A prospective study of amniotic fluid cholinesterases: comparison of quantitative and qualitative methods for the detection of open neural tube defects

A comparison of amniotic fluid alpha-fetoprotein and acetylcholinesterase in the prenatal diagnosis of open neural tube defects and anterior abdominal wall defects

Amniotic fluid samples received for routine prenatal diagnosis of open neural tube defects were used for a study to compare amniotic fluid acetylcholinesterase (AChE) determination using a monoclonal antibody (4F19) enzyme antigen immunoassay and amniotic fluid alpha-fetoprotein (AFP) measurement as diagnostic tests for open neural tube defects. The study was based on 9964 women with singleton pregnancies and known outcome (including 6 with anencephaly and 18 with open spina bifida) having an amniocentesis at 14-23 weeks of gestation. The AChE immunoassay yielded detection rates for anencephaly of 100 per cent (95 per cent confidence interval (CI) 54.07-100 per cent), for open spina bifida of 100 per cent (95 per cent CI 81.47-100 per cent), for anterior abdominal wall defects of 20 per cent (95 per cent CI 0.51-71.64 per cent), and a false-positive rate of 0.22 per cent (95 per cent CI 0.14-0.34 per cent) excluding anencephaly, open spina bifida, and anterior abdominal wall defects. For similar detection rates the false-positive rate of the AFP test was significantly higher, 0.74 per cent (95 per cent CI 0.58-0.94 per cent). On the basis of these findings, it is recommended that the technically simple AChE immunoassay should be used on all samples; the AFP test should only be used on the 0.5 per cent of the samples with concentrations of AChE activity > or = 8.5 nkat/l for clear samples and blood-stained samples becoming clear after centrifugation, and > or = 25.0 nkat/l for blood-stained samples that are discoloured after centrifugation; an AFP cut-off level of 2.0 MOM is recommended for this policy. Thereby, the detection rates for anencephaly, open spina bifida, and anterior abdominal wall defects would be 100, 100, and 20 per cent, respectively (95 per cent CIs 54.07-100, 81.47-100, and 0.51-71.64 per cent, respectively), and the false-positive rate would be 0.08 per cent (95 per cent CI 0.03-0.16 per cent) (excluding anencephaly, open spina bifida, and anterior abdominal wall defects).

Ultrasonographic and biochemical abnormalities associated with the prenatal diagnosis of epignathus

A case of epignathus, detected prenatally in a 20-week fetus, is described. The pathological features are discussed in relation to the biochemical and ultrasonographic findings.

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.

Neural tube defect-specific acetylcholinesterase: its properties and quantitation in the detection of anencephaly and spina bifida

Amniotic fluid from neural tube defect-affected pregnancies (NTD) contains three forms of acetylcholinesterase (AChE), the major species of which is present only in trace amounts in normal pregnancies or those associated with a non-NTD fetal malformation. The activity of this 'specific' AChE is increased 62-fold in the presence of NTD and its measurement provides a sensitive and specific test for the biochemical detection of this disorder. The sedimentation coefficient of 'NTD specific' AChE (10.3S) indicates that it is a tetrameric species, and that the two additional forms present, (4.0S and 5.5S) are monomer and dimer, respectively. Gel filtration studies also support these findings. Combining these data, the molecular masses of monomer, dimer and tetramer are shown to be 78, 126 and 256 kDa (+/- 10%). 'NTD-Specific' AChE does not react with the detergent Triton X-100, indicating that it is a soluble, and probably secreted, species without membrane associating properties.

Amniotic fluid gel acetylcholinesterase determination in prenatal diagnosis: dark field illumination as a method for improving the detection of precipitation bands

This paper describes a sensitive method for the detection of precipitation bands in the qualitative acetylcholinesterase (AChe) gel test. This method--dark field illumination--is compared with two existing methods of detection: epi-illumination of unstained gels and transillumination of dithio-oxamide stained gels. The comparison has been carried out on a selected series of 271 amniotic fluid samples, taken before the 22nd week of gestation. All 96 samples from normal pregnancies were scored as negative with all three visualization methods. For the detection of neural tube defects (NTD) and omphaloceles, dark field illumination proved to be more efficient than epi-illumination or dithio-oxamide staining. In case of another congenital defect, we obtained a positive result with dark field illumination, and a negative one with the other detection methods. It is concluded that dark field illumination was shown to be the most sensitive method for the detection of precipitation bands.

Amniotic band disruption syndrome associated with elevated amniotic AFP and normal acetylcholinesterase gel test

Ultrasonography in an 18 week pregnancy selected for detailed scanning because of a single elevated maternal serum AFP result suggested the presence of anencephaly. Amniocentesis at 19 weeks yielded a clear AF with an elevated AFP result (5.4 MOM) and no evidence of an acetylcholinesterase band of neural origin on PAG electrophoresis. At termination, the fetus had cephalo-amniotic fusion and multiple abnormalities including bilateral cleft lip and palate and digital amputations characteristic of the amniotic band syndrome. The cranial defect was completely sealed by attachment of the amniotic surface of the placenta to the skull.

Predictive value of amniotic acetylcholinesterase analysis in the diagnosis of fetal abnormality in 3700 pregnancies

The value of acetylcholinesterase (AChE) analysis as an adjunctive test to amniotic alpha fetoprotein (amAFP) for the diagnosis of fetal abnormality has been investigated in a series of 3785 amniotic fluid samples. Quantitative analysis of AChE performed retrospectively on a selected group of 541 amniotic fluid samples failed to discriminate between normal and open neural tube defect pregnancies. Qualitative analysis of AChE by polyacrylamide gel (PAG) electrophoresis in the same series of 541 fluids correctly identified 251 of the 255 pregnancies with open neural tube defect and 29 of the 31 pregnancies with false positive amAFP results. The failure of the test to diagnose 4 cases of open neural tube defect was probably attributable to the age and condition of the stored AF samples. Routine diagnostic testing of AChE isoenzymes in a further 3244 AF samples successfully identified all 170 cases of open neural tube defect and 20 cases with other fetal defects. Thirteen fluids gave false positive AChE results (0.4 per cent) compared to 59 of the series in which there were false positive amAFP results (1.8 per cent). Six of the 13 false positive AChE cases had AChE bands of low intensity which would not be regarded as diagnostic of fetal abnormality, and in five the AChE band may have been the result of significant blood contamination. False positive AChE results contributed to the decision to abort three apparently normal fetuses, but a normal AChE result undoubtedly helped to save a number of pregnancies with false positive amAFP results. Our experience suggests that repeating the amniocentesis may help in resolving the rare diagnostic difficulty of a positive AChE result with or without an elevated amAFP in the absence of ultrasound evidence of fetal abnormality, particularly where there is blood contamination of the amniotic fluid sample.