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. One-thousand seventy-seven urine specimens were examined by the two methods and the results compared with results of quantitative urine cultures. Two hundred three samples produced growth of greater than 10-5 organisms per ml. urine. The Limulus assay detected 86.2% of these specimens, and 98.8% of urines that contained greater than 10-5 Gram-negative bacilli per ml. The Gram stain procedure detected only 69.5% of urines containing greater than 10-5 organisms per ml. and 74.5% of specimens with greater than 10-5 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.