Toraymyxin and Other Endotoxin Adsorption Systems

Role and Measurement of Endotoxin in Septic Patients

Endotoxin, also known as lipopolysaccharide, is a major cell wall constituent of gram-negative bacteria ( Fig. 193.1 ). The lipid A portion of endotoxin is conserved across bacterial species and is responsible for many of the pathogenic effects observed in sepsis. Endotoxin in the blood comes from several potential sources, a bacterial infection or translocation of bacteria that reside normally in the commensal flora of the gastrointestinal tract during periods of hypotension and impaired gut perfusion in shock. Endotoxin causes the release of cytokines such as interleukin-6 and tumor necrosis factor-alpha and activates complement and coagulation factors resulting in multiple organ failure and even death. High levels of endotoxin in the blood are responsible for many of the symptoms seen during sepsis such as fever and elevated white blood cell count. Severe sepsis occurs when the body is overwhelmed by the inflammatory response and body organs begin to fail. Sepsis may develop as a result of infections acquired in the community, such as pneumonia, or may be a complication that develops during the treatment of trauma or cancer or after major surgery. Increased levels of endotoxin can be associated with other conditions in critically ill patients, such as cardiac surgery and burns.

The historical assay for the detection of lipopolysaccharide in non–blood-based or non–plasma-based fluids has been the limulus amebocyte lysate (LAL) assay. It is based on the observation in 1956 that killed gram-negative bacteria caused the blood of the horseshoe crab to clot. This test performs well in matrices such as crystalloids, food products, or solutions such as dialysate. In plasma and in whole blood, however, lipopolysaccharide exists in a variety of different forms and binds to lipopolysaccharide-binding protein and other substances including lipoproteins, so various extraction and pretreatment strategies have been developed to attempt to release lipopolysaccharide from its binding sites in whole blood or plasma and to neutralize inhibitors including fungal products and other proteins that confound LAL technologies. These changes have not been successful in overcoming all issues, however. In fact the LAL test has never been approved by regulatory agencies in North America for clinical use in humans. Nevertheless, some clinical data has been collected with it, demonstrating an association between high levels of endotoxin in sepsis and mortality.

The Endotoxin Activity Assay (EAA) was cleared by the U.S. Food and Drug Administration for the measurement of endotoxin in whole blood in 2004 to assess patients for risk of severe sepsis.

The test is done as a rapid assay in whole blood and relies on the following factors:

• Endotoxin reacts with a high-affinity antibody specific to lipid A.

• The antibody-antigen complex activates complement and is amplified by the patient's neutrophils in whole blood.

• The amplification results in an enhanced respiratory burst in the presence of zymosan; the burst is detected by luminal chemiluminescence.

• The magnitude of the priming influence is proportional to the concentration of antigen-antibody complex.

The test is feasible, accurate, and reliable, providing in less than 40 minutes a measurement of blood endotoxin concentration. In a multicenter clinical trial involving approximately 1000 patients, the test was shown to have excellent test characteristics in critically ill patients with suspected sepsis, and its results correlated strongly with adverse outcomes, including death and organ dysfunction. Elevations of endotoxin are associated significantly with the development of a clinical diagnosis of severe sepsis. When endotoxin levels are moderately high, the odds ratio for development of severe sepsis is 2.0; when endotoxin levels are very high, the odds ratio rises to 3.0. Results of this trial suggest that the EAA could be used as a biomarker of the risk for development of sepsis as well as a trigger for targeted endotoxin-directed therapies.