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Unit Code 9287:
Acetylcholinesterase, Amniotic Fluid (AChE-AF), Amniotic Fluid

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Useful For

Open tube defects, and to a lesser degree, ventral wall defects

Clinical Information

Acetylcholinesterase (AChE) is 1 of 2 species of enzymes of the

hydrolase class that hydrolyze the acetylcholine cation to acetate

and choline. The pseudocholinesterase (PChE) found in serum

constitutes one of these classes and will not be considered here

(see #8518 "Pseudocholinesterase, Total, Serum"). The true

cholinesterase activity is found in nerve tissue, erythrocytes,

skeletal muscle, and cerebrospinal fluid. Small amounts of this

enzyme are also found in fetal serum. Normal amniotic fluid does

not contain true AChE, unless contributed by the fetus as a result

of open communication between fetal central nervous system, or

to a lesser degree, fetal circulation. Thus, open neural tube defects

result in the appearance of AChE in the amniotic fluid.

 

The embryologic events leading to the fusion of the dorsal-lateral

edges of the central neural groove to form the neural tube may fail,

resulting in neural tube defects of various types depending on the

site of the failed fusion. These defects include anencephaly where

the brain is replaced by amorphous neural tissue and the skull does

not form, exencephaly where the brain forms exterior to the skull, and

encephalocele where various portions of the brain can extend to the

skull and scalp. Midline defects in fusion of the neural tube may result

in spina bifida. Various other more subtle neural tube defects may also

occur. The term "open" neural tube defect is applied to defects that are

not skin covered and thus are diagnosable by the measurement of either

alpha-fetoprotein (AFP) or AChE. Collectively, the neural tube defects

affect approximately 6,000 infants each year in the United States.

 

The usual events leading to AChE testing in amniotic fluid are 2 positive

serum AFP results (high) and a positive amniotic fluid AFP. In practice,

amniotic AChE is usually ordered with the amniotic AFP. If positive, level II

ultrasound should be performed.

Reference Values

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.

Interpretation

The presence of AChE in amniotic fluid is positive for a neural tube

defect if fetal contamination can be ruled out for reasons detailed

under  "Cautions".

Cautions

False-positive AChE may occur from a bloody tap, particularly if

fetal blood is present. All positives have a fetal hemoglobin

performed. This is even more problematic for the AFP test since

AFP is essentially the fetal form of albumin.

 

Because of the mechanics of obtaining amniotic fluid in the

laboratory for AChE testing, a cytogenetics laboratory may handle

the specimen first in order to set up amniocyte cultures.  Contamination

of the specimen with fetal-calf serum can occur and will lead to an

elevated AChE activity of bovine origin. Fortunately, the migration of

the bovine enzyme on polyacrylamide is sufficiently unique to be

detected if the operator is well versed in the procedure.  Again, this is

also a problem for AFP testing, yielding false-positives.

Special Instructions and Forms

• MayoConnect Additional Test Information 

Clinical Reference

Wald N, Cuckle H, Nanchahal K:  Amniotic fluid acetylcholinesterase

measurement in the prenatal diagnosis of open neural tube defects.

Second report of the Collaborative Acetylcholinesterase Study.

Prenatl Diagn 1989;9:813-829

Key