Test Catalog

Test ID: URNS    
Bacterial Culture, Aerobic, with Antimicrobial Susceptibilities, Urine

Useful For Suggests clinical disorders or settings where the test may be helpful

Diagnosis of urinary tract infections


Quantitative culture results may be helpful in discriminating contamination, colonization, and infection


Determining the in vitro antimicrobial susceptibility of potentially pathogenic aerobic bacteria, if appropriate

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

When this test is ordered, the reflex tests may be performed and charged. Antimicrobial agent appropriate to the organism and specimen source will be tested according to Mayo's practice and the laboratory's standard operating procedures.


See Special Instructions to review tables that provide a listing of the antimicrobials routinely tested in the laboratory as well as antimicrobials that may be tested upon request. These tables are organized by isolate groups and are not all inclusive. Call 800-533-1710 and ask to speak to the Bacteriology Antimicrobial Susceptibility Testing Laboratory if the organism or antimicrobial of interest are not listed in these tables.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Urinary tract infection (UTI) encompasses a broad range of clinical entities that vary in their clinical presentation, degree of tissue invasion, epidemiologic setting, and antibiotic therapy requirements. There are 4 major types of UTIs: urethritis, cystitis, acute urethral syndrome, and pyelonephritis. UTIs may also be classified as uncomplicated or complicated. Escherichia coli is the leading cause of uncomplicated community-acquired UTI. Risk factors that predispose one to complicated UTIs include: underlying diseases that are associated with kidney infection (eg, diabetes), kidney stones, structural or functional urinary tract abnormalities, and indwelling urinary catheters. Another classification of UTIs is as upper UTI (related to the kidney, renal pelvis, or ureter) or lower UTI (urinary bladder and urethra). The classic symptoms of upper UTI are fever (often with chills) and flank pain. Frequent painful urination, urgency, and dysuria are more often associated with lower UTI.


Antimicrobial susceptibility testing determines the minimum inhibitory concentration (MIC) value of selected antimicrobial agents against isolated potentially pathogenic bacteria. The MIC is the lowest antimicrobial concentration (of a series of increasing concentrations) that inhibits growth of the bacterium. Agar dilution MIC testing is performed by testing for growth of bacteria on agar plates containing varying concentrations of antimicrobial agents.


For each organism-antimicrobial agent combination, the Clinical and Laboratory Standards Institute provides interpretive criteria for determining whether the MIC should be interpreted as susceptible, susceptible-dose dependent, intermediate, nonsusceptible, resistant, or epidemiological cutoff value (ECV).

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

No growth, Organism present <10,000 cfu/mL, or mixed flora.


Identification of probable pathogens with colony count ranges.


When antimicrobial susceptibilities are performed, results are reported as minimal inhibitory concentration (MIC) in mcg/mL. Breakpoints (also known as "clinical breakpoints") are used to categorize an organism as susceptible, susceptible-dose dependent, intermediate, resistant, or nonsusceptible according to the Clinical and Laboratory Standards Institute (CLSI) guidelines.


In some instances an interpretive category cannot be provided based on available data and the following comment will be included: "There are no established interpretive guidelines for agents reported without interpretations."


Susceptible (S):

A category defined by a breakpoint that implies that isolates with an MIC at or below the susceptible breakpoint are inhibited by the usually achievable concentrations of antimicrobial agent when the dosage recommended to treat the site of infection is used, resulting in likely clinical efficacy.


Susceptible-Dose Dependent (SDD):

A category defined by a breakpoint that implies that susceptibility of an isolate is dependent on the dosing regimen that is used in the patient. In order to achieve levels that are likely to be clinically effective against isolates for which the susceptibility testing results are in the SDD category, it is necessary to use a dosing regimen (ie, higher doses, more frequent doses, or both) that results in higher drug exposure than that achieved with the dose that was used to establish the susceptible breakpoint. Consideration should be given to the maximum literature-supported dosage regimens, because higher exposure gives the highest probability of adequate coverage of a SDD isolate. The drug label should be consulted for recommended doses and adjustment for organ function.


Intermediate (I):

A category defined by a breakpoint that includes isolates with MICs within the intermediate range that approach usually attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates.

Note: The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated or when a higher than normal dosage of a drug can be used. This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major discrepancies in interpretations, especially for drugs with narrow pharmacotoxicity margins.


Resistant (R):

A category defined by a breakpoint that implies that isolates with an MIC at or above the resistant breakpoint are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules and/or that demonstrate MICs that fall in the range in which specific microbial resistance mechanisms are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.


Nonsusceptible (NS):

A category used for isolates for which only a susceptible breakpoint is designated because of the absence or rare occurrence of resistant strains. Isolates for which the antimicrobial agent MICs are above the value indicated for the susceptible breakpoint should be reported as nonsusceptible.

Note: An isolate that is interpreted as nonsusceptible does not necessarily mean that the isolate has a resistance mechanism. It is possible that isolates with MICs above the susceptible breakpoint that lack resistance mechanisms may be encountered within the wild-type distribution subsequent to the time the susceptible-only breakpoint was set.


Epidemiological Cutoff Value (ECV):

The MIC that separates microbial populations into those with and without acquired resistance (non-wild-type or wild-type, respectively). The ECV defines the highest MIC for the wild type population of isolates. ECVs are based on in vitro data only, using MIC distributions. ECVs are not clinical breakpoints, and the clinical relevance of ECVs for a particular patient has not yet been identified or approved by CLSI or any regulatory agency.


When an ECV is reported, the following comment will be included: "This MIC is consistent with the Epidemiological Cutoff Value (ECV) observed in isolates (WITH/WITHOUT) acquired resistance; however, correlation with treatment outcome is unknown."


(CLSI: Performance Standards for Antimicrobial Susceptibility Testing. 29th edition. CLSI supplement M100. Wayne, PA: Clinical and Laboratory Standards Institute; 2019)

Interpretation Provides information to assist in interpretation of the test results

In general, the isolation of more than 100,000 cfu/mL of a urinary pathogen is indicative of urinary tract infection (UTI). Isolation of 2 or more organisms with more than 10,000 cfu/mL may suggest specimen contamination. For specimens contaminated with the usual bacterial flora, bacteria that are potentially pathogenic are identified.


A "susceptible" category result and a low minimum inhibitory concentration value indicate in vitro susceptibility of the organism to the antimicrobial tested.


Refer to the Reference Values section for interpretation of various categories.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Although urine is normally sterile, contamination by organisms normally present in the urethra or on periurethral surfaces can allow a proliferation of these organisms yielding misleading urine culture results.


Urine held at ambient temperature for more than 30 minutes supports the growth of both pathogens and contaminants, leading to potentially inaccurate colony counts.


Urine obtained from catheter bags at the bedside and Foley catheter tips are unacceptable for culture.


When antimicrobial susceptibilities are performed, in vitro susceptibility does not guarantee clinical response. Therefore, the decision to treat with a particular agent should not be based solely on the antimicrobial susceptibility testing result.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Forbes BA, Sahm DF, Weissfeld AS: Infections of the urinary tract. In Bailey and Scott's Diagnostic Microbiology. 12th edition. St. Louis, MO, Mosby, 2007, pp 842-855

2. Cockerill FR: Conventional and genetic laboratory tests used to guide antimicrobial therapy. Mayo Clin Proc 1998;73:1007-1021

3. Procop GW, Church DL, Hall GS, et al: Koneman's Color Atlas and Textbook of Diagnostic Microbiology. Seventh Edition. Philadelphia: Wolters Kluwer Health, Lippincott, Williams and Wilkins; 2017. Chapter 2, Introduction to Microbiology Part II: Guidelines for the Collection, Transport, Processing, Analysis, and Reporting of Cultures From Specific Specimen Sources; p. 66-110

4. CLSI: Performance Standards for Antimicrobial Susceptibility Testing. 29th edition. CLSI supplement M100. Wayne, PA: Clinical and Laboratory Standards Institute; 2019, pp 3-5, 246

Special Instructions Library of PDFs including pertinent information and forms related to the test