Platelets, or thrombocytes, are the smallest formed elements in blood. They promote coagulation and the formation of a hemostatic plug in vascular injury.
Platelet count is one of the most important screening tests of platelet function. Accurate counts are vital
- To evaluate platelet production
- To assess effects of chemotherapy or raditation therapy on platelet production.
- To diagnose and monitor severe thrombocytosis or thrombocytopenia
- To confirm a visual estimate of platelet number and morphology from a stained blood film
- Explain to the patient that this test is used to determine if the patient's blood clots normally.
- Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
- Inform the patient that food or fluids need not be restricted before the test.
- Reassure the patient that drawing a blood sample will take less than 3 minutes.
- Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
- Check the patient's history for use of medications that may affect test results, including acetazolamide, acetohexamide, antineoplastics, brompheniramine maleate, carbamazepine, chloramphenicol, ethacrynic acid, furosemide, gold salts, hydroxychloroquine, indomethacin, isoniazid, mephenytoin, mefeamic acid, methazolamide, methimazole, methyldopa, oral diazoxide, oxyphenbutazone, penicillamine, penicillamine, phenylbutazone, phenytoin, pyrimwthamine, quinidine sulfate, quinine, salicylates, streptomycin, sulfonamides, thiazide and thiazide-like diuretics, and tricyclic antidepressants. Check for heparin, which causes transient, reversible thrombocytopenia thrombocytopenia. Notify the laboratory if such drugs have been used.
Procedure and posttest care
- Perform a venipuncture and collect the sample in a 7-ml lavender-top tube.
- If a hematoma develops at the venipuncture site, apply warm soaks.
- To prevent hemolysis, avoid excessive probing at the venipuncture site and handle the sample gently.
- Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly
Normal platelet counts range from 140,000 to 400,000/u1.
A decreased platelet count (thrombocytopenia) can result from aplastic or hypoplastic bone marrow; infiltrative bone marrow disease, such as carcinoma, leukemia, or disseminated infection; megakaryocytic hypoplasia; in effective thrombopoiesis due to folic acid or vitamin B12 deficiency; pooling of platelets in an enlarged spleen; increased platelet destruction due to drugs or immune disorders; disseminated intravascular coagulation; Bernard-Soulier syndrome; or mechanical injury to platelets.
A platelet count that falls below 50,000/u1 can cause spontaneous bleeding. When it drops below 5,000, fatal central nervous system bleeding or massive GI hemorrhage is possible.
An increased platelet count (thrombocytosis) can result from hemorrhage, infectious disorders, malignant disease, iron deficiency anemia, recent surgery, pregnancy, splenectomy, or inflammatory disorders such as collagen vascular disease. In such cases, the platelet count returns to normal after the patient recovers from the primary disorder. However, the count remains elevated in primary thrombocythemia, myelofibrosis with myeloid metaplasia, polycythemia vera, and chronic myelogenous leukemia.
When the platelet count is abnormal, diagnosis usually requires further studies, such as complete blood count, bone marrow biopsy, direct antiglobulin test (direct Coombs' test), and serum protein electrophoresis.
- Failure to use the proper anticoagulant, or to mix the sample and anticoagulant promptly and adequately.
- Hemolysis due to rough handling of the sample or excessive probing at the venipuncture site.
- Heparin (decrease)
- Acetazolamide, acetohexamide, antineoplastics, brompheniramine maleate, carbamazepine, chloramphenicol, ethacrynic acid, furosemide, gold salts, hydroxychloroquine, indomethacin, isoniazid, mephenytoin, mefenamic acid,methazolamide, methimazole, methyldopa, oral diazoxide, oxyphenbutazone, penicillamine, penicillin, phenylbutazone, phenytoin, pyrimethamine, quinidine sulfate, quinine, salicylates, streptomycin, sulfonamides, thiazide and thiazide-like diuretics, and tricyclic antidepressants (possible decrease).
- High altitudes, persistent cold temperatures, strenuous exercise, or excitement (increase).
- Menstruation (possible decrease just before onset).
- Testing delayed more than 3 hours after sample collection.
- White blood cell count> 100,000/u1.