Urinary Nitrites and Leukocyte Esterase
Tests to detect nitrites and leukocyte esterase have become part of a routine urinalysis. Most species of bacteria that colonize in the urine cause nitrates, which are derived from dietary metabolites, to be converted to nitrites. In healthy people, both the urinary nitrite test and the leukocyte esterase (LE) tests are negative. A negative nitrite test does not necessarily mean that the urine is free of all bacteria, particularly if there are clinical symptoms, because some bacteria do not produce nitrites. Optimal results for a urinary nitrite test are obtained by analyzing an early morning. specimen that has been incubating in the bladder for 4 hours or more. A clean catch or mid-stream specimen is important to reduce bacterial contamination.
White cells in the urine usually indicate a urinary tract infection. The leukocyte esterase (LE) test detects esterase, an enzyme released by white blood cells. Positive test results are clinically significant. The LE test is also used to screen for gonorrhea and for amniotic fluid infections. The combination of the LE test with the urinary nitrite test provides an excellent screen for predicting a urinary tract infection. A urine sample that tests positive for both nitrate and leukocyte esterase should be cultured for pathogenic bacteria.
These tests are indirect ways of detecting bacteria in the urine. Significant urinary tract infections may be present in patients who do not experience other symptoms. If undetected and untreated they can cause potentially severe kidney disease. The urinary nitrite and LE tests are often ordered to screen patients at high-risk for urinary tract infections, including pregnant women, school-age females, elderly patients, and persons with a history of urinary tract infections.
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A negative nitrite and leukocyte esterase test indicates that a patient does not have a urinary tract infection.