Diagnosis of Endotoxemia with Gram-Negative Bacteremia Is Bacterial Species Dependent: a Meta-Analysis of Clinical Studies

Endotoxemia is undetectable for up to 60% of cases of bacteremia caused by gram-negative (GN) species, a discordance attributed to the limitations of the Limulus assay for endotoxemia. The lipid A structure of the endotoxin molecule is critical for the sensing of GN bacteria by the host immune system although not so for sensing by the Limulus assay. The lipid A structure of commensal Enterobacteriaceae is hexa-acyl, whereas non-Enterobacteriaceae have a broader range of structures. By using a previously published classification of lipid A structures (R. S. Munford, Infect. Immun. 76:454-465, 2008), the association of endotoxemia with bacteremia caused by GN organisms is reexamined for 580 GN bacteremic patients from 46 studies. Endotoxemia was less commonly detected for cases of bacteremia caused by Salmonella enterica serovar Typhi (four studies; 15 of 55 cases of bacteremia [27%]) than for cases of bacteremia caused by Neisseria meningitidis (five studies; 69 of 84 cases [82%]) and Pseudomonas pseudomallei (one study; 38 of 41 cases [93%]) among studies restricted to those with specified cases of bacteremia caused by GN organisms. Among 23 unrestricted studies, endotoxemia was less commonly detected for cases of bacteremia with a commensal member of the Enterobacteriaceae (104 of 240 cases [43%]) than with non-Enterobacteriaceae (59 of 100 cases [59%]) (summary odds ratio, 0.53 [90% confidence interval, 0.33 to 0.85]). This finding is consistent across all the unrestricted studies, even including studies with seemingly contrary results for endotoxemia diagnosis among cases of bacteremia caused by GN bacteria overall. Surprisingly, with bacteremia caused by commensal Enterobacteriaceae, the diagnosis of endotoxemia appears to be unrelated to the Limulus assay sensitivity. Across these 45 studies, the association of endotoxemia with GN bacteremia is variable but consistent for different types of GN bacteremia.There are key structural differences between the lipid A components of the endotoxin molecule (lipopolysaccharide) of different gram-negative (GN) bacteria. Members of the Enterobacteriaceae characteristically have a lipid A structure with a hexa-acyl structure, whereas other lipid A structures are present for non-Enterobacteriaceae (45). These differences in lipid A structure are now known to be critically important for the recognition of GN bacteremia by the host immune system (45) by the MD-2-Toll-like receptor 4 receptor but not for sensing by the clotting proteins of the blood cells of the Limulus polyphemus horseshoe crab, from which the Limulus amebocyte lysate assay is derived (56). If the recognition of hexa-acyl lipid A by the host immune system has a role in the pathogenesis of bacteremia, it might be expected that the proportion with detectable endotoxemia among patients with GN bacteremia might depend on the lipid A structure of the isolate.Attempts to define the concordance between GN bacteremia and endotoxemia in patients with sepsis have been elusive. Indeed, two of those studies (15, 54), with over a hundred patients each, concluded that there is no concordance. The purpose of this review is to attempt to reconcile the disparate findings from studies of clinically detected sepsis by examining the proportion with endotoxemia detected by using the Limulus assay for patients with different types of GN bacteremia for which the lipid A structures are known. Of particular interest is the proportion with endotoxemia among cases of bacteremia caused by commensal Enterobacteriaceae versus non-Enterobacteriaceae. The statistical techniques of meta-analysis are used to derive study-specific and summary estimates of these proportions expressed as an OR and more so to obtain estimates of the consistency in these ORs across the panel of studies (24).