Rapid Detection of Gram-Negative Bacteriuria by Use of the Limulus Endotoxin Assay

Abstract

The Limulus in vitro endotoxin assay was evaluated as a possible method for the prompt detection of significant gram-negative bacteriuria in children. This assay is capable of detecting endotoxin associated with intact cell walls of viable gram-negative bacteria as well as free endotoxin. Quantitative results are obtained following a 1-h incubation of Limulus lysate and 10-fold dilutions of otherwise untreated urine. A standard curve of Limulus activity and viable cell counts of Escherichia coli and Klebsiella pneumoniae in urine demonstrated that a positive Limulus reaction at a dilution of 1:100 or 1:1,000 indicated a colony count of at least 100,000 bacteria/ml. A positive Limulus reaction only from undiluted urine or at a dilution of 1:10 indicated less than 100,000 cells/ml. These experimental observations were confirmed by comparing the Limulus test with quantitative plate counts on 209 urine specimens from a mixed pediatric population. These results indicate that the Limulus assay is a simple, accurate method for rapid presumptive detection of gram-negative bacteriuria in patients where an immediate diagnosis is needed. This test would also seem promising for screening large patient populations for bacteriuria or for monitoring the effectiveness of treatment of urinary tract infections.