Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1130 page)

   Attachment of antibodies to platelets is difficult to measure because platelets have normally cell-bound immunoglobulins attached to them. In addition, platelets do not lend themselves to agglutination methodology, as used for RBC antibody detection (see p. 884, DAT). The use of different proposed methodologies remains difficult to standardize, and practicality is limited. Solid-phase methodologies, such as those using ELISA immunoassays, are used by some laboratories to detect IgG antibodies against HLA, ABO, and HPA antigens.

PLATELET COUNT
^†

   Definition

   Platelets are small discoid blood corpuscles, the primary link in achieving hemostasis. They are counted by automated counters (rarely manually) that also report mean platelet volume. Their morphology is studied on peripheral blood smear. Automated counters flag abnormal platelet count or appearance.

   
Normal range:
140–440 (× 10
⁻⁶
cells/L). Platelets can be estimated on peripheral blood smear (number of platelets/100× oil immersion field × 10,000); for accuracy, platelets in at least 10 different fields should be counted.

   Interpretation

Causes of Increases

   Clonal bone marrow disorders such as myeloproliferative neoplasms

   Reactive: after acute hemorrhage, in malignancies (about 50% of patients with “unexpected” thrombocytosis are found to have a malignancy), after splenectomy, severe trauma, infections, chronic inflammatory disorders, drug reactions, and many miscellaneous conditions

Causes of Decreases

   Immune destruction such as in ITP, reaction to certain drugs, neonatal alloimmune thrombocytopenia, aplastic anemia, leukemias, lymphoproliferative diseases, hypersplenism, DIC, or TTP/HUS and with extracorporeal circulation

   Following chemotherapy, posttransfusion thrombocytopenia

   Numerous congenital conditions, which may be associated with low platelet counts

   Limitations

   Interference and limitations of testing are more numerous with platelets than with RBC and WBC. Preanalytic errors occur if the blood was not admixed well with anticoagulant upon drawing; as soon as the clotting is activated, the platelets are consumed.