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A comparative analysis between the rFC and LAL endotoxin assays for agricultural air samples




Krause, Laura Ann, author
Reynolds, Stephen J., advisor
Schaeffer, Joshua W., committee member
Ellis, Robert P., committee member

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Agricultural workers experience increased exposure to inhalable dust and endotoxins, which make up the outer membrane of Gram-negative bacteria species. Endotoxin has specifically been linked to an increased degree of pro-inflammatory symptoms from inhaled dust, leading to a variety of lung diseases. Because there is no standardized method of collection or analysis of endotoxin, there are paramount gaps in the knowledge of how best to collect and analyze samples. The aims of this study were to: (1) assess the recovery from PVC filters spiked with known endotoxin concentrations; and (2) compare two different biological endotoxin assay kits: Lonza rFC and Associates of Cape Cod Pyrochrome Chromogenic, in order to detect any significant variation in measured endotoxin concentrations and potentially establish a conversion factor for interstudy comparison purposes. The LAL assay uses a component found in the blood of horseshoe crabs in order to detect and quantify endotoxin concentrations. This process poses some concern with variability, as the reactivity of lysate with endotoxin can vary greatly between individual horseshoe crabs. The newer rFC assay offers an additional option for endotoxin analysis that does not require the use of horseshoe crabs. Because all of the materials are produced in a laboratory, the consistency between kits is much higher. In Aim 1, PVC filters in replicates of five were liquid-spiked with 5 levels of known amounts of endotoxin. To simulate effects of sampling and handling, each filter was then desiccated for 24 hours and loaded into SKC Button Aerosol Samplers where air was pulled through them for 4 hours at a flowrate of 4 L/min to mimic field sampling conditions. Samples were then frozen at -80ºC, thawed, and extracted. Each sample was analyzed for endotoxins using the rFC assay. For Aim 2, a combination of personal, area, and field blanks were collected from two Colorado dairy farms from 2013-2014 in conjunction with a larger study for a total sample size of n=31. Samples were desiccated for 24 hours, frozen at -80ºC, thawed, and extracted. Each sample was then analyzed using the rFC and LAL assay and the results were compared. Using the rFC assay, measurements for endotoxin concentrations were on average several magnitudes lower than the anticipated concentration. Spike recoveries ranged from 1-8%. It is likely that the hydrophobic properties of the PVC filters did not allow complete absorption of the liquid spikes, but rather evaporated into the air. For aim 2, there was no statistical difference found between the rFC and LAL assay for the total sample set (p-value 0.7146) using an alpha=0.10. There was also no statistical difference between assay types for the personal sample subset (p-value 0.3788). However, there was a statistically significant difference for the area sample subset (p-value 0.0698) and the lab and field blank sample subset (p-value 0.06638). Due to the small sample size, the power had to be adjusted to accommodate an alpha value of 0.10. The correlation between observations for all samples was found to be reasonably high with an r value of 0.867. The R2 coefficient value was found to be 0.7524. This indicates that 75.24% of the variability in LAL assay data can be explained by rFC assay data. The rFC assay serial dilution of standards gives a much more broad detection range of 0.005-5.0 EU/ml. The LAL standards only cover a fraction of this range, going from 0.005-0.04 EU/ml, 0.02-0.16 EU/ml, and 0.16-1.28 EU/ml. This dramatically decreases the chances of correctly identifying the dilution factor on the first attempt, and can create additional costs to use extra kits for re-analysis. The rFC assay can also be a considerably more cost effective option when purchasing in bulk of 20-30 kits at a time; however, when purchased individually, the LAL assay is less expensive. Overall, the development of the rFC assay greatly reduces the amount of horseshoe crabs harvested and bled for collection, reduces the costs of formulating the lysate enzyme, and most significantly, reduces the inconsistency in endotoxin measurement results.


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