Acetylcholinesterase, Amniotic Fluid (AChE-AF), Amniotic Fluid

CPT CODE:

USEFUL FOR:

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

SPECIMEN REQUIRED:

1 mL of amniotic fluid. Do not centrifuge. Send specimen refrigerated. A specimen from the 16 to 18 week gestational periodof pregnancy is preferred. Amniotic fluid from the 14 to 21 weekgestational period is acceptable. Note:    1. Gestational age is required on request form for                      processing.                  2. If chromosome studies are also requested, see                       #8426 "Chromosome Analysis, Amniotic Fluid."  The                       specimen for AChE testing, when requested with                       chromosome analysis, cannot be frozen.                  3. If ordering electronically, please complete and submit                      a "MayoConnect Additional Test Information Form"                      (Supply T357 or see Special Instructions) with the specimen.                      If not ordering electronically, please complete and submit                      a "Cytogenetics/AFP Congenital Disorders Request Form"                      (Supply T238) with the specimen.

TRANSPORT TEMPERATURE:

Refrig\Ambient <96 hours OK\Frozen OK

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-lateraledges of the central neural groove to form the neural tube may fail,resulting in neural tube defects of various types depending on thesite of the failed fusion. These defects include anencephaly wherethe 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.

CLINICAL INTERPRETATION:

The presence of AChE in amniotic fluid is positive for a neural tubedefect if fetal contamination can be ruled out for reasons detailedunder  "Cautions".

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.