Determination of nitrite in urine
Nitrites are determined in urine as an indirect sign of bacteriuria. Normal urine does not contain them at measurable concentrations. Some primarily Gram-negative bacteria, such as Escherichia coli, Proteus, Klebsiella, staphylococci and others, have the ability to reduce the nitrates present in the urine to nitrites. Diagnostic strips for indirect detection of bacteriuria use nitrites in the so-called Griess reaction. Its essence is the diazotization of sulfanilamide with nitrites in the sample to form the diazonium salt. This is followed by azocoupling of the resulting salt with a coupling agent, developing a pink to purple colour.
The nitrite urine test (urinalysis for nitrites) should be performed in the first morning urine, as in this case a sufficiently long time necessary for bacterial reduction of nitrates to nitrite in the bladder is guaranteed. Another recommendation is to consume enough vegetables (containing nitrates) the day before the examination. A positive urinary nitrite test confirms bacteriuria, while a negative one does not rule it out.
Indirect evidence of bacteriuria is indicative and not a substitute for microbiological examination.
