A new procedure for stable quantification of endotoxin in dialysate fluid using limulus reagent

For reliable determination of endotoxin, the activation of enzymes in lysate before measurement should be prevented, and the authors have designed a new procedure to effect this by dissolving the enzymes in lysate in a buffer solution of low pH. A given amount of the enzymes in lysate was dissolved in a lower pH buffer solution (pH 6.1-6.3) and the substrate was dissolved in a higher pH buffer solution (pH 8.0). After standing for 0-24 hr, both solutions were mixed with the sample solution. Data on blank absorbance and calibration line slope obtained by the new procedure were compared with those obtained by the conventional procedure. In the conventional procedure, blank absorbance increased with standing time, reaching approximately seven times the initial value in 24 hr, whereas in the improved procedure, it increased by 1.5 times at a standing time of 3 hr, after which it was independent of standing time. The change in slope of the calibration line with standing time was more gradual in the improved procedure than in the conventional procedure. The authors conclude that the activation of enzymes in lysate can be prevented by dissolving the enzymes in a buffer solution of low pH, and that this procedure is effective for long- term monitoring of endotoxin concentration.