On an urine dipstick for suspected UTI, it is not uncommon to find positive nitrites. Nitrates are normally excreted by the kidney, however nitrites are not normally found in urine. Nitrites will be found when nitrate reductase organisms are present in the 
patient’s urine. These organisms are gram-negative bacteria including E. coli and other bacteria in the family Enterobacteriaceae (i.e Klebsiella).
NOTE: However, several other common urinary pathogens do not produce nitrites, such as: enterococci, S. saprophyticus, and Pseudomonas!! Therefore negative nitrites DOES NOT RULE OUT BACTERIURIA.
WATCH OUT FOR FALSE (-):
- If a patient has insufficient nitrates (i.e low vegetable intake in their diet), there may not be sufficient material to be converted by the bacteria
- It takes at least 4 hours for bacteria to convert nitrates to detectable levels of nitrite, so this test may be negative in patients with UTIs whose urinary frequency does not provide adequate time for bacterial conversion of nitrates to nitrites
REFERENCES
- Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. Am Fam Physician. 2005 Mar 15;71(6):1153-62.
- Chapter 104. Urinalysis and Urine Electrolytes. Principles and Practice of Hospital Medicine. dam C. Schaffer, MD