How To 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
How It Is Done
The health professional drawing blood
- 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 the vein.
- Clean the needle site with
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- Put a gauze pad or cotton ball over the needle site as
the needle is removed.
- Put pressure to the site and then a
How It Feels
The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
There is very little chance of a problem from
having blood sample taken from a vein.
- You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
- 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 taken.
Prothrombin time (PT) is a blood test
that measures how long it takes blood to clot.
The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
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.