Nitrates are ions that are in the greenvegetables that we consume in our diets. Nitrates are converted to nitriteswhen there is a bacterial infection in the urinary tract. If a patient issuffering from a urinary tract infection that is caused by gram-positivebacteria the nitrite dip-strip test may come out as negative. This falsenegative is seen because certain gram-positive bacteria may not produce enzymesneeded to convert nitrate to nitrite. Falseresults may also be seen if the urine sample contains a high concentration ofascorbic acid. Themost common way to detect nitrites in urine is by using a dip-strip test.
Thereagent strip is founded on the Griess test and involves a diazo reaction.Nitrite reacts with an aromatic amine (such as p-arasnilic or sulfanilic acid)in an acid medium to create a diazonium salt. This salt will then react withanother aromatic ring to produce an azo dye. The azo dye will produce a pink orred color on the strip, and this color is what signifies a positive result fornitrite in the urine. Chromatography can also be used to determinenitrite levels in urine. Ion chromatography, gas chromatography, and high-performanceliquid chromatography are all examples of chromatography that could be used inthis situation.
Othernitrite tests include electron paramagnetic resonance, spectrophotometry,infrared spectrophotometry, fluorescence probe, chemiluminescence and capillaryelectrophoresis. These tests are not cost-effective and take a long time tocomplete and therefore, are rarely used. Inconclusion, the negative nitrite results for this patient, whom is sufferingfrom an Escherichia coli infection wasdue to the fact the patient was also suffering from frequent urination. Frequent urination causes the urine sample collectedto not be retained in the bladder long enough for the Escherichia coli to release enzymes to convert nitrate intonitrite.
To get an accurate urinalysis, the urine sample must be retained inthe bladder for at least four hours, which ensures that bacteria in the urinehad enough time to release reducing enzymes to convert nitrate into adetectable amount of nitrite.