Mechanism: The nitrite test detects nitrite converted from dietary nitrates by Gram-negative bacteria such as E. coli.
A urinary tract infection may be associated with negative nitrite test if the causative bacteria does not convert nitrates to nitrites (Gram-positive bacteria, Mycobacteria, Neisseria gonorrheae, others).
False negative nitrite test in the presence of suitable Gram negative bacteria:
(1) diet low in nitrates (present in spinach and other vegetables)
(2) diuresis or frequent urination, with insufficient time for urinary bacteria to convert nitrates to nitrites (it usually takes at least 4 hours for sufficient conversion to take place)
(3) deterioration of chemicals on dipstick in high humidity
(4) high urine specific gravity
(5) high urinary levels of ascorbic acid
(6) prolonged storage of urine at room temperature prior to testing, with conversion of nitrite to nitrogen gas
(7) low levels of bacteria (a positive result is usually seen with > 10^5 organisms/mL)
False positives are rare:
(1) improperly stored urine with overgrowth of Gram-negative bacteria
(2) possibly ingestion or inhalation of nitrites (for angina, as drug abuse, in cured meats)
