Positive nitrates in urine but negative culture

Topic Resources

Urinalysis involves

• Chemical tests to detect and measure the level of various substances in the urine

• Often, examining urine under a microscope

Chemical tests look for protein, glucose (sugar), ketones, blood, and other substances. These tests use a thin strip of plastic (dipstick) impregnated with chemicals that react with substances in the urine and quickly change color. Sometimes the test results are confirmed with more sophisticated and accurate laboratory analysis of the urine.

Examining the urine under a microscope may be done to check for the presence of red and white blood cells, crystals, and casts (impressions of the kidney tubules created when urinary cells, protein, or both precipitate out in the tubules and are passed in the urine).

Protein in the urine (proteinuria) can usually be detected by dipstick when present in large amounts. Protein may appear constantly or only intermittently in the urine, depending on the cause. Proteinuria may occur normally after strenuous exercise, such as marathon running, but is usually a sign of a kidney disorder. Small amounts of protein in the urine may be an early sign of kidney damage due to diabetes. Such small amounts may not be detected by dipstick. In these cases, urine will need to be collected over a period of 12 or 24 hours and tested by a laboratory.

Ketones in the urine (ketonuria) can often be detected by dipstick. Ketones are formed when the body breaks down fat. Ketones can appear in the urine as a result of starvation or uncontrolled diabetes mellitus and occasionally after drinking significant amounts of alcohol.

Blood in the urine (hematuria) is detectable by dipstick and confirmed by viewing the urine with a microscope and other tests. Sometimes the urine contains enough blood to be visible, making the urine appear red or brown.

Leukocyte esterase (an enzyme found in certain white blood cells) in the urine can be detected by dipstick. Leukocyte esterase is a sign of inflammation, which is most commonly caused by a urinary tract infection.

The acidity (pH) of urine is measured by dipstick. Certain foods, chemical imbalances, and metabolic disorders may change the acidity of urine. Sometimes a change in acidity can predispose the person to kidney stones.

The concentration of urine (also called the osmolality, roughly indicated by specific gravity) can vary widely depending on whether a person is dehydrated, how much fluid a person has drunk, and other factors. Urine concentration is also sometimes important in diagnosing abnormal kidney function. The kidneys lose their capacity to concentrate urine at an early stage of a disorder that leads to kidney failure. In one special test, a person drinks no water or other fluids for 12 to 14 hours. In another test, a person receives an injection of vasopressin (also called antidiuretic hormone). Afterward, urine concentration is measured. Normally, either test should make the urine highly concentrated. However, in certain kidney disorders (such as nephrogenic diabetes insipidus Nephrogenic Diabetes Insipidus In nephrogenic diabetes insipidus, the kidneys produce a large volume of dilute urine because the kidney tubules fail to respond to vasopressin (antidiuretic hormone) and are unable to reabsorb... read more ), the urine cannot be concentrated even though other kidney functions are normal.

Sediment in urine can be examined under a microscope to provide information about a possible kidney or urinary tract disorder. Normally, urine contains a small number of cells and other debris shed from the inside of the urinary tract. A person who has a kidney or urinary tract disorder usually sheds more cells, which form a sediment if urine is spun in a centrifuge (a laboratory instrument that uses centrifugal force to separate components of a liquid) or allowed to settle.

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Abstract

Background and objectives

To evaluate whether initial urinalysis (UA) and urinary nitrite results can be used as a proxy for choosing empiric antibiotic therapy.

Materials and methods

A retrospective study was conducted in an urban inner city community hospital in New York City (NYU Woodhull Medical Center). We reviewed the charts of patients seen in the Emergency Department and Pediatric Clinic who had a diagnosis of urinary tract infection (UTI) during a 3 year time period (January 2010–December 2012). Statistical analysis was performed using SPSS 20.0 statistical software.

Results

Between January 2010 and December 2012, a total of 378 patients had a diagnosis of UTI. Seventy-five (19.8%) were males and 203 (80.2%) were females. Of the 378 patients with a diagnosis of UTI, the most common isolated pathogen was Escherichia coli, which was detected in 283 (74.9%) isolates. Other bacteria included Klebsiella spp 30 (7.9%), Proteus 21 (5.6%), Enterococcus 14 (3.7%), and others 30 (7.9%). The resistance rate was higher in the nitrite positive group for the following antibiotics: TMP/SMX and ampicillin with or without sulbactam. No significant correlation was found with the remaining studied antibiotics. No significant correlation was found between leukoesterase and the resistance patterns in all of the studied antibiotics, except cefazolin.

Conclusion

Urinary nitrite results are not helpful in choosing an initial antibiotic to treat a UTI. Leukocytosis in the blood or urine or the presence of a fever cannot be used to predict bacterial resistance. The use of nitrofurantoin or cephalexin for the treatment of cystitis was optimum, and in the presence of negative leukoesterase, nitrofurantoin was preferable to cephalexin.

Keywords

Pediatrics

Infectious disease-bacteria

Renal

Emergency department management

Abbreviations

UTI

urinary tract infection