This report describes a method of continuously, stably, and inexpensively measuring endotoxin (ET) concentrations in dialysate fluid using an ET sensor with intermittent injection of limulus reagent. An ET solution simulating dialysate fluid was sampled in a single tube at a flow rate of 260 μl/min and mixed with 30 μl of limulus reagent intermittently injected into the tube. The absorbance of the solution was measured after the limulus reaction at 313 or 318° K at 26 min. A good linear relationship (r = 0.98) between peak area of absorbance and ET concentration at ET concentrations ranging from 0 to 0.12 endotoxin unit (EU)/ml was obtained, using a spectrophotometer with a cell volume of 8 μl. The baseline rose after the measurements were taken because the cell volume was so small that the cell was stuffed with gel. A good linear relationship (r = 1.00) at ET concentrations of 0.1-0.25 EU/ml was also obtained, and the baseline was unchanged after measurements, using a metal free spectrophotometer with a cell volume of 420 μl. In conclusion, to measure ET concentrations below 0.1 EU/ml, the cell volume of a metal free spectrophotometer should be minimal.
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