A urinary tract infection showing leukocytes but negative nitrite is most commonly caused by which organism?

• A. Escherichia coli
• B. Enterococcus faecalis
• C. Klebsiella pneumoniae
• D. Pseudomonas aeruginosa

Answer: (B)

Leukocytes in the urine indicate infection, while the nitrite test detects bacteria that convert urinary nitrates to nitrites. Many typical urinary pathogens are Gram-negative and reduce nitrate, so the nitrite test is often positive with them. When you have pyuria but a negative nitrite result, it suggests organisms that do not reduce nitrate to nitrite. Enterococcus faecalis is a common cause of urinary tract infections and does not perform this nitrate reduction, so the nitrite test remains negative. In that setting, Enterococcus becomes the most likely culprit, especially in older adults, hospitalized patients, or those with catheters. Staph saprophyticus can also be nitrite negative but is more characteristically seen in young sexually active women and is less common overall.