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Subject: Platelet Count
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Platelets are small discoid blood corpuscles, the primary link in achieving hemostasis. They are counted by automated counters (rarely manually) that also report mean platelet volume. Their morphology is studied on peripheral blood smear. Automated counters flag abnormal platelet count or appearance.
Normal range: 140–440 (× 10−6 cells/L). Platelets can be estimated on peripheral blood smear (number of platelets/100× oil immersion field × 10,000); for accuracy, platelets in at least 10 different fields should be counted.
Clonal bone marrow disorders such as myeloproliferative neoplasms
Reactive: after acute hemorrhage, in malignancies (about 50% of patients with “unexpected” thrombocytosis are found to have a malignancy), after splenectomy, severe trauma, infections, chronic inflammatory disorders, drug reactions, and many miscellaneous conditions
Immune destruction such as in ITP, reaction to certain drugs, neonatal alloimmune thrombocytopenia, aplastic anemia, leukemias, lymphoproliferative diseases, hypersplenism, DIC, or TTP/HUS and with extracorporeal circulation
Following chemotherapy, posttransfusion thrombocytopenia
Numerous congenital conditions, which may be associated with low platelet counts
Interference and limitations of testing are more numerous with platelets than with RBC and WBC. Preanalytic errors occur if the blood was not admixed well with anticoagulant upon drawing; as soon as the clotting is activated, the platelets are consumed.
Platelets cannot be accurately counted after being stored at 4°C for more than 24 hours. In some cases, and for no known reason, the EDTA used for anticoagulation of the CBC may clump platelets, reducing their number. In such situations, the blood must be drawn with a different anticoagulant, usually 3.2% sodium citrate. A similar situation resulting in low counts is platelet satellitism (platelet adherence to neutrophils).
Other sources of error, especially in automated counters, are giant platelet (may be counted as RBC), white cell fragments, very small red cells, or red cell fragments, counted by automated counters as platelets.