Chapter 73: Thrombocytopenia

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 73–1.

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% 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.

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 Figure 117–1 in Williams Hematology, 9th ed). 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 blood directly into diluting fluid at 37°C and performing counts by phase-contrast microscopy.

Clinical Features

• The ...