Fetal and neonatal alloimmune thrombocytopaenia (FNAIT) appears to be substantially under-identified in Canada, according to a national review of cases referred for testing published recently in Vox Sanguinis.
Researchers found a large gap between the number of cases expected based on birth rates and the number actually investigated, suggesting many affected families may never receive a diagnosis.
The analysis reviewed all FNAIT investigations sent to the National Platelet Immunology Reference Laboratory at Canadian Blood Services between January 20, 2014, and December 31, 2024. Over 11 years, 1,986 samples were received, including 1,076 maternal, 620 paternal and 290 neonatal samples. The median maternal age was 31.7 years. Among maternal samples, 130 of 1,076 cases, or 12.1%, had detectable human platelet antigen antibodies linked to FNAIT. A total of 135 antibodies were identified, most commonly anti-HPA-1a and anti-HPA-5b.
Read more about testing and diagnosis of FNAIT
Based on published estimates that FNAIT affects about 1 in 1,000 live births, the number of confirmed cases was far lower than expected. Across Canada, excluding Alberta and Quebec, the observed incidence was about 1 in 24,500 live births. For example, Ontario recorded 60 positive cases during the study period despite more than 1.5 million live births. Similar patterns were seen in other provinces. Annual referrals rose modestly but did not track with national birth numbers.
Genetic testing showed that risk varies depending on a mother’s platelet antigen type. Among mothers with the HPA-1b1b genotype, 75.0% had antibodies and 60.6% were incompatible with paternal samples. In contrast, only 0.3% of HPA-1a1a mothers had antibodies. For HPA-5b5b mothers, antibody positivity reached 35.0%, with 60.0% paternal incompatibility. However, genetic mismatch alone did not always lead to antibody formation.
Researchers explained that several factors may explain why FNAIT is missed. Mild newborn thrombocytopenia may resolve without further testing. Other conditions such as prematurity or infection may mask the diagnosis. In severe cases, fetal death may occur before testing is performed. Incomplete family testing also limits confirmation of incompatibility.
“Improved recognition of FNAIT through increased clinician awareness, standardized national investigation algorithms and routine antenatal screening may help reduce missed diagnoses and improve clinical management,” explained the authors of this study.
For patients, these findings highlight the importance of awareness. Earlier recognition of FNAIT can guide monitoring in pregnancy, inform delivery planning and reduce the risk of serious bleeding in newborns. Improved screening pathways and standardized investigation could help ensure affected families receive timely diagnosis and care, potentially improving outcomes for future pregnancies.
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