PLATELETS REVISISTED

 

1. The normal platelet count in adults ranges from 150,000 to 450,000/microL.
2. The mean value in males is 237,000/microL.
3. The mean value in females is 266,000/microL.
4. Thrombocytopenia or low platelet count is defined as a platelet count less than 150,000/microL.
5. About 2.5 percent of the normal population has platelet count lower than 150,000 /microl (as a normal variant).
6. A recent fall in the platelet count by one–half is abnormal even though it may still be in the normal range.
7. Thrombocytopenia is not usually detected clinically until the platelet count has fallen to levels below 100,000/microL.
8. Variation of the platelet count in a given individual is limited. Differences in the absolute platelet count greater than 70 to 90,000/microL will occur by chance less than one percent of the time.
9. Surgical bleeding due solely to a reduction in the number of platelets does not generally occur until the platelet count is less than 50,000/microL, and clinical or spontaneous bleeding does not occur until the platelet count is less than 10,000 to 20,000/microL.
10. Platelets survive in the circulation for 8 to 10 days, after which they are removed from the circulation by cells of the monocyte–macrophage system, as a result of programmed apoptosis.
11. The youngest platelets in the circulation are larger and more hemostatically active. Thrombocytopenic patients, who do not have serious bleeding, suggest that the small numbers of young platelets in these patients are more hemostatically active than mixed age platelets in normal subjects.
12. In dengue no transfusion is needed unless the count is lower than 2% of the baseline levels.
13. Platelet count can be falsely low in a number of clinical situations:
    • If anticoagulation of the blood sample is inadequate, the resulting thrombin–induced platelet clumps can be counted as white cells by automated cell counters. The WBC count is rarely increased by more than 10 percent.
    • Approximately 0.1 percent of normal subjects have EDTA–dependent agglutinins which can lead to platelet clumping and spurious thrombocytopenia and spurious leukocytosis.
    • Pseudothrombocytopenia can also occur after the administration of the abciximab.
    • EDTA–induced platelet clumping can be diagnosed by examination of the peripheral smear. One should do a repeat count in a non–EDTA anticoagulant.
    • If platelet clumping is observed, the platelet count is repeated using heparin or sodium citrate as an anticoagulant. If citrate is used, one should remember to correct the platelet count for dilution caused by the amount of citrate solution used; no such correction is needed for heparin. Alternatively, one can use freshly–shed non–anticoagulated blood pipetted directly into platelet counting diluent fluid.
    • Patients with cirrhosis, portal hypertension, and spleen enlargement may have significant degrees of "apparent" thrombocytopenia (with or without low white cells and anemia), but rarely have clinical bleeding, since their total available platelet mass is usually normal.