The Sensitivity and Specificity of Nitrite Testing for Bacteriuria

Abstract

The sensitivity and specificity of the urine nitrite test were investigated using urine samples from 2,000 patients in order to determine the effectiveness of the test as a screening indicator for bacteriuria in a hospital clinical laboratory. Six of these patients had greater than 2+ microscopic bacteriuria and/or more than 10 WBC/HPF in their urine. Cultures were performed. Comparisons of urine samples that demonstrated positive and negative nitrite tests, pyuria and microscopic bacteriuria with urine culture results revealed disagreements. Among the 45 urines cultured, ten had truepositive nitrite test results, three had false-positive results, eight had false-negative results and 24 had true-negative results. Greater than 105 organisms/mL, including Escherichia coli, Pseudomonas aeruginosa, Proteus mirabilis , Group A beta-streptococci and enterococci, were found in eight of the 40 nitrite-negative urines. Three of the 20 urine samples with positive nitrite tests showed no growth on culture. Of 100 patient urines that had negative nitrite tests, 0-2+ microscopic bacteria and fewer than 10 WBC/HPF, six grew pathogenic organisms. It appears that routine use of the urine nitrite test for detection of occult urinary tract infection in hospitalized patients is neither as specific nor as sensitive as might be desired.