False positive gel‐acetylcholinesterase results in blood‐stained amniotic fluids

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Abstract
Summary. The effect of blood contamination on the gel‐acetylcholinesterase (AChE) test used in the diagnosis of fetal open neural‐tube defects was studied with amniotic fluid samples artificially contaminated with fetal or maternal blood in concentrations covering a range exceeding that usually found in clinical practice. Amniotic fluid samples contaminated with maternal blood gave negative gel‐AChE results at all concentrations. Contamination with fetal blood yielded positive results if the erythrocyte concentration was greater than about 60 × 106 cells/ml. Thus contamination of amniotic fluid with blood is only likely to cause false positive gel‐AChE results if this critical concentration is exceeded. Such samples will occur only rarely in clinical practice but when they do the diagnosis should be made with caution.