Endotoxin determination in viscous opaque solutions of iron dextran by Limulus amebocyte lysate

LAL would not form a clot when mixed with a viscous, opaque parenteral preparation of iron dextran spiked with endotoxin. However, recoverable precipitate could be obtained by diluting the LAL iron dextran mixture with PBS and centrifuging. Although the pellet so formed was red colored the protein present could be quantitated by dissolving it in a Coomassie Blue stain solution. The very rapid change in color from reddish black to deep blue was measured quantitatively in a spectrophotometer and was sigmoidally related to the amount of endotoxin used to spike the iron dextran. This method is suggested to be generally useful to measure quantitatively endotoxin concentrations too low to form a clot with LAL but high enough to precipitate recoverable protein from LAL.