Neonatal alloimmune thrombocytopenia (NAIT) is the most common cause of severe thrombocytopenia in the first few days of life of a healthy newborn.

Epidemiology

• Incidence - 1/1,000-1/2,000
• Age - can occur as early as 18-20 weeks gestation
• Sex - M:F equal

Risk Factors

• Previous child with NAIT or family history of NAIT
• Primiparous female
  • 50% of cases occur during a first pregnancy

Pathophysiology

• Maternal isoimmunization against fetal-specific platelet antigens
• The alloantigen on the fetal platelets is inherited from the father
• Most commonly occurs in women lacking human platelet antigen (HPA) 1a
  • 80-90% of cases are due to HPA 1a negative mothers
  • 98% of the U.S. population is HPA 1a positive
  • HPA-1a causes the most severe form of the disease, followed by HPA-3a

Clinical Presentation

• Otherwise healthy newborn
• Widespread petechiae or purpura
• Visceral hemorrhage - gastrointestinal or bladder
• Intracranial hemorrhage - usually present at birth in approximately 20% of neonates
  • Fatal in 10% of patients
  • Causes neurologic impairment in 20-30% of patients
• Recurrence rate is high in subsequent pregnancies

Treatment

• Platelet infusion with compatible platelets (antigen negative)
• Intravenous immune globulin

Diagnosis

• CBC with platelet count based on clinical presentation - <50,000 and patient may have anemia; however, the mother has a normal platelet count
• If NAIT is suspected by clinical presentation, do maternal platelet antibody testing - specific HPAs are 1, 3 and 5
  • Fetal platelet typing confirms diagnosis
• If NAIT is suspected in utero, maternal and paternal platelet antigen testing may be useful

Differential Diagnosis

• Infection, sepsis
• Disseminated intravascular coagulation (DIC)
• Other autoimmune destruction
• Inherited disorders such as von Willebrand disease
• Rh hemolytic disease