement therapy
(HRT), and vitamin K supplements.
Having severe diarrhea or vomiting that causes fluid loss and dehydration.
This may make the PT time longer. If diarrhea is caused by poor absorption
of nutrients, vitamins, and minerals from the intestinal tract
(malabsorption syndrome), the PT may be longer because of a lack of vitamin
Eating foods that have vitamin K, such as beef liver, pork liver, green tea,
broccoli, chickpeas, kale, turnip greens, and soybean products.
- Drinking a lot of alcohol.
- Taking laxatives.
- Taking some herbal products or natural remedies.