Detection of endotoxin in antibiotic solutions with Limulus amoebocyte lysate

Twenty-eight antibiotics were tested with the Limulus amoebocyte lysate assay to determine their non-inhibitory concentrations (NICs). The Limulus amoebocyte lysate assay was found to be a valid test for most of the antibiotics tested; the NICs were found to be greater than the minimum valid test concentrations. Borderline results were obtained with cefamandole nafate and neomycin sulfate. Polymyxin B and colistimethate contained too much endotoxin to permit determination of NICs. The NIC of tetracycline hydrochloride was dependent on the initial concentration of antibiotic. This dependence was most likely caused by the amount of base required to adjust the pH before testing.