Quantitation of endotoxin in products using the LAL kinetic turbidimetric assay.

The data presented here show the kinetic turbidimetric LAL assay to be a highly quantitative and effective method for determining endotoxin concentrations in products. The assay allows for the accurate assessment of inhibiting or enhancing effects in products when related to a LRW standard curve. However, designating some products as inhibitors or enhancers can be both misleading and erroneous unless qualified as to the dilution and/or endotoxin concentration. Our results demonstrate that some products can yield both inhibiting and enhancing results when related to water. Due to the enhanced resolution of the kinetic turbidimetric assay, these complicating inhibition/enhancing effects can usually be avoided by diluting to the WED. Alternatively, products could be related to a PSC in which the endotoxin response is defined and quantified within the product itself. The practicality of a PSC, however, depends upon the pass/fail limit established, the "cleanliness" of the product used as a standard and the degree of product "lot to lot" variability. Current FDA Guidelines consider a PSC valid providing the value of the "unspiked" product extrapolated from the regression line of the PSC is less than 10% of lambda, the lowest endotoxin concentration used to construct the standard. All products in which LAL kinetics have not been previously analyzed will require a characterization similar to that used with the four products described. From these data, the optimal methodology for kinetically testing the product (dilution to a WED or generation of a PSC) can be determined. Although analysis of endotoxin in a product will always require a characterization of the kinetics of the LAL-endotoxin-product reaction, subsequent testing should be rapid and straightforward. More importantly, the kinetic turbidimetric assay allows the user to quantitatively assess product endotoxin levels with a degree of precision greater than that of any other methodology currently in use.