Limulus amebocyte lysate (LAL) detection of endotoxin in human blood

Abstract

Limulus amebocyte lysate (LAL) has been applied to the detection of endotoxin in human serum, plasma and blood since the early 1970s. Although the diagnostic potential of LAL for endotoxemia was recognized immediately, the assay's modest sensitivity and specificity (for Gram-negative sepsis/bacteremia) were perceived as limiting the clinical usefulness of LAL. In an attempt to overcome these drawbacks, many studies have been conducted since the initial work by Levin and his colleagues. Numerous attempts have been made to improve the sensitivity of the assay by changing the formulation of the LAL and assay methodology. The original gel-clot method has for the most part been replaced with turbidimetric or chromogenic methods. The amount of endotoxin detectable within a 1 h incubation period has gone from the nanogram to the picogram range. Since blood (plasma) components interfere with the test, various methods to remove inhibition and/or enhancement have been developed. The chloroform extraction technique of Levin and co-workers has been replaced with acid extraction or with dilution and heating. Partitioning of endotoxin in blood may also influence the assay (recovery). Many recent investigators use platelet-rich plasma instead of ordinary plasma, while a few studies have used whole blood. Even with all the improvements, the specificity and related diagnostic usefulness of the LAL assay for Gram-negative sepsis remain an obstacle for regulatory acceptance. This may have more to do with our understanding of the septic process than with the ability of LAL to detect endotoxin. Although a recent study indicates that the type of Gram-negative bacteremia may be a critical determinant for clinical utility of the LAL test, the presence of endotoxin is not highly predictive of Gram-negative sepsis and vice versa. However, with the potential availability of anti-endotoxin therapy, the diagnosis of endotoxemia, with or without bacteremia, may be extremely important for timely and effective treatment modalities. It is concluded that the LAL test and accompanying sample preparation has evolved into a clinically useful test for the detection of circulating endotoxins and even its modest predictability for sepsis may have some clinical utility.