Do blood and meconium affect the detection of endotoxin in amniotic fluid with the limulus amebocyte gel clot assay?

The limulus amebocyte lysate (LAL), assay is the most sensitive technique for the detection of endotoxin in biological fluids. Because endotoxin is a component of gram-negative bacteria, the assay has been employed in the detection of gram-negative bacterial contamination of biological fluids. The LAL assay is rapid, inexpensive, easy to perform, and requires little laboratory expertise. When used in conjunction with the gram stain examination of amniotic fluid, it improves the detection of intra-amniotic infection before the availability of culture results. However, the usefulness of the LAL assay in the detection of endotoxin in other body fluids is limited by the presence of an inhibitor to the gelation of the assay. The studies reported in this communication were undertaken to establish if amniotic fluid contains such an inhibitor. Sterile amniotic fluid (AF) samples obtained from 93 patients by transabdominal amniocentesis before labor were used to determine the ED 50 dose of endotoxin necessary for a positive LAL result. The ED 50 dose of endotoxin required for gelation was significantly higher when AF--rather than pyrogen-free saline--was used as the diluent, implying that inhibitors are in fact present (ED 50 = 58.3 pgm/ml). The presence of blood or meconium in the AF did not enhance inhibition significantly: ED 50 doses were 58.3 pgm/ml and 56.2 pgm/ml, respectively. This is not significantly different from the ED 50 of clear amniotic fluid.