Neonatal Alloimmune Thrombocytopenia (NAIT)
Neonatal Alloimmune Thrombocytopenia (NAIT) results from maternal human platelet antibodies (HPA) against fetal platelet antigens inherited from the father.
Background
Neonatal Alloimmune Thrombocytopenia (NAIT) results from maternal human platelet antibodies (HPA) against fetal platelet antigens inherited from the father. In contrast to rhesus haemolytic disease, platelet allo-immunization can occur during the first pregnancy.
NAIT presents commonly in the newborn with unexpected bruising and purpura or can be the cause of severe intracranial haemorrhage.
The possibility of allo-immunization during pregnancy is particularly high for HPA-1a, -5b and -15b among Europeans, for HPA-2b and HPA-3a among Maori, for HPA -4b among Asians, and for HPA-6b among Polynesians. The picture becomes more complex for families with mixed ethnic backgrounds.¹
The incidence of NAIT is reported to be between 1:800 and 1:1000. In clinically presenting cases, the rate of intracranial haemorrhage is ~20% followed by death in 10% and neurological sequelae in 20% of these newborns.²
Presentation
The commonest mode of presentation is the well neonate with bruises or petechiae, but the spectrum of disease ranges from sub-clinical moderate thrombocytopenia to catastrophic intracranial haemorrhage and death. A high index of suspicion is essential in all cases of active bleeding, but also in asymptomatic laboratory diagnosed thrombocytopenia. A history of thrombocytopenia in a previous sibling makes the diagnosis almost certain.
Modes of presentation
Bruising/bleeding neonate
Excessive haematoma at injection site
Previously affected sibling
Recurrent fetal loss and stillbirth
Antenatal ICH/hydrocephalus
Disseminated intravascular coagulopathy
Postnatal ICH (e.g.: silent, full fontanel, seizures)
Differential Diagnosis
Also see Neonatal Thrombocytopenia guideline
| Timing | Most Common Aetiology |
| Fetal | Allo-immune (e.g. NAIT) Congenital infection Aneuploidy Autoimmune (e.g. ITP, SLE) Inherited (e.g. Wiskott-Aldrich syndrome) |
| Early-Onset | Placental insufficiency Perinatal asphyxia Early onset sepsis Congenital infection DIC Allo-immune Autoimmune Kasabach-Merritt syndrome Metabolic disease Inherited |
| Late-Onset | Sepsis NEC Infection Autoimmune Kasabach-Merritt syndrome Metabolic disease Inherited Thrombus |
Investigation
| Infant | FBC to confirm platelet count If no paternal blood available or paternity is uncertain, send 1 x infant EDTA (purple, 1 mL) tube for genotyping |
| Mother | FBC: Normal platelet count during pregnancy 4 x CPDA (yellow) tubes for genotyping and 1x plain (red) tubes for anti-platelet antibodies. |
| Father | 4 x CPDA (yellow) tubes for genotyping |
If platelet transfusion is needed, ring haematologist on call to discuss.
Definitive diagnosis of NAIT depends on parental testing.
Ideally, matching with maternal antibodies is preferred prior to platelet transfusion.
Phone laboratory with specimen queries (523 5731).
Management
| Platelet Count (x109) | Action |
| <30 | Transfuse |
| 30-49 | Transfuse if any bleeding or high risk |
| 50-99 | Transfuse if major bleeding |
| >99 | Do not transfuse |
Transfusion
If transfusion is needed, discuss the case with a clinical haematologist.
Urgent transfusion of 10 mL/kg of platelets (over 30-60 min) is needed if the infant is bleeding or at high risk of bleeding. Infants are at significant risk for ICH in the first days of life.
Matched platelets are first choice, but may not be available in a timely fashion.
If matched platelets are not available in a timely fashion, unmatched platelets (consider addition of IvIg) should be given.
IvIg
Consider IvIg (1-2 g/kg) in combination with unmatched platelets if matched platelets are not available in a timely fashion.⁵
IvIg as stand-alone treatment may be considered, but response can be delayed by 24-36 hours, leaving a window of risk for ICH.
Other considerations
Ensure Vitamin K has been administered
Urgent head ultrasound
Infants are at significant risk of ICH in the first days of life.
If the platelet count fails to rise in response to matched platelets consider alternative diagnoses, and check for rarer maternal antibody types.
Affected infants should have a FBC checked daily until platelet count is stable over 100x10⁹/L.
Maternal antibody levels fall towards term, so repeat testing 6 weeks post-partum is required in some cases.
It is essential that parental investigation is initiated prior to discharge and adequate follow up arrangements made.
The risk of a subsequent pregnancy being affected is 100% if the father is homozygous for the reacting antigen, and 50% if heterozygous.
In general, subsequent pregnancies are at least as severely affected as the first. Antenatal therapy in subsequent pregnancies remains contentious, but options include intrauterine platelet transfusion, maternal immunoglobulin and steroids.⁵ If NAIT is confirmed, future pregnancies should by managed by the Maternal Fetal Medicine Team.