How to Monitor Platelet Inhibitors

Platelets are blood cells produced in the bone marrow---along with white and red cells---that are crucial in clotting and bleeding. Platelets become active whenever there is an injury or some other cause of bleeding, They help to stop the bleeding and to lay the foundation for scar tissue that heals the injury. High platelet counts enable the blood to clot faster, while lower counts mean clotting takes longer.

In treating certain diseases, such as heart attack, a lower count is needed to prevent further vessel blockages. Anticoagulants, such as coumadin, are often administered to maintain a lower platelet count, but not so low as to cause uncontrolled bleeding. Thus, the monitoring of platelet counts is important to judge the effectiveness and dosage of platelet inhibitors.

Instructions

Collecting and testing blood

• 1

  Use an antiseptic to clean the site where a vein will be punctured to draw the blood sample. The site will usually be upper arm on the inside of the elbow or the back of the hand. Then apply a tourniquet above where the blood will be drawn to cause the vein to become engorged with blood.

  Gently insert the needle into the vein. Use a tube that contains sodium citrate to prevent clotting before the prothrombin test can be performed. Once the blood is collected, remove the needle, swab the site again with antiseptic and bandage.

• 2

  Add the patient's plasma to a blood protein known as thromboplastin that converts prothrombin to thrombin---a chemical necessary for clotting. Keep the mixture in a warm water bath at 37 degrees C for one or two minutes. Add calcium chloride to the mixture to neutralize the sodium citrate in the tube used to collect the blood.

  The prothrombin time test (PT test) is used to measure the clotting capability of blood in patients who are taking anticoagulant medications such as coumadin (warfarin) and a variety of other drugs such as chemotherapy agents, heparin, streptomycin and others.

  The test is also useful as a diagnostic tool for hemophilia (inherited tendency to bleed uncontrollably) as well as deficiencies in clotting factors and vitamin K. as well as liver disease.

• 3

  Timing for the test starts when the calcium chloride is added to the mixture and ends when the plasma clots. The resulting value is the prothrombin time. The normal time for clotting is 11 to15 seconds. Clotting time within this range means the patient has normal amounts of clotting factors.

  Count the number of platelets per microliter of blood. The use of platelet inhibitors affects clotting time as well as concentration of platelets. Normal ranges for platelets are from 150,000 to 400,000 platelets per microliter. A lower platelet count is an indication of the effectiveness of platelet inhibitors, and thus a slower clotting time.

  However, a clotting time that is greater than 30 seconds or longer is cause for concern regarding the possibility of hemorrhage (unusual bleeding).