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Chapter 74: Thrombocytopenia

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INTRODUCTION

  • Thrombocytopenia is defined as a platelet count below the lower limit of normal for the specific method used (eg, <150 × 109/L).

  • The types and causes of thrombocytopenia are listed in Table 74–1.

TABLE 74–1CLASSIFICATION OF THROMBOCYTOPENIA
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TABLE 74–1 CLASSIFICATION OF THROMBOCYTOPENIA

  1. Pseudo (spurious)-thrombocytopenia

    1. Antibody-induced platelet aggregation

    2. Platelet satellitism

    3. Antiphospholipid antibodies

    4. Glycoprotein IIb/IIIa antagonists

    5. Miscellaneous

  2. Thrombocytopenia resulting from impaired platelet production

    1. Inherited platelet disorders

    2. Acquired marrow disorders

      1. Nutritional deficiencies and alcohol-induced thrombocytopenia

      2. Clonal hematologic diseases (myelodysplastic syndrome, leukemias, myeloma, lymphoma, paroxysmal nocturnal hemoglobinuria)

      3. Aplastic anemia

      4. Marrow metastasis by solid tumors

      5. Marrow infiltration by infectious agents (eg, HIV, tuberculosis, brucellosis)

      6. Hemophagocytosis

      7. Immune thrombocytopenia (ITP)

      8. Drug-induced thrombocytopenia

      9. Pregnancy-related thrombocytopenia

  3. Thrombocytopenia resulting from increased platelet destruction

    1. Immune thrombocytopenia

      1. Autoimmune thrombocytopenia (primary and secondary ITP)

      2. Alloimmune thrombocytopenia

    2. Thrombotic microangiopathies (TTP, hemolytic uremic syndrome [HUS])

    3. Disseminated intravascular coagulopathy (DIC)

    4. Pregnancy-related thrombocytopenia

    5. Hemangiomas (Kasabach-Merritt phenomenon)

    6. Drug-induced immune thrombocytopenia (quinidine, heparin, abciximab)

    7. Artificial surfaces (hemodialysis, cardiopulmonary bypass, extracorporeal membrane oxygenation)

    8. Type 2B von Willebrand disease

  4. Thrombocytopenia resulting from abnormal distribution of the platelets

    1. Hypersplenism

    2. Hypothermia

    3. Massive blood transfusions

    4. Excessive fluid infusions

  5. Miscellaneous Causes

    1. Cyclic thrombocytopenia, acquired pure megakaryocytic thrombocytopenia

SPURIOUS THROMBOCYTOPENIA (PSEUDOTHROMBOCYTOPENIA)

  • A false diagnosis of thrombocytopenia can occur when laboratory conditions cause platelets to clump, or platelets satellite around neutrophils, resulting in artificially low platelet counts as determined by automated counters. This occurs in 0.1% to 0.2% of automated platelet counts. Occasionally, if a high proportion of platelets are unusually large, the automated count can be spuriously low.

  • Blood films should always be carefully examined to confirm the presence of thrombocytopenia and eliminate the possibility of platelet clumping or neutrophil satellitism.

Etiology and Pathogenesis

  • Falsely low platelet counts are caused by platelet clumping most often occurring in blood samples collected in EDTA anticoagulant. Blood collected in citrate will often confirm the spurious nature of the thrombocytopenia, although clumping may occur in any anticoagulant.

  • Platelets may attach to each other to form clumps or may form clumps with leukocytes, usually neutrophils.

  • Platelet clumping is usually caused by a low-titer IgG antibody reacting with an epitope exposed on platelet glycoprotein (GP) IIb/IIIa by in vitro conditions.

Laboratory Features

  • A film made from blood anticoagulated with EDTA demonstrates more platelets than expected from the platelet count, but many are in large pools or clumps (see Williams Hematology, 10th ed, Chap. 116, Figure 116–1). Correct platelet counts can be obtained by placing fingerstick blood directly into diluting fluid at 37°C and performing counts by phase-contrast microscopy.

  • Pseudothrombocytopenia is often accompanied by a falsely elevated white count because some platelet clumps are sufficiently large to be detected as leukocytes by an automated counter.

  • Correct platelet counts can be obtained by placing fingerstick blood directly into diluting fluid at 37°C and performing ...

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