Clotting is what prevents excessive bleeding when you cut yourself. But 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. This can cause a heart attack, stroke, or even death.
Coagulation tests measure your blood’s ability to clot, and how long it takes to clot. Testing can help your doctor assess your risk of excessive bleeding or developing clots (thrombosis) somewhere in your blood vessels.
Learn more: Bleeding disorders »
Coagulation tests are similar to most blood tests. Side effects and risks are minimal. A medical professional will take a blood sample and send it 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 normally
Coagulation tests are useful in monitoring people who take medications that affect clotting ability. Coagulation tests are also sometimes recommended before surgery.

There are many types of coagulation tests. The sections below include explanations of several of them.

Complete blood count (CBC)

Your doctor may order a complete blood count (CBC) as part of your routine physical. The results of the test can alert your doctor if you have anemia or a low platelet count, which can interfere with your ability to clot.

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 level

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 hypofibrinogenemia test.

Prothrombin time (PT or PT-INR)

Prothrombin is another protein your liver produces. 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 (Coumadin).
Results are given in the number of seconds it takes the blood to clot. Sometimes the PT test uses a calculation called the international normalized ratio (INR) to compare results of different laboratories.
Your doctor will usually order the PT test along with another clotting test called an activated partial thromboplastin time (aPTT).

Platelet count

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.
Anemia, primary thrombocythemia, or chronic myelogenous leukemia (CML) may cause an abnormally high number of platelets.

Thrombin time

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.

Bleeding time

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 won’t be deep and will generally feel like scratches.
Your healthcare provider will remove the cuff when it’s deflated and briefly place blotting paper 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 need 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. They will insert a needle into a vein. Most people feel a minor stick.
Your healthcare provider will draw and collect your blood. Then they’ll more likely place a bandage 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 experience excessive bleeding, the procedure will be carefully monitored.
The sample will be sent to a laboratory for testing and analysis.