Clotting is what prevents excessive bleeding when you cut yourself. However, blood moving through your blood vessels should not clot. If such clots form they can travel through your bloodstream to your heart, lungs, or brain, causing a heart attack or stroke.
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 vessels.
Most coagulation tests are 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, he or she may recommend one or more coagulation tests. These tests measure various proteins and how they function.
Conditions that cause coagulation problems include liver disease, thrombophilia (excessive clotting), and hemophilia (inability to clot normally). 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 as part of a routine physical. It can alert your doctor to a coagulation problem.
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, or disseminated intravascular coagulation (DIC).
Fibrinogen is a protein made by your liver. This test measures how much is in your blood. Abnormal results may be a sign of hemorrhage, fibrinolysis, or placental abruption (separation 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 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 is 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 something called the international normalized ratio (INR) to compare results of different laboratories.
Another name for this test is activated partial thromboplastin time (aPTT). Another version, 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 are 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 is 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 ceases.
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.
Side effects are generally minor. You may have slight soreness or bruising at the site. Risks of blood tests include light-headedness, pain, and infection.
If you have experienced excessive bleeding, the procedure will be carefully monitored.
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 are diagnosed with a clotting disorder, treatment will depend on the specific diagnosis.