Evaluation of the Limulus amebocyte lysate and recombinant factor C assays for assessment of airborne endotoxin

As a potent inflammatory agent, endotoxin is a key analyte of interest for studies of lung ailments in domestic environments and occupational settings with organic dust. A relatively unexplored advance in endotoxin exposure assessment is the use of recombinant factor C (rFC) from the Limulus pathway in a fluorometric assay. In this study, we compared airborne endotoxin concentrations in laboratory- and field-collected parallel air samples using the kinetic Limulus amebocyte lysate (LAL) assay and the rFC assay. Air sampling was performed using paired Institute of Occupational Medicine (IOM) samplers, Button samplers, closed-face cassettes, and cyclone samplers. Field sampling was performed in 10 livestock production facilities, including those housing swine, chicken, turkey, dairy cows, cattle, and horses. Laboratory sampling was performed in exposure chambers using resuspended airborne dust collected in five livestock facilities. Paired samples were extracted in pyrogen-free water with 0.05% Tween 20 and analyzed using LAL and rFC assays. In 402 field sample pairs there was excellent agreement between endotoxin concentrations determined by LAL and rFC (r = 0.93; P < 0.0001). In 510 laboratory sample pairs there was also excellent agreement between the two assays (r = 0.86; P < 0.0001). Correlations for subgroups of facility or dust type ranged from 0.65 to 0.96. Mixed-model analysis of variance (ANOVA) for the field studies showed significant interactions of facility-sampler and facility-assay. rFC/LAL ratios of the geometric means were 0.9 to 1.14 for the samplers (not significantly different from 1.0). The data from this study demonstrate that the LAL assay and the rFC assay return similar estimates of exposure in livestock facilities. Both methods provided suitable lower limits of detection such that all but 19 of 1,824 samples were quantifiable.