Assays / Protein Reference Units / Acetylcholinesterase Isoenzyme (Amniotic Fluid)

Clinical use: Polyacrylamide gel electrophoresis of amniotic fluid cholinesterase isoenzymes has proved a useful adjunct to the estimation of alphafetoprotein in the early antenatal diagnosis of open neural tube defects. A collaborative study which investigated the potential of the qualitative isoenzyme technique concluded that it could reduce the number of AFP-based false positive diagnoses without significant loss of sensitivity. It would not appear, however, to be a substitute for amniotic fluid AFP in the diagnosis of neural tube defects.
The specific AChE isoenzyme of neuronal origin is demonstrated as a 'fast' electrophoretic band on polyacrylamide gel electrophoresis. The specificity of the band is confirmed by inhibition with the specific inhibitor BW284C51. The 'fast' isoenzyme may be seen in cases of severe exomphalos and in intrauterine fetal death but is not apparent in cases of fetal blood contamination - a common cause of false positive AFP elevation. False positive AChE bands have been reported in cases of contamination of amniotic fluid with fetal calf serum during the harvesting of cells for cytogenetic studies.
The specific AChE can also be detected by an enzyme immunoassay using the monoclonal antibody to the isoenzyme - Novoclone AChE-4f19. The sensitivity of the EIA for open neural tube defect in clear amniotic fluids approaches 100%. Positive results may also be seen in about 50% of major anterior abdominal wall anomalies. Slight elevations are seen with gross blood contamination and more marked elevations in case of IUD. False positive reactions due to fetal calf serum contamination are NOT seen.

Sample requirement: 1 mL amniotic fluid.

Mode of reporting: Results are reported as POSITIVE indicative of fetal abnormality, NEGATIVE, or EQUIVOCAL. The latter will require interpretation in the light of high resolution ultrasonography.

Centre offering this assay: Sheffield

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