A positive (+) result of nitrites in urine (nitrituria) is indicative of the presence of bacteria in the urine (bacteriuria) and it is usually related to a urinary tract infection (UTI).
Nitrates are normally present in urine. However, some types of bacteria are capable of converting nitrates, which are derived from dietary metabolites, to nitrites.
The most common species of bacteria that cause the conversion of nitrates to nitrites are from the enterobacteriacea family such as those of the Escherichia coli, Klebsiella, Proteus, and Serratia genera (all of them gram-negative organisms).
The presence of nitrites in the urine (nitrites positive +) usually signifies the presence of bacteria in the urine (bacteriuria) due to a urinary tract infection (UTI).
However, a negative nitrite test does not provide proof that the urine is free of all bacteria because not all bacteria are capable of converting nitrate to nitrite. That is the case for bacteria from enterococcus, staphylococcus or streptococcus genus (gram-positive organisms) and yeast of the Candida genera.
The nitrite urine test to detect bacteria in the urinary tract is usually combined with the leukocyte esterase test.
This test is performed along with other urine tests. The patient must collect a sample of the urine in a specific container using a special kit. This sample will be sent to the lab for the analysis. The urine should be collected by a clean-catch midstream to avoid bacterial contamination.
Optimal results are obtained by using a first morning specimen that has been incubating in the bladder for at least 4 hours or more.
Nitrites in urine can be tested either with a dipstick or with a tablet that change of color in the presence of a high amount of nitrites in the urine.
In laboratories, there are automated machines that perform the analysis, but it is also possible to buy strips that let you analyze a urine sample at home. They comprise up to 10 or more different reagents or chemical pads to perform different urine tests at the same time.
The strips are calibrated so that they turn pink within 30 seconds if bacteria are present (105 or more organisms per mL in the urine specimen). The color may be different depends on the test used.
The urine nitrite test must be negative to be considered within normality. As previously commented, a negative nitrite test does not mean that there is no bacterial infection.
A positive result of nitrites (nitrites in urine +) in the urine is helpful for diagnosis.
The most common causes for a positive result are:
A positive result from the nitrite test is a sign of a significant bacteriuria and a urine culture is needed to know the responsible bacteria.
A bacteria count over 100,000/mL (bacteriuria) is considered evidence of a bacterial infection.
Some drugs or medical conditions may give false positive in the nitrite urine test: