Nitrites Information and Courses from MediaLab, Inc.

The combination of decarboxylation of arginine (red color in the 2nd tube from left compared to the yellow color of the control to its left), the hydrolysis of esculin (black pigment in the esculin agar tube) and the reduction of nitrates to nitrites (red color in last tube on the right) are biochemical characteristics confirmatory for S. anginosus ("milleri").

False Positive BilirubinFalse positive results may occur when patients are on large doses of chloropromazine, and may occur in the presence of metabolites of phenazopyridine. When these compounds are present, the urine becomes red. Metabolites of Lodine® (etodolac) may cause false positive or atypical results. False Negative BilirubinFalse negative bilirubin dipstick results are often due to testing a specimen that is not fresh. Bilirubin breaks down when exposed to light. Indoxyl sulfate (Indican) can produce a yellow orange-to-red color response which may interfere with the interpretation of a positive or negative reaction. Positive nitrites due to a urinary tract infection may also cause a false negative result.

False Positives:A false positive result for blood on the urine chemical reagent strip can occur when oxidizing contaminants, such as hypochlorite (bleach), remain in collection bottles after cleaning. Contamination of the urine with provodine-iodine, a strong oxidizing agent, used in surgical procedures can also result in a false positive reaction. Microbial peroxide found in association with urinary tract infections may also cause false-positive results. Capoten® (Captopril) can cause decreased reactivity.The muscle tissue form of hemoglobin, myoglobin is a well-known cause of false-positive reactions on the blood portion of the reagent strip. When tissue hemoglobin is present, the urine specimen has a clear red appearance. Patients suffering from muscle-wasting disorders or muscular destruction due to trauma, prolonged coma, or convulsions or individuals engaging in extensive exertion may have myoglobin in their urine. Specific tests for myoglobin, such as immunodiffusion techniques or protein electrophoresis, are needed to confirm the presence of this substance in a urine specimen. Levels of ascorbic acid normally found in urine do not interfere with this test. False Negatives:False negative results may occur in some analysis methods when the concentration of ascorbic acid is greater than 5 mg/dL. The sensitivity of the blood portion of the test strip is decreased in specimens with a high specific gravity and increased protein. High levels of nitrites may delay the reaction, causing a false negative to be reported. If the pH of a urine sample is below 5, hemolysis of red cells as part of the test reaction is inhibited which results in a false negative reaction. An improperly mixed specimen may test negative if the red blood cells are in the sediment.

Bladder infections are usually caused by gram-negative bacteria. These bacteria reduce nitrates to nitrites when urine remains in the bladder three to four hours. Nitrite is not present in urine under normal circumstances. When present, nitrites react with p-arsanilic acid to produce a diazonium compound. The diazonium compound in turn couples with 3-hydroxy-1,2,3,4 tetrahydrobenzo-(h)-quinolin to produce a pink color. A first morning, clean, voided midstream specimen is optimal for detecting nitrites in urine.

False Positives:A false positive nitrite test result may occur when a urine specimen has remained at room temperature for an extended period of time, allowing bacterial contaminants to multiply and produce measurable levels of nitrites. Interference from some medications that cause the urine to become red or orange may lead to an incorrect reading of positive for nitrite. False Negatives:False negative results may occur in urine specimens that did not remain in the bladder for the sufficient length of time needed for the bacteria to reduce a measurable quantity of nitrate to nitrite. Other reasons for false negative results include high specific gravity, ascorbic acid levels above 25mg/dL or a low pH (<6). Less frequently, the cause may be due to a lack of sufficient nitrate in the diet (green vegetables) or further reduction of nitrite to nitrogen when large numbers of bacteria are present. In patients receiving antibiotics, the metabolism of the bacteria may be inhibited which would also produce a false negative reaction.

False negative bilirubin dipstick results are often due to testing a specimen that is not fresh. Bilirubin breaks down when exposed to light. Indoxyl sulfate (Indican) can produce a yellow orange-to-red color response which may interfere with the interpretation of a positive or negative reaction. Positive nitrites due to a urinary tract infection may also cause a false negative result.

The nitrites portion of the reagent strip provides a rapid screening test for the presence of gram-negative bacteria that are often responsible for urinary tract infections. Although urine cultures are still needed to confirm the diagnosis and monitor any urinary tract or kidney infection, the need for a culture may not be obvious because in some cases of early bladder infection, the symptoms may be vague or the patient may be asymptomatic. Diagnosis and treatment of cystitis (bladder infection) is important because if left untreated it may result in kidney damage, impairment of renal function, hypertension and/or septicemia.

This test is sensitive to 0.06-0.1 mg/dL nitrite ion in urines with a low specific gravity and ascorbic acid concentrations of less than 25 mg/dL. Pink spots or pink edges should not be interpreted as a positive result because some medications can color urine red or turn red in an acid environment. Any degree of uniform pink color should be considered positive, suggesting the presence of 105 organisms/mL. Detection of low levels of nitrite ion may be enhanced by comparing the activated test strip to a white background. It is important to note that color development is NOT proportional to the number of bacteria present. The test is specific for nitrites and does not react with any other substances normally present in urine. Negative results do not necessarily rule out a urinary tract infection because yeasts or gram-positive bacteria unable to reduce nitrites may be the causative agent.