The most accurate diagnosis of ONTD requires amniocentesis with measurement of AF AFP and AChE (24,70). Both of these tests are diagnostic of ONTD, and the existence of two such tests allows for their selected combined application to maximize diagnostic performance. Abnormal AFAFP results are usually defined as greater than 2.0 MoM. In these patients, AChE can add to the diagnostic certainty and reduce false positives to a dramatically low level. AChE can also be performed when a positive family history or ultrasound finding exists despite a normal AFAFP result. Table 4 provides the expected performance of AFAFP and AChE in the diagnosis of ONTD in patients with elevated MSAFP (71).
Conditions other than ONTD that are associated with an elevated AFAFP and/or detectable AChE include normal variance in a healthy fetus (AFAFP only), fetal blood contamination, open ventral wall defects detectable by ultrasound (gastroschesis, and less commonly omphalocele), congenital nephrosis (AFAFP only), and fetal demise.
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