An HPA antibody screening is a key diagnostic test used to diagnose fetal and neonatal alloimmune thrombocytopenia (FNAIT). It is not part of the standard prenatal screening protocol and is only conducted when FNAIT is suspected during pregnancy.
The test determines whether the mother’s blood contains antibodies against fetal human platelet antigens (HPA). If positive, the pregnancy is identified as being at risk of FNAIT.
What is FNAIT?
Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare but serious condition that affects 0.1% of pregnancies in which a pregnant mother’s immune system produces antibodies against the platelets of her fetus. This occurs when a fetus inherits platelet antigens from the father that are not compatible with the mother, typically involving a protein called human platelet antigen (HPA). The mother’s immune system recognizes the fetal platelets as foreign, attacking and destroying them, leading to low platelet levels (thrombocytopenia) in the fetus or newborn.
What does HPA antibody screening involve?
HPA antibody screening isn’t part of standard prenatal testing because FNAIT is a rare disease with mostly mild symptoms, affecting only one in 1000 live births. Including HPA screening in the national prenatal screening program is not a priority for many at the moment, as the clinical benefit of screening has not yet been proven.
Learn more about FNAIT testing and diagnosis
An HPA antibody screening test is conducted during pregnancy if there is a family history, a previous FNAIT affected pregnancy or signs of bleeding or hemorrhage in the fetus.
Here is what often happens when an HPA antibody screening is ordered by a physician:
- A blood sample is taken and sent to a specialized laboratory.
- The laboratory analyzes the blood, checking it against specific HPA markers, most commonly HPA-1a.
- If HPA antibodies are confirmed, fetal HPA genotyping is performed to check if the fetus has the antigen.
Noninvasive fetal genotyping is the preferred option and involves further testing of the mother’s blood sample. Fetal cell-free DNA found in the mother’s blood is also tested to find out if the fetus inherited the HPA from its father.
What does HPA antibody screening tell us?
If no HPA antibodies are detected, the risk for FNAIT is low. If HPA antibodies are found, but the fetus shares the same HPAs as the mother, there is no risk. However, if the fetus has the same HPAs as the father, there is a risk of FNAIT. The presence of HPA antibodies to the fetus’s HPAs changes the level of prenatal care, with closer monitoring and proactive medical treatment.
In pregnancies at risk of FNAIT, the administration of intravenous immunoglobulin (IVIG) with or without steroids is effective in delaying or preventing the onset of severe thrombocytopenia. Invasive medical treatment is no longer recommended, as the risk of uncontrolled bleeding or hemorrhage is too high, potentially leading to miscarriage.
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