The presence of nitrites in the urine is indicative of a urinary infection. It is not always routinely requested, but may be used by the doctor to guide the diagnosis when in doubt. Let’s see in this article how it is done.
Nitrites in the urine are substances that are of diagnostic importance to guide the suspicion of urinary infection. Normally, there should be no nitrites in human urine. Its presence is abnormal.
Urine is the fluid made by the kidneys to expel waste that is useless for the body. Through the expulsion to the outside it is possible to maintain the filtration of the blood as a defense method.
Among the substances that are expelled with the urine are nitrates. Although they sound similar to nitrites, they are not the same. Chemically there are differences between them that determine that some are normal to find and others are not.
Nitrates in the urine, when they meet certain bacteria, are converted to nitrites. For this reason, the nitrite test is associated with urinary infections, since bacteria must be present for them to exist as such.
Nitrites do not exist only because of bacteria. Substances such as potassium nitrite or sodium nitrite are used in the food industry to enhance the red color of meats, for example, artificially.
Vegetables, on the other hand, usually have nitrates among their compounds. Leafy greens, beets, and radishes are rich sources of nitrates. The world health organization – WHO – has established a daily consumption limit of no more than 3.7 milligrams for each kilogram of people’s weight.
Nitrates that the body cannot use and that are excreted in the urine can be taken up by certain bacteria to become nitrites, as we already said. But not all bacteria have this capacity, although the ones that most frequently because urinary infections are:
- Escherichia coli.
- Proteus mirabilis.
When is a urine nitrite test indicated?
There are symptoms that are characteristic of a urinary infection and that raise suspicion. Doctors orient their diagnosis towards cystitis when the person reports certain signs that are classic. Among these symptoms we have:
- Dysuria: Difficulty urinating.
- Pain or burning when urinating.
- Changes in urine color.
- Pollakiuria: Urgent need to urinate all the time.
Faced with a clinical picture with any of these characteristics, the professional may request a urine nitrite test. In itself, this determination is not made in isolation, but is part of the complete biochemical examination of urine.
It is common to combine the urine nitrite test with the leukocyte esterase test. A positive result in both is highly suggestive of a urinary tract infection.
Leukocyte esterase is an enzyme produced by neutrophils, basophils, eosinophils, and monocytes. All of these are white blood cells. A positive result in the urine indirectly indicates that there are white blood cells due to an infection.
How is the urine nitrite test performed?
Once the physician has ordered the complete urine test, including nitrites, the task of collecting the sample begins. This point is very important to ensure that the work of the biochemist reaches reliable results.
The collection must discard the first stream of urine, which is flushed down the toilet, and collect the second part in a container. If a sample is obtained that comes from several hours before without urinating it will be more reliable, for example, in the morning when waking up.
The biochemist inserts a test strip into the urine to be tested. Test strips contain substances that change color when they detect what they want to measure.
In general, test strips do not measure a single thing, but contain several reagents to make the process more efficient. With a single test strip, nitrites, glucose, esterase and proteins can be detected.
Although brand dependent, urine nitrite reagents often turn pink when detected. In any case, each manufacturer specifies the colors and their interpretation on the packaging of the strips.
How to interpret the results?
So, what happens if the urine nitrite test is positive? Let’s say a person consults for burning urination and fever, the doctor requests a general urine test, and the laboratory reports positive nitrites. What to do?
A positive result suggests a urinary infection. If we add a positive leukocyte esterase to this, then there is practically a diagnosis. The professional may indicate an antibiotic treatment without waiting for the results of a urine culture. In any case, the possibility of modifying the antibiotic chosen later will be reserved.
It is important to note that positive nitrites do not preclude the performance of a urine culture. Culture continues to be the test that confirms the diagnosis of urinary tract infection and defines the type of antibiotic to use.