Prothrombin Time and INR
A method of standardizing prothrombin time results, called
the international normalized ratio (INR) system, has been developed so the
results among labs using different test methods can be understood in the same
way. Using the INR system, treatment with blood-thinning medicine
(anticoagulant therapy) will be the same. In some labs, only the INR is
reported and the PT is not reported.
Prothrombin time (PT) and international normalized
|Prothrombin time (PT):|
The warfarin (Coumadin) dose is changed so that the
prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal
value or INR values 2 to 3). Prothrombin times are also kept at longer times
for people with artificial heart valves, because these valves have a high
chance of causing clots to form.
- A longer-than-normal PT can mean a lack of or
low level of one or more blood clotting factors (factors I, II, V, VII, or X).
It can also mean a lack of vitamin K; liver disease, such as
cirrhosis; or that a liver injury has occurred. A
longer-than-normal PT can also mean that you have
disseminated intravascular coagulation (DIC), a
life-threatening condition in which your body uses up its clotting factors so
quickly that the blood cannot clot and bleeding does not stop.
longer-than-normal PT can be caused by treatment with blood-thinning medicines,
such as warfarin (Coumadin) or, in rare cases, heparin.
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- 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 therapy (HT), and vitamin K
- 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
- Eating foods that have vitamin K, such as broccoli, chickpeas, kale, turnip greens, and soybean
- Drinking a lot of alcohol.
- Taking some herbal products or natural remedies.
What To Think About
- The U.S. Food and Drug Administration (FDA) has
approved a home test for prothrombin time (PT). If you need a PT test frequently and for a long time, you may want to ask your doctor if this home test is an option for you.
- A PT is done at the same time of day
each time so test results can check whether the right dose of medicine is being
used to prevent blood clots.
- 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. To learn more, see the topic
Partial Thromboplastin Time.
levels are checked along with other liver tests, such as aspartate
aminotransferase and alanine aminotransferase to check how the liver is