Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (390 page)

   C
¹⁴
-platelet serotonin release (PSR) is considered the gold standard for the diagnosis of HIT. It has excellent sensitivity and specificity. Because it uses radioactive serotonin as its principal reagent, this assay is performed only in a few reference laboratories. Because of the delay in obtaining results, the assay is useful only for final confirmation of the diagnosis.

   Heparin-induced platelet aggregation is an alternative to the PSR. The assay can be performed in laboratories that offer platelet aggregometry, but it is not well standardized, and although it has excellent specificity (>90%), it lacks sensitivity.

NEONATAL THROMBOCYTOPENIA

   Classification

Thrombocytopenias in the newborn can be classified as due to increased destruction or decreased production.

Increased Destruction

   Neonatal alloimmune thrombocytopenia (NAIT) occurs when fetal platelets contain an antigen inherited from the father that the mother lacks. Fetal and neonate thrombocytopenia is the platelet equivalent of Rh disease; the most commonly involved platelet antigen is HPA-1a or Pl
^A1
. When the mother is exposed to fetal platelets during pregnancy, anti-HPA-1a antibodies are generated; they traverse the placenta, and the result is fetal thrombocytopenia. Intracranial hemorrhage is a potentially serious complication.

   Laboratory studies

   Platelet counts in the neonate are often <50,000/μL.

   Platelet antigens are tested in mother and father to establish the incompatibility. HPA 1, 3, and 5 should be screened in all potential cases, as well as HPA 4 if the patient is of Asian descent. Testing must demonstrate both a platelet antigen incompatibility between the parents and a maternal antibody directed against the antigen. The assays must be performed in very experienced laboratories. It is unclear if antenatal screening should be instituted.

   Autoimmune thrombocytopenia in the newborn is the result of the mother having ITP with antibodies that cross the placenta and react with the platelets of the fetus.

   Most neonates whose mothers have ITP have mild thrombocytopenia (counts >50,000/μL), but occasionally they may be severely affected.

   Other causes of thrombocytopenia due to increased platelet destruction in the neonate:

   Disseminated intravascular coagulation (DIC) as a complication of an acute underlying illness or as the result of consumption in large hemangiomas (Kasabach-Merritt syndrome)

   Severe infection