Test ID: ACHE_
Acetylcholinesterase, Amniotic Fluid (AChE-AF), Amniotic Fluid

Diagnosing open neural tube defects, and to a lesser degree, ventral wall defects

Birth defects involving openings along the brain and spine are called neural tube defects (NTDs). They develop in the early embryonic period when closure of the neural tube fails. Anencephaly is the most severe NTD, and occurs when the cranial end fails to form resulting in an absence of the forebrain, the area of the skull that covers the brain, and the skin. Most infants with anencephaly are stillborn or die shortly after birth. NTDs along the spine are referred to as spina bifida. Individuals with spina bifida may experience hydrocephalus, urinary and bowel dysfunction, club foot, lower body weakness, and loss of feeling or paralysis. Severity varies depending upon whether the NTD is covered by skin, whether herniation of the meninges and spinal cord are present, and the location of the lesion. NTDs not covered by skin are referred to as open NTDs and are typically more severe than closed NTDs. Likewise those presenting with herniation and higher on the spinal column are typically more severe.

Most NTDs occur as isolated birth defects with an incidence of approximately 1/1,000 to 2/1,000 live births in the United States. Rates vary by geographic region with lower rates being observed in the North and West than the South and East. A fetus is at higher risk when the pregnancy is complicated by maternal diabetes, exposed to certain anticonvulsants, or there is a family history of NTDs. Studies have shown a dramatic decrease in risk as a result of maternal dietary supplementation with folic acid. The March of Dimes currently recommends that all women of childbearing age take 400 micrograms of folic acid daily beginning at least 1 month prior to conception. For women who have had a prior pregnancy affected by an NTD, the recommended dose is 10 times this.

When an NTD is suspected based upon maternal serum alpha-fetoprotein (AFP) screening results or diagnosed via ultrasound, analysis of AFP and acetylcholinesterase (AChE) in amniotic fluid are useful diagnostic tools. AChE is primarily active in the central nervous system with small amounts of enzyme found in erythrocytes, skeletal muscle, and fetal serum. Normal amniotic fluid does not contain AChE, unless contributed by the fetus as a result of an open NTD.

Negative (reported as negative [normal] or positive [abnormal] for inhibitable AChE)

Reference values were established in conjunction with alpha-fetoprotein testing and include only amniotic fluids from pregnancies between 14 and 21 weeks gestation.

The presence of acetylcholinesterase in amniotic fluid is positive for a neural tube defect if fetal hemoglobin contamination can be ruled out.

False-positive acetylcholinesterase (AChE) results may occur when blood is present in the amniotic fluid specimen. For this reason, fetal hemoglobin analysis is performed on all specimens with a positive AChE result.

Muller F: Prenatal biochemical screening for neural tube defects. Childs Nerv Syst 2003 Aug;19(7-8):433-435 Epub 2003 Jul 12

• Informed Consent for Genetic Testing
• Second Trimester Maternal Screening Alpha-Fetoprotein (AFP)/Quad Screening Patient Information Sheet