Clotting is what prevents excessive bleeding when you cut
yourself. However, the blood moving through your vessels shouldn’t clot. If
such clots form, they can travel through your bloodstream to your heart, lungs,
or brain, causing a heart attack, stroke, or even death.
Coagulation tests measure your blood’s ability to clot, as well
as how long it takes. Testing can help your doctor assess your risk of
excessive bleeding or developing clots (thrombosis) somewhere in your blood
Coagulation tests are usually given in the same manner as most
blood tests. Side effects and risks are minimal. A blood sample is sent to a
laboratory for testing and analysis.
Clotting disorders can cause a dangerous amount of bleeding or
clotting. If your doctor suspects you have a clotting disorder, they may
recommend one or more coagulation tests. These tests measure various proteins
and how they function.
Conditions that can cause coagulation problems include:
- liver disease
- thrombophilia, which is excessive clotting
- hemophilia, which is an inability to clot
Coagulation tests are also useful in monitoring people who take
medications that affect clotting ability.
Coagulation tests are sometimes recommended before surgery.
There are many types of coagulation tests. Several of them are
explained in the sections below.
Complete Blood Count (CBC)
Your doctor may order a complete blood count (CBC) as part of
your routine physical. It can alert your doctor if you have a coagulation
Factor V Assay
This test measures Factor V, a substance involved in clotting. An
abnormally low level may be indicative of liver disease, primary fibrinolysis (a
breakdown of clots), or disseminated intravascular coagulation (DIC).
Fibrinogen is a protein made by your liver. This test measures
how much fibrinogen is in your blood. Abnormal results may be a sign of excessive
bleeding or hemorrhage, fibrinolysis, or placental abruption, which is a separation
of the placenta from the uterine wall.
Other names for this test include factor I and hypo-fibrinogenemia
Prothrombin Time (PT or PT-INR)
Prothrombin is another protein produced in your liver. The prothrombin
time (PT) test measures how well and how long it takes your blood to clot. It
normally takes about 25 to 30 seconds. It may take longer if you take blood
thinners. Other reasons for abnormal results include hemophilia, liver disease,
and malabsorption. It’s also useful in monitoring those who take medications
that affect clotting, such as warfarin.
Results are given in the number of seconds it takes to clot.
Sometimes the PT test uses a calculation called the international normalized
ratio (INR) to compare results of different laboratories.
The PT test is usually ordered with another clotting test called
an activated partial thromboplastin
time (aPTT). Another version called the model for end stage liver disease (MELD) is used to assess people who
need a liver transplant.
Platelets are cells in the blood that help your blood clot. You
may have an abnormally low number if you’re on chemotherapy, take certain
medications, or have had a massive blood transfusion. Other causes of a low
platelet count are celiac disease, vitamin K deficiency, and leukemia.
An abnormally high number of platelets may be caused by anemia,
primary thrombocythemia, or chronic myelogenous leukemia.
Thrombin time measures how well fibrinogen is working. Abnormal
results may be due to inherited fibrinogen disorders, liver disease, some
cancers, and medications that affect clotting.
This test analyzes how quickly small blood vessels in your skin close
up and stop bleeding. It’s performed differently than the other blood tests.
A blood pressure cuff will be placed on your upper arm and
inflated. Your healthcare provider will make a couple of tiny cuts on your
lower arm. The cuts are not deep and generally feel like a scratch.
The cuff is then deflated. Blotting paper is briefly placed on
the cuts every 30 seconds until bleeding stops.
Bleeding usually lasts between one to nine minutes. The test is
considered safe and carries few side effects or risks.
Coagulation tests are conducted the same way as most blood tests.
You may be required to discontinue taking certain medications prior to the
test. No other preparation is necessary.
Your healthcare provider will sterilize a spot on the back of
your hand or inside your elbow. A needle will be inserted into a vein. Most
people feel a minor stick.
Blood will be drawn and collected. A small bandage may be placed
on the puncture site.
The side effects of a coagulation test are generally minor. You
may have slight soreness or bruises at the site. The risks include lightheadedness,
pain, and infection.
If you have experienced excessive bleeding, the procedure will be
The sample will be sent to a laboratory for testing and analysis.
Results from blood tests are sent from the laboratory to your
doctor. Values can vary from one laboratory to another, so ask your doctor to
explain the results. If you’re diagnosed with a clotting disorder, treatment
will depend on the specific diagnosis.