Page 2211 - Hematology_ Basic Principles and Practice ( PDFDrive )

1958 Part XII Hemostasis and Thrombosis

TABLE Laboratory Tests Used to Investigate a Patient With treatment for thrombocytopenia that does not exist. A much less
132.3 Thrombocytopenia common (one in 10,000 blood samples) antibody-mediated pseudo-
thrombocytopenic disorder is platelet satellitism, in which rosette-like
Test Rationale clusters of platelets surround neutrophils. This entity is produced by
Common Tests immunoglobulin G (IgG) antibodies that recognize EDTA-induced
CBC Isolated thrombocytopenia usually is cryptic epitopes on both platelet GPIIb/IIIa and neutrophil FcγIII
caused by platelet destruction, but receptors.
involvement of all cell lines BM examination can be helpful for assessment of platelet produc-
suggests underproduction or tion, particularly if megakaryocytes are reduced in number or
sequestration abnormal in appearance. Examination of the BM can be diagnostic
in some disorders (e.g., leukemia, metastatic tumor, Gaucher disease,
Examination of the blood film Pseudothrombocytopenia (platelet megaloblastic anemia).
clumps) Elevated platelet-associated IgG (PAIgG) can be detected in
Toxic changes and granulocyte “left patients with either immune or non-ITP; therefore, this assay is not
shift” suggest septicemia useful diagnostically. In contrast, GP-specific platelet antibody assays,
Atypical lymphocytes suggest viral such as the monoclonal antibody immobilization of platelet antigens
infection (MAIPA) assay or antigen capture enzyme immunoassay, are relatively
RBC fragments suggest TTP or HUS specific for detection of autoimmune thrombocytopenic disorders.
Parasites (e.g., in malaria) These assays can also be adapted for detection of drug-dependent
White cell inclusions suggest GP-reactive antibodies.
hereditary macrothrombocytopenia When the mechanism of chronic thrombocytopenia is unclear, an
111
Blood cultures Bacteremia, fungemia autologous platelet survival study using In-labeled platelets may be
ANA test Systemic lupus erythematosus informative. Three patterns can be seen: (1) normal platelet survival
and recovery (underproduction), (2) marked reduction in the platelet
Direct antiglobulin test Exclude immune hemolysis life span (increased destruction), and (3) reduced recovery but a
accompanying ITP (Evans normal or near-normal life span (sequestration). However, platelet
syndrome) survival studies are rarely performed.
Coagulation Assays
aPTT, PT (INR), thrombin DIC
time, fibrinogen, D-dimer Therapy
assay
LA assay (nonspecific aPL antibody syndrome The risk of bleeding in patients with thrombocytopenia can be
inhibitor), anticardiolipin reduced by avoiding drugs that impair hemostasis (e.g., alcohol,
and anti-β 2 -glycoprotein I antiplatelet agents, anticoagulants) and invasive procedures (e.g.,
assays intramuscular injections). If drug-induced thrombocytopenia is sus-
Serum protein ITP associated with pected, as many medications as possible, especially those started
electrophoresis; IgG, IgM, lymphoproliferative disorder within the preceding 5–14 days, should be stopped. Life-threatening
IgA levels (monoclonal); hypersplenism bleeding episodes should be treated with platelet transfusion regard-
associated with chronic hepatitis less of the mechanism of the thrombocytopenia.
The underlying cause and anticipated natural history of the
(polyclonal) thrombocytopenic disorder influence the decision about prophylactic
HIV serologic studies HIV-associated thrombocytopenia platelet transfusion. As a general rule, patients with chronic throm-
BM aspiration, biopsy Assess megakaryocyte numbers and bocytopenic disorders characterized by increased platelet destruction
morphology; exclude primary BM (e.g., chronic ITP) or chronic underproduction (e.g., aplastic anemia
disorder or myelodysplasia) can tolerate long periods of severe thrombocyto-
Specialized Tests penia without major bleeding. In addition, prophylactic platelet
GP-specific platelet antibody Relatively specific assay for primary transfusions can trigger alloimmunization against human leukocyte
assays (e.g., MAIPA) and secondary ITP antigen (HLA) or platelet antigens, thereby jeopardizing future thera-
peutic platelet transfusions. Consequently, prophylactic platelet
Drug-dependent increase in Specific assay for D-ITP transfusions are seldom indicated for such patients except when they
platelet-associated IgG
are at risk of bleeding because of trauma or major surgery. When
Drug-dependent platelet HIT platelets are given, the platelet count should be maintained above 50
activation test (e.g., × 10 /L. Invasive procedures such as thoracentesis, paracentesis, and
9
platelet serotonin release liver biopsy are not usually associated with excess bleeding if the
assay) or PF4–heparin (or platelet count is greater than 50 × 10 /L.
9
PF4-polyanion) ELISA Prophylactic platelet transfusions should not be given to patients
Radionuclide platelet life Define the mechanism of with strongly suspected or confirmed HIT, TTP, or HUS because
span study with imaging thrombocytopenia; identify an they may exacerbate platelet-mediated thrombotic complications,
(e.g., 111 In platelet survival “accessory” spleen postsplenectomy and, particularly with HIT, mucocutaneous bleeding is uncommon.
study) However, bleeding in the setting of severe thrombocytopenia may
justify platelet transfusion even in these disorders.
ANA, Antinuclear antibody; aPL, antiphospholipid; aPTT, activated partial
thromboplastin time; BM, bone marrow; CBC, complete blood count;
DIC, disseminated intravascular coagulation; D-ITP, drug-induced immune
thrombocytopenia; ELISA, enzyme-linked immunosorbent assay; ANATOMY AND PHYSIOLOGY
GP, glycoprotein; HIT, heparin-induced thrombocytopenia; HIV, human
immunodeficiency virus; HUS, hemolytic uremic syndrome;
IgG, immunoglobulin G; INR, international normalized ratio; ITP, idiopathic The Spleen: Anatomy and Function
(immune) thrombocytopenic purpura; LA, lupus anticoagulant;
MAIPA, monoclonal antibody immobilization of platelet antigens; PF4, platelet The spleen is a small, well-perfused organ that receives about 5% of
factor 4; PT, prothrombin time; RBC, red blood cell; TTP, thrombotic the total cardiac output. In adults, the spleen weighs between 150
thrombocytopenic purpura.
and 200 g and measures approximately 11 cm in length.