Comparative evaluation of the Limulus assay and the direct gram stain for detection of significant bacteriuria

A double blind study comparing the Limulus in vitro endotoxin assay with the direct Gram stain of uncentrifuged urine for detection of significant bacteriuria was performed. A total of 1077 urine specimens were examined by the two methods and the results compared with results of quantitative urine cultures. 203 samples produced growth of >10⁵ organisms per ml urine. The Limulus assay detected 86.2% of these specimens, and 98.8% of urines that contained >10⁵ Gram negative bacilli per ml. The Gram stain procedure detected only 69.5% of urines containing >10⁵ organisms per ml and 74.5% of specimens with >10⁵ Gram negative bacteria per ml urine. The Limulus assay demonstrated both greater sensitivity and greater specificity than the Gram stain procedure. Moreover, the Limulus test is much less susceptible to errors of interpretation than methods involving microscopy.