Neonatal Alloimmune Thrombocytopenia - NAIT

Neonatal alloimmune thrombocytopenia (NAIT), also referred to as fetal and neonatal alloimmune thrombocytopenia or perinatal alloimmune thrombocytopenia, is the most common cause of thrombocytopenia in an otherwise healthy newborn. NAIT is caused by alloantibodies formed by the mother in response to a paternally inherited human platelet antigen (HPA) on the surface of fetal platelets. These maternal alloantibodies pass through the placenta to the fetus, attack the fetal platelets, and cause fetal platelet destruction. Many cases of NAIT are mild, resulting in only skin manifestations. Left untreated, mild cases may resolve within 2 weeks of birth. NAIT can also cause severe thrombocytopenia (platelet count <50,000/µL), which can result in intracranial hemorrhage (ICH) and even infant death, although severe hemorrhages are uncommon. Laboratory testing is used to diagnose NAIT in an affected infant and to determine the risk of NAIT in future pregnancies. Testing involves evaluation of maternal serum for platelet alloantibodies and genotyping or phenotyping of the maternal, paternal, and/or neonatal platelets. NAIT can occur in first pregnancies. After diagnosis, subsequent pregnancies should be monitored and antenatal therapy can be used to avoid bleeding complications.

Quick Answers for Clinicians

When should neonatal alloimmune thrombocytopenia be suspected?

Neonatal alloimmune thrombocytopenia (NAIT) should be suspected in newborns with incidentally detected thrombocytopenia or manifestations of a bleeding complication (eg, ultrasound anomalies in utero that may suggest intracranial hemorrhage [ICH], petechiae, bruising, or purpura shortly after birth, hematomas that develop at sites of injection, or bleeding following circumcision), and in all pregnancies of women who have had a previously affected infant.

What are the primary laboratory tests used to investigate neonatal alloimmune thrombocytopenia?

The primary tests for neonatal alloimmune thrombocytopenia (NAIT) are an initial CBC with platelet count to confirm thrombocytopenia, followed by a maternal serum test for platelet alloantibodies. (Neonatal samples are discouraged for circulating antibody testing due to insensitivity.) Platelet genotyping is performed to assess maternal, paternal, and/or neonatal human platelet antigen (HPA) incompatibilities. The identification of an HPA incompatibility along with the presence of a corresponding maternal alloantibody establishes a diagnosis of NAIT.

What is the role of screening for neonatal alloimmune thrombocytopenia?

Broad screening in first pregnancies is not currently recommended, although groups such as the International Collaboration for Transfusion Medicine have proposed that universal screening of first pregnancies be considered. Currently, testing for neonatal alloimmune thrombocytopenia (NAIT) is recommended during pregnancy in women with at-risk pregnancies (eg, women with a previously affected child), and antenatal therapy is recommended in such pregnancies to avoid bleeding complications. See Antenatal Testing below.

What are some of the challenges of diagnostic testing for neonatal alloimmune thrombocytopenia?

Diagnosis of neonatal alloimmune thrombocytopenia (NAIT) can be challenging when rare antiplatelet antibodies are involved. Cross-matching between maternal serum and paternal platelets can be performed in specialized laboratories and may detect less common or unique antigen incompatibilities. Delayed, low-titer, or low-affinity alloantibodies can also present diagnostic challenges.

Indications for Testing

Laboratory testing for NAIT is appropriate for:

Diagnosis of a fetus with bleeding on ultrasound (especially if suggestive of ICH) or a neonate with bleeding symptoms or early-onset, severe thrombocytopenia
Diagnosis of a fetus of a woman with a previously affected child
Risk assessment in women planning a pregnancy who have a history of an affected pregnancy or 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. However, there are 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 (ie, antiplatelet antibody tests and platelet antigen genotyping) if there is clinical suspicion.

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 the 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 cases. 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 enables 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.