Platelets (thrombocytes) are anucleate cell fragments derived from megakaryocytes in the bone marrow. They play a central role in hemostasis, forming the initial platelet plug at sites of vascular injury and providing a surface for coagulation factor assembly. The CBC includes the platelet count, and careful assessment of platelet parameters is essential for evaluating bleeding risk, thrombotic tendency, and bone marrow function.
Platelet Count
The normal platelet count is 150–450 × 10⁹/L. Platelet counts are determined by impedance or optical methods in automated analyzers, with immunoplatelet counting (CD61/CD41 labeling by flow cytometry) serving as the reference method for severely thrombocytopenic samples. Counts may be falsely decreased due to platelet clumping (EDTA-induced pseudothrombocytopenia), platelet satellitism (platelets adhering to neutrophils), giant platelets counted as RBCs, or incomplete clot formation. Falsely elevated counts may result from RBC or WBC fragments, cryoglobulin precipitation, or bacterial contamination.
Thrombocytopenia
Thrombocytopenia is defined as a platelet count below 150 × 10⁹/L. Severity is graded: mild (100–149 × 10⁹/L), moderate (50–99 × 10⁹/L), severe (20–49 × 10⁹/L), and very severe (< 20 × 10⁹/L). Spontaneous bleeding risk increases significantly below 20 × 10⁹/L. Etiologies are categorized by mechanism: decreased production (bone marrow failure, leukemia, chemotherapy, aplastic anemia, myelodysplasia), increased destruction (immune thrombocytopenia (ITP), drug-induced, heparin-induced thrombocytopenia (HIT), post-transfusion purpura, hypersplenism, DIC), and sequestration (hypersplenism with splenomegaly). Laboratory investigation includes review of the peripheral blood smear to verify the count and assess platelet morphology, followed by testing for specific causes.
Thrombocytosis
Thrombocytosis is a platelet count above 450 × 10⁹/L. Reactive (secondary) thrombocytosis is more common and occurs in response to infection, inflammation (CRP/IL-6-driven), iron deficiency, malignancy, hemolytic anemia, post-splenectomy, and acute blood loss. Essential thrombocythemia (ET) is a myeloproliferative neoplasm driven by JAK2, CALR, or MPL mutations, presenting with sustained thrombocytosis, normal iron stores, and no reactive cause. The distinction between reactive and primary thrombocytosis is made through clinical assessment, inflammatory markers (CRP, ferritin), iron studies, and mutation testing.
Mean Platelet Volume (MPV)
MPV measures average platelet size and correlates with platelet production. Larger platelets (high MPV) are younger, more reactive, and contain more dense granules. MPV is increased in ITP (due to accelerated platelet turnover with increased thrombopoiesis) and myeloproliferative neoplasms. MPV is decreased in aplastic anemia and after chemotherapy. MPV has been studied as a marker for cardiovascular risk, though its clinical utility remains limited due to pre-analytical variability (EDTA causes platelet swelling over time) and lack of standardization.
Platelet Morphology on Smear
The peripheral blood smear provides critical qualitative information about platelets. Normal platelets appear as small (2–4 µm) discoid fragments with pale blue cytoplasm and fine reddish-purple granules. Large platelets (giant platelets, > 5 µm) suggest increased turnover or inherited disorders (Bernard-Soulier syndrome). Gray platelets (lack of granules) indicate gray platelet syndrome or myelodysplasia. Platelet clumps suggest EDTA-induced pseudothrombocytopenia. Schistocytes with thrombocytopenia raise concern for thrombotic microangiopathy (TTP, HUS), a medical emergency requiring prompt plasma exchange.
Clinical Context
The platelet count is interpreted in conjunction with other CBC parameters and clinical presentation. Isolated thrombocytopenia suggests ITP or drug-induced causes. Thrombocytopenia with microangiopathic hemolytic anemia (schistocytes, elevated LDH, low haptoglobin) is a TTP/HUS emergency. Thrombocytopenia with abnormal WBC differential suggests leukemia or myelodysplasia. Thrombocytosis after splenectomy is expected and benign. Coordination with hemostasis and coagulation testing is essential when bleeding symptoms are present.
