Test Overview
Partial thromboplastin
time (PTT) is a blood test that measures the time it takes your blood to clot.
A PTT test can be used to check for bleeding problems.
Blood
clotting factors are needed for blood to clot
(coagulation). The partial thromboplastin time is an important test because the
time it takes your blood to clot may be affected by:
- Blood-thinning medicine, such as heparin.
Another test, the activated partial thromboplastin time (APTT) test, may be used 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, that
affect the clotting factors.
- An increase in the use of the clotting
factors.
Another blood clotting test, called prothrombin time (PT) or INR
(international normalized ratio),
measures other clotting factors. Partial thromboplastin time and prothrombin
time are often done at the same time to check for bleeding problems caused by a
problem with the clotting factors.
Why It Is Done
Partial thromboplastin time (PTT) is
done to:
- Find a cause of abnormal bleeding or
bruising.
- Check for low levels of blood clotting factors. The lack
of some clotting factors can cause bleeding disorders such as
hemophilia.
- Check for conditions that
cause clotting problems. Conditions such as antiphospholipid antibody syndrome
or lupus anticoagulant syndrome develop when the immune system makes
antibodies that attack blood clotting factors. This
can cause the blood to clot easily in veins and arteries.
- Check if it is safe to do a procedure or surgery that might cause bleeding.
- Check how well the liver is working.
The activated partial thromboplastin time (APTT) test is
used after you take blood-thinners to see if the right dose of medicine is
being used. If the test is done for this purpose, an APTT may be done every few
hours. When the correct dose of medicine is found, you will not need so many
tests.
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.
How It Is 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 the vein.
- 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 removed.
- Put pressure to the site and then a
bandage.
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
pinch.
Risks
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
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 (such as 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.
Results
Partial thromboplastin time (PTT) is a
blood test that measures the time it takes your blood to clot.
Normal
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.
Partial thromboplastin time1| Partial thromboplastin time (PTT): | 60–70 seconds |
|---|
| Activated partial thromboplastin time
(APTT): | 30–40 seconds |
|---|
The heparin dose is changed so that the PTT or APTT
result is about 1.5 to 2.5 times the normal value.1
Abnormal values
- A longer-than-normal PTT or APTT can mean a
lack of or low level of one of the blood clotting factors or another substance
needed to clot blood. This can be caused by bleeding disorders, such as
hemophilia or
von Willebrand's disease.
- A
longer-than-normal PTT or APTT can be caused by liver disease, kidney disease
(such as
nephrotic syndrome), or treatment with blood thinners,
such as heparin or warfarin (Coumadin).
- A longer-than-normal PTT
may be caused by conditions such as antiphospholipid antibody syndrome
or lupus anticoagulant syndrome. These conditions happen when the immune system makes
antibodies that attack blood clotting factors. This
can cause the blood to clot easily in veins and arteries.
- The PTT
can get longer when you are using heparin, so your PTT value needs to be
closely checked. If you have a longer PTT, you may have a higher risk of bleeding.
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- Taking some herbal products or natural
remedies.
- Taking some medicines, such as antihistamines.
What To Think About
- The partial thromboplastin time (PTT) and
activated partial thromboplastin time (APTT) may be normal in people who have
inherited bleeding disorders but have only mild symptoms.
- The APTT
is used to check treatment of people who are using heparin or other
blood-thinning medicine to prevent blood clots.
- Sometimes people
who use heparin have a higher APTT because of other substances in their blood
and not because of blood-thinning medicines. A test called the heparin
neutralization assay may be done to see if this is true.
- Another
blood clotting test, called prothrombin time (PT), measures other clotting
factors. Partial thromboplastin time and prothrombin time are often done at the
same time to check for bleeding problems. For more information, see the topic
Prothrombin Time and INR.
- An PTT or APTT is done
regularly in people who have bleeding or clotting problems. The tests are also
done before procedures or surgeries where too much bleeding may be a
concern.
References
Citations
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | E. Gregory Thompson, MD - Internal Medicine |
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| Specialist Medical Reviewer | Joseph O'Donnell, MD - Hematology, Oncology |
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| Last Revised | September 6, 2012 |
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