Coagulation Studies

When a break occurs in a blood vessel wall—due to a skin cut or abrasion, for example—a process called coagulation occurs that leads to the formation of a blood clot, which stops the bleeding. First, blood cells called platelets aggregate, or clump, at the injury site; at the same time, the injured cells and platelets both trigger the release of special substances, called clotting factors, into the bloodstream. The clotting factors activate a complex series of events that results in the production of a long-stranded protein called fibrin. At the injury site, strands of fibrin interlock to form a meshwork that traps platelets and red blood cells, completing the blood clot. Blood coagulation studies—the most common ones are covered here—assess whether your blood clots normally and help to identify the origin of any clotting problems.

Prothrombin time (PT) and activated partial thromboplastin time (APTT) are tests that evaluate the release of specific clotting factors by measuring the time it takes for a clot to form after certain reagents are added to a blood sample. The PT test is often performed periodically to monitor patients who regularly take the anticoagulant drug warfarin (Coumadin, Panwarfin), while APTT is used to evaluate therapy with the anticoagulant heparin (which is usually used to treat patients after an acute episode such as a blood clot or heart attack).

Platelet count—one of the most essential tests of platelet function—estimates the number of platelets in your blood using an automated system. The results are confirmed with a visual estimate by a pathologist who examines a blood sample under a microscope.

Bleeding time measures the time required for a clot to form and stop bleeding after a standardized skin incision. It is used to evaluate platelet function and is typically performed, along with a platelet count, in people with a personal or family history of bleeding disorders, or as a preoperative safety measure before a scheduled surgery.

Purpose of Blood Clotting Tests


  • To evaluate blood coagulation
  • To screen patients for bleeding tendencies or thrombotic (blood clot) episode before a scheduled surgery
  • In cases of recurrent miscarriages (may be associated with anticardiolipin or antiphospholipid antibodies)
  • To monitor the response of people taking anticoagulant drugs
  • To monitor heparin anticoagulant therapy
  • To evaluate extrinsic and common pathways of coagulation cascade

Platelet count:

  • To help diagnose bleeding disorders and monitor patients who are being treated for any disease
  • To evaluate platelet production; assess the effects of chemotherapy or radiation treatment on platelet production; and aid in the diagnosis of decreased (thrombocytopenia) or increased (thrombocytosis) number of platelets

Bleeding time:

  • To evaluate platelet function
  • To screen patients for bleeding tendencies before a scheduled surgery

Who Performs Blood Clotting Tests

  • A nurse or a technician draws the blood sample.

Special Concerns about Blood Clotting Tests

  • A variety of medications may affect the results of coagulation studies in people who take anticoagulant medication. In addition, a diet rich in green, leafy vegetables may cause a shortened PT time in those receiving treatment with anticoagulants.
  • The bleeding time test should not be performed in people with extremely low platelet counts, those who are taking anticoagulant medication, and those who have had lymph nodes dissected (for example, breast cancer patients) on the test side (the other arm should be used, if possible).
  • The bleeding time test will leave two small, hairline scars that will be nearly invisible after they have healed. More severe scarring may occur in people who tend to form large scars, or keloids.

Before the Blood Clotting Tests

  • Report to your doctor any medications, herbs, or supplements you are taking. You may be advised to discontinue certain of these agents before the test.

What You Experience

PT, APTT, and platelet count:

  • A sample of your blood is drawn from a vein in your arm and sent to a laboratory for analysis.

Bleeding time:

  • A blood pressure cuff will be wrapped around your upper arm and then inflated so that it feels tight.
  • An area high on your forearm will be cleansed with an antiseptic solution. After it has dried, 1 or 2 small incisions are made in the skin. (Alternatively, 3 small punctures are made with a lancet, or a puncture is made in the earlobe.)
  • The examiner will start a stopwatch, and then blot the drops of blood with filter paper every 30 seconds until bleeding stops.
  • The incisions and tightness of the blood pressure cuff may cause some discomfort.
  • The test takes about 10 minutes.

Risks and Complications of Blood Clotting Tests

  • The bleeding time test carries a small risk of excessive bleeding or skin infection. In addition, scars may occur in people who are prone to keloid formation.

After Blood Clotting Tests

  • Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture or incision site. (For people on anticoagulant therapy, additional pressure may be needed to control the bleeding.)
  • After a bleeding time test, the incisions are covered with a small piece of gauze and an adhesive bandage. (However, in people with bleeding disorders such as hemophilia, a pressure bandage is kept over the incisions for 24 to 48 hours to prevent further bleeding.)
  • After bleeding has stopped, you may leave the testing facility.
  • Resume taking any medications that were withheld before the test, according to your doctor’s instructions.
  • Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.

Blood Clotting Test Results

  • The blood sample is sent to a laboratory for analysis. Your doctor will review the results for evidence of a blood clotting abnormality.
  • A wide variety of conditions may cause abnormal results on coagulation studies. For example, people with liver disease often have a prolonged PT, since this test screens for clotting factors that are produced by the liver. An abnormal platelet count and prolonged bleeding time may be caused by bone marrow disease, chemotherapy or radiation therapy for cancer, and liver disease. Additional tests—such as further blood clotting studies, a complete blood count, or a bone marrow biopsy—may be necessary to pinpoint the cause of the problem.
  • Prolonged PT or APTT results—meaning the blood takes too long to clot—in people taking anticoagulants may indicate the need


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 10 Jan 2012

Last Modified: 10 Jan 2012