The identification of false‐positive amniotic fluid acetylcholinesterase results due to fetal calf serum contamination

All 103 amniotic fluids with positive gel‐acetylcholinesterase (AChE) test results, obtained in Oxford between July 1980 and March 1983, were tested for the presence of fetal calf serum using a counter‐immunoelectrophoresis test for bovine serum albumin. None of the samples from pregnancies associated with neural‐tube defect (85), exomphalos (nine) and intrauterine death (four) had a positive fetal calf serum test, suggesting little or no immunological cross‐reaction between the antiserum used in the test and amniotic fluid proteins. Among the remaining five samples, from unaffected pregnancies (two bloodstained and three clear), two had positive fetal calf serum test results (both were clear samples). The fetal calf serum test is therefore a useful technique capable of identifying false‐positive AChE test results, without mis‐classifying true‐positive results as false‐positive.