Neonatal Alloimmune Thrombocytopenia - NAIT

Indications for Testing

Laboratory testing for NAIT is appropriate in the following circumstances:

• Diagnosis in a fetus with bleeding on ultrasound (especially if suggestive of ICH) and in neonates with bleeding symptoms or early-onset, severe thrombocytopenia  
• Diagnosis in the fetus of a woman with a previously affected child 
• Risk assessment in women planning a pregnancy who have a history of an affected pregnancy, a history of posttransfusion purpura,   or who have a sister or another relative with a previously affected pregnancy 

Laboratory Testing

Diagnosis

Antenatal Testing

If NAIT is suspected during pregnancy, the mother can be tested regularly for the development of alloantibodies; however, alloantibody levels can fluctuate and have not been demonstrated to correlate with NAIT severity.  Platelet genotyping is used to identify potential genetic incompatibilities. See Platelet Antigen Genotyping and Phenotyping below.

Postnatal Testing

Initial Testing

A CBC with platelet count is an initial test for infants suspected of having NAIT; in addition, a maternal CBC with platelet count should be performed to rule out low platelets in the mother, which might indicate autoimmune thrombocytopenia.  The International Society on Thrombosis and Haemostasis (ISTH) considers a platelet count <100,000/μL at birth or within 7 days after birth, with no other cause, as diagnostic for NAIT in the first affected pregnancy.  There are, however, often many potential causes of neonatal thrombocytopenia. Thrombocytopenia with a platelet count of <50,000/μL is considered severe ; in some cases the count may be as low as <20,000/μL.  Platelet counts continue to decline after birth in infants with NAIT but often recover and become normal within a couple of weeks.   A CBC with platelet count that confirms thrombocytopenia should be followed by specific tests to establish a diagnosis of NAIT if there is clinical suspicion (ie, antiplatelet antibody tests and platelet antigen genotyping).

Antiplatelet Antibody Tests

Antiplatelet antibody tests and test interpretation should be performed by an experienced platelet immunology reference laboratory (see specific laboratories listed below). Definitive diagnosis of NAIT requires a combination of tests because laboratory diagnosis of NAIT is based on both detection of anti-HPA alloantibodies and determination of the causative antigen.  It is important to note that false-negative antibody test results can occur due to delayed, low-titer, or low-affinity antibodies, so absence of a detectable maternal antibody does not rule out NAIT. 

A maternal sample is preferred for evaluation of the presence of anti-HPA antibodies; neonatal serum/plasma samples are considered less sensitive when testing for circulating antibodies and are not recommended.  Antiplatelet antibody tests are designed to detect maternal alloantibodies to antigens associated with NAIT, for example, anti-HPA-1, anti-HPA-3, and anti-HPA-5.  More rare HPAs also have been implicated in NAIT, but antibodies targeting these less common HPAs are not detected by tests that use commercial kits and require testing by a specialized laboratory.

Reference laboratories that perform comprehensive testing for NAIT include the following:

• Platelet Immunology Laboratory, Bloodworks Northwest
• BloodCenter of Wisconsin (part of Versiti)

Platelet Antigen Genotyping and Phenotyping​

Testing for NAIT also involves genotyping or phenotyping of the mother’s and father’s and/or neonatal platelets.  The paternal platelet genotype is often used as a surrogate for the fetal or neonatal genotype, but typing paternal platelets may be uninformative in some settings. A physician experienced in the diagnosis and treatment of NAIT should provide guidance regarding the testing (whom to test and which tests to use). Genotyping enables detection of incompatibilities between HPAs. Ideally, genotyping involves both parents and the neonate to obtain comprehensive information.  Genotyping also allows for more accurate risk assessment for future pregnancies because paternal zygosity for the specific HPA in question will affect risk.  Platelet antigen incompatibility is necessary but not sufficient for NAIT (in many cases, genetic incompatibilities do not result in antibody formation). Close correlation between clinical information and the results of platelet antibody testing is necessary for definitive diagnosis.

If the father is heterozygous for the relevant HPA, the genotype of the fetus in future pregnancies will be indeterminate. Fetal samples may be tested, if testing is indicated, or treatment may be administered empirically if clinically warranted. 

Platelet phenotyping is now used less frequently than molecular genotyping, but phenotyping may be helpful to resolve discrepant or inconclusive genotyping results. It is available in specialized laboratories.