Positive amniotic fluid acetylcholinesterase: distinguishing between open spina bifida and ventral wall defects

The association between 'faint-positive' amniotic fluid acetylcholinesterase and fetal malformations

The finding of a 'faint-positive' acetylcholinesterase band in amniotic fluid samples of women at 15 weeks' gestation or above is associated with an increased risk of fetal abnormalities, most commonly gastroschisis. This finding warrants a targeted sonographic evaluation, in order to rule out significant fetal malformations.

False-positive amniotic fluid acetylcholinesterase analysis in the third trimester

Thirty-two third-trimester amniotic fluid samples were studied according to the indication for amniocentesis, result of acetylcholinesterase (AChE) analysis, and outcome, in order to address the issue of the effectiveness of AChE testing late in gestation. The results indicate that third-trimester AChE analysis is less effective than second trimester in distinguishing open neural tube defects (ONTDs) and ventral wall defects (VWDs) from other abnormalities. False-positive results occurred in cases of isolated hydrocephaly (four of seven cases), polyhydramnios, and intrauterine growth retardation (IUGR). Caution is recommended in interpreting third-trimester AChE tests, particularly when neither an ONTD nor a VWD is observed by ultrasound.

Ratio of immunochemically determined amniotic fluid acetylcholinesterase to butyrylcholinesterase in the differential diagnosis of fetal abnormalities

A total of 111 amniotic fluid samples, clear or blood stained, with elevated levels of alpha-fetoprotein and acetylcholinesterase was analysed by immunoassays specific for acetylcholinesterase and butyrylcholinesterase and the acetylcholinesterase/butyrylcholinesterase-ratios determined. Samples from 40 pregnancies associated with anencephaly, 47 pregnancies associated with open spina bifida or encephalocele and six pregnancies with fetal intrauterine death or miscarriage all had ratios of greater than 0.14. All 11 pregnancies with fetal ventral wall defects had ratios less than 0.14 as had four pregnancies with normal outcome and elevated levels of alpha-fetoprotein and acetylcholinesterase. Three fetuses with both open spina bifida and ventral wall defects were associated with ratios above 0.14. These results suggest that immunochemical determination of acetylcholinesterase and butyrylcholinesterase can be used to distinguish pregnancies complicated by anencephaly, open spina bifida, encephalocele and miscarriage from those with ventral wall defects and samples with false positive elevated levels of alpha-fetoprotein and acetylcholinesterase. The procedure is accurate and simple to carry out and well suited to routine use in a clinical chemistry laboratory.

Amniotic fluid acetylcholinesterase in the prenatal diagnosis of open neural tube defects and abdominal wall defects: a comparison of gel electrophoresis and a monoclonal antibody immunoassay

Stored amniotic fluid samples collected in Oxford and East Birmingham as part of the Collaborative Acetylcholinesterase Study were assayed for the presence of acetylcholinesterase (AChE) using a monoclonal antibody (4F19) enzyme antigen immunoassay. These results were compared with the results of a gel AChE which had been performed earlier. A total of 5689 samples from singleton pregnancies were analysed (including 36 with anencephaly, 77 with open spina bifida and 17 with anterior abdominal wall defects). The gel test yielded detection rates of 97% for anencephaly, 99% for open spina bifida and 94% for abdominal wall defects; the false positive rate (excluding pregnancies associated with serious abnormalities, miscarriages and intrauterine deaths) was 0.24%. The monoclonal test yielded similar results; using appropriate cut-off values to allow for differences in acetylcholinesterase levels in blood stained and clear samples, a similar false-positive rate of 0.22% was associated with detection rates of 97%, 95% and 71% respectively for the three types of defect. Although the detection rates and the false positive rate were slightly higher for the gel test, a result that might be explained by a decrease in AChE activity caused by storage of the samples, the monoclonal test has the advantages of requiring less interpretative expertise, it can be performed on a larger number of samples a day and it is not affected by contamination with fetal calf serum.

Amniotic fluid acetylcholinesterase measurement in the prenatal diagnosis of open neural tube defects. Second report of the Collaborative Acetylcholinesterase Study

Seventeen centres from Australia, Britain, France, and the United States collaborated in a study to compare amniotic fluid acetylcholinesterase (AChE) determination by gel electrophoresis and amniotic fluid alpha-fetoprotein (AFP) measurement as diagnostic tests for open neural tube defects. The study was based on 32,642 women with singleton pregnancies (including 428 with open spina bifida and 238 with anencephaly) who had an amniocentesis at 13-24 weeks' gestation. The AChE test yielded a detection rate for open spina bifida of 99 per cent (95 per cent confidence interval 98-100 per cent), 98 per cent for anencephaly (95 per cent confidence interval 96-100 per cent), and a false-positive rate of 0.34 per cent (95 per cent confidence interval 0.28-0.40 per cent) excluding miscarriages, intrauterine death, and serious fetal abnormalities. The false-positive rate was 0.30 per cent among the 13 centres that used a specific AChE inhibitor in the test. Comparable rates for the AFP test were less favourable. (For example, the open spina bifida detection rate was 90 per cent and the false-positive rate was 0.46 per cent using the cut-off levels specified in the U.K. Collaborative AFP Study.) The AChE false-positive rate was lower in samples that were not bloodstained (0.16 per cent) than in those that were (2.4 per cent). It was higher in women who had an amniocentesis on account of a raised maternal serum AFP level (0.56 per cent) than in those who had one for other reasons (0.29 per cent). The best results were obtained by a combination of the two tests, an effective and economical policy being to perform the AFP measurement on all amniotic fluid samples and an AChE test on samples with AFP levels greater than or equal to 2.0 multiples of the normal median (about 5 per cent of all samples). Using this policy, the open spina bifida detection rate was 96 per cent and the false-positive rate was 0.14 per cent (0.06 per cent for samples that were not bloodstained and 1.2 per cent for those that were; 0.40 per cent for women with raised serum AFP levels and 0.09 per cent for other women). This policy offers a useful improvement to the prenatal diagnosis of open spina bifida.

Amniotic fluid acetylcholinesterase ratios in prenatal diagnosis of fetal abnormalities

This study examined 141 pregnancies with acetylcholinesterase in the amniotic fluid to determine if an acetylcholinesterase/pseudocholinesterase ratio cutoff could be set to distinguish between neural tube defects, ventral wall defects, other fetal defects, and pregnancy problems. Amniotic fluid specimens with positive or faintly positive acetylcholinesterase bands were studied. They were scanned and analyzed for the acetylcholinesterase/pseudocholinesterase ratio. In 62 of 65 fluid samples from open neural tube defects, the acetylcholinesterase/pseudocholinesterase ratios were greater than 0.13. All cases with ventral wall defects (n = 29), fetal blood contamination (n = 16), or fetal ascites (n = 2) had acetylcholinesterase/pseudocholinesterase ratios less than 0.13. Samples of cystic hygroma fluid (n = 10) had acetylcholinesterase/pseudocholinesterase ratios greater than 0.13. Two patients with fetal teratomas had intermediate values. All patients with normal outcomes but positive acetylcholinesterase had ratios of less than 0.12. Acetylcholinesterase/pseudocholinesterase ratios are a valuable part of a prenatal program to accurately diagnose fetal abnormalities.

Secretion of acetylcholinesterase and butyrylcholinesterase from the guinea-pig isolated ileum

1. Strips of longitudinal muscle from guinea-pig ileum, retaining Auerbach's plexus, were superfused with oxygenated Krebs solution. Addition of 50 mM KCl led to a pronounced Ca2+-dependent increase in the activities of both acetylcholinesterase and non-specific cholinesterase (butyrylcholinesterase) in the perfusate but with no change in lactate dehydrogenase activity. 2. No release of acetylcholinesterase, either spontaneous or K+-evoked was observed in tissue freed of the nerve plexus, although release of butyrylcholinesterase still occurred. 3. Carbachol induced a marked Ca2+-dependent increase in the release of acetylcholinesterase but had no effect on the release of butyrylcholinesterase or lactate dehydrogenase. This carbachol-evoked increase in acetylcholinesterase release was blocked by hexamethonium but not by atropine. 4. Four readily soluble molecular forms of acetylcholinesterase and three soluble molecular forms of butyrylcholinesterase were present in innervated longitudinal muscle strips, but insignificant amounts of acetylcholinesterase were detected in denervated strips of muscle. Only one of the four molecular forms of acetylcholinesterase was recovered in the perfusates. 5. It is concluded that acetylcholinesterase is secreted from the nerves of Auerbach's plexus in response to depolarizing stimuli or to nicotinic cholinergic stimulation, while butyrylcholinesterase is secreted from non-neural elements, possibly the longitudinal muscle cells, of guinea-pig ileum in response to a depolarizing stimulus.

Amniotic fluid acetylcholinesterase: implications of an inconclusive result

The implications of an "inconclusive" acetylcholinesterase test (a faint but true band) in amniotic fluid were studied over a 2 1/2-year period in our laboratory. One thousand one hundred fifty-four amniotic fluid samples were tested for acetylcholinesterase and alpha-fetoprotein; the rate of an inconclusive acetylcholinesterase result was 3.3% (38 of 1154). Fourteen such results were found in patients with a high amniotic fluid alpha-fetoprotein level (23.3%), and 24 results were associated with normal amniotic fluid alpha-fetoprotein levels (2.19%). The rates of congenital fetal malformation associated with an inconclusive acetylcholinesterase result in the two groups were 57.14% and 37.5%, respectively. In amniotic fluid samples obtained before 15 weeks' gestation, there was a higher rate of inconclusive acetylcholinesterase tests (9.29%), but a lower percentage of malformed fetuses were found compared with later in pregnancy (2.46% and 56%, respectively). Thus we suggest the terminology "equivocal" for early specimens and "suspicious" for later specimens. If obtained in early second trimester and the ultrasound scan is normal, such findings implicate the need for a careful search for fetal malformations. A positive pregnancy outcome may be expected in most cases.