Neonatal alloimmune thrombocytopaenia (NAIT) (platelet count <50×109/L) results from maternal alloimmunisation against foetal platelet antigens inherited from the father, which are different from those present in the mother. This results in the destruction of foetal platelets by maternal IgG antibodies and presents as a severe isolated thrombocytopaenia in otherwise healthy new-borns. Approximately 20% of new-born NAIT cases present with intracranial haemorrhage which can be fatal if not treated promptly. Mothers are typically healthy with no previous history of thrombocytopaenia themselves or autoimmune disorders.
Infants with NAIT are at risk of haemorrhage throughout their thrombocytopaenic period, including intracranial bleeding and must be transfused with platelets even before laboratory results are available to confirm the diagnosis. Transfusion before laboratory results are available is with platelets that are HPA-1a5b neg. as these are the two main HPA antibodies implicated in NAIT.
For ongoing support, HPA typing of donors is required to identify donors lacking the antigen against which maternal antibodies have been formed. Mothers with previous NAIT babies need to be closely monitored in subsequent pregnancies.
H&I laboratory investigations of suspected cases of NAIT would involve HPA antibody testing of the mother, typically by either the flowcytometric Platelet Immunofluorescence Test (PIFT) and/or the Monoclonal Antibody Specific Immobilisation of Platelet Antigens (MAIPA) test. The father or child may be HPA typed to determine if they express the cogent HPA antigen against which antibodies have been formed. As presence of the HLA type HLA-DRB3*01 in the mother has been implicated as the antigen presenting molecule, HLA typing of the mother may also be carried out.