Diagnostic testing for fetal and neonatal alloimmune thrombocytopenia (FNAIT) during pregnancy can sometimes be inaccurate. False positives and false negatives may occur in testing for FNAIT and can cause challenges in correctly treating symptoms and create unnecessary anxiety for families.
Fortunately, doctors rely on multiple tests to confirm FNAIT in fetuses, while post-delivery diagnostic tests provide the most accurate FNAIT diagnosis.
What is FNAIT?
Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare but serious condition 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 is a false positive vs a false negative?
A false negative may occur when testing doesn’t detect FNAIT-related antibodies or incompatibility, while FNAIT is actually present. This can be more of a concern if retesting isn’t performed later in pregnancy, as early medical treatment to protect the fetus may be missed.
A false positive can indicate the presence of FNAIT-related antibodies or incompatibility, without the baby being directly affected. Platelet levels may be normal, even if antibodies are detected. False positives are less common than false negatives. False negatives may lead to additional monitoring, regular blood tests and unnecessary anxiety during pregnancy.
Learn more about FNAIT testing and diagnosis
How do false positives and false negatives happen?
A false positive may occur if a test detects antibodies that are not clinically significant or if technical limitations impact highly sensitive methods of testing.
A false negative can occur if antibody levels are too low to detect or if antibodies develop after testing. Certain laboratories may also not have the necessary resources to test for less common platelet antigens.
How can the risk of false positives and false negatives in FNAIT be reduced?
FNAIT testing is only one piece of the diagnostic puzzle. In addition to testing for FNAIT-related maternal antibodies or antigen incompatibility, doctors also consider family history, imaging, symptoms and platelet counts. If there are any doubts about FNAIT testing, a specialized platelet immunology laboratory should be used to test samples.
The most accurate testing for FNAIT occurs post-delivery, and it can take several weeks for a diagnosis to be confirmed. If FNAIT is suspected in a newborn due to visible symptoms and platelet counts, treatment will begin without a confirmed diagnosis.
Diagnostic testing includes maternal antibody testing and platelet antigen typing of both parents, which are then compared with blood taken from the newborn. This information will be essential when planning for future pregnancies to help avoid or plan for any FNAIT-related health issues for the baby.
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