Thrombocytopenia is defined as a platelet count below the lower limit of normal for the specific method used; e.g., less than 150 × 109/L.
The causes of thrombocytopenia are listed in Table 74–1.
TABLE 74–1CLASSIFICATION OF THROMBOCYTOPENIA |Favorite Table|Download (.pdf) TABLE 74–1 CLASSIFICATION OF THROMBOCYTOPENIA
|Platelet agglutination |
|Platelet satellitism |
|Antiphospholipid antibodies |
|GP IIa-IIIa antagonists |
|Giant platelets |
|Miscellaneous associations |
|Impaired platelet production |
| Autosomal dominant |
| MYH9-related |
| May-Hegglin anomaly |
| Fechtner syndrome |
| Epstein syndrome |
| Sebastian syndrome |
| Mediterranean macrothrombocytopenia |
| Familial platelet syndrome with predisposition to acute myelogenous leukemia |
| Thrombocytopenia with linkage to chromosome 10 |
| Paris-Trousseau syndrome |
|Thrombocytopenia with radial synostosis |
| Autosomal recessive |
| Congenital amegakaryocytic thrombocytopenia |
|Thrombocytopenia with absent radius (TAR) syndrome |
| Bernard-Soulier syndrome |
| Gray platelet syndrome |
| X-linked thrombocytopenias |
| Wiskott-Aldrich syndrome |
| X-linked thrombocytopenia |
| X-linked thrombocytopenia with dyserythrocytosis |
| Marrow infiltration |
|Infectious disease |
| HIV |
| Parvovirus |
| Cytomegalovirus |
| Radiotherapy and chemotherapy |
| Folic acid and vitamin B12 deficiency |
| Paroxysmal nocturnal hemoglobinuria |
| Acquired aplastic anemia |
| Myelodysplastic syndromes |
| Acquired pure megakaryocytic thrombocytopenia |
| Accelerated platelet destruction |
| Immune-mediated thrombocytopenia |
| Autoimmune thrombocytopenic purpura |
| Secondary (infections, pregnancy-related, lymphoproliferative disorders, collagen vascular diseases) |
| Alloimmune thrombocytopenia |
| Neonatal thrombocytopenia |
| Posttransfusion purpura |
| Nonimmune thrombocytopenia |
|Thrombotic microangiopathies |
| Thrombotic thrombocytopenic purpura and hemolytic uremic syndrome |
| Disseminated intravascular coagulopathy |
| Kasabach-Merritt syndrome |
| Platelet destruction by artificial surfaces |
| Hemophagocytosis |
| Abnormal platelet distribution or pooling |
| Splenomegaly |
| Hypersplenism |
| Hypothermia |
| Massive transfusion |
|Drug-induced thrombocytopenia |
| Heparin-induced thrombocytopenia |
| Other drug-induced thrombocytopenias |
SPURIOUS THROMBOCYTOPENIA (PSEUDOTHROMBOCYTOPENIA)
A false diagnosis of thrombocytopenia can occur when laboratory conditions cause platelets to clump, resulting in artificially low platelet counts as determined by automated counters. This occurs in 0.1 to 0.2 percent 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.
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 GP IIb/IIIa by in vitro conditions.
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. A blood film made directly from a fingerstick sample accurately reflects the true count.
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 ...
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