Helicobacter pylori Culture with Antimicrobial Susceptibilities
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Helicobacter pylori is a spiral-shaped gram-negative bacterium that may cause chronic gastritis, peptic ulcer disease, or gastric neoplasia. In adults of industrialized countries, an estimated 0.5% of the susceptible population becomes infected each year, although the incidence has been decreasing over time. The organism may asymptomatically colonize humans. In suspected Helicobacter pylori-associated disease, the noninvasive stool antigen or urea breath test is recommended. If patients fail to respond to treatment and antimicrobial resistance is suspected, gastric biopsy, gastric brushings, or gastric aspirate may be cultured to attempt to recover the organism for antimicrobial susceptibility testing to assess for resistance.
Multidrug regimens are required to attain successful cure of Helicobacter pylori infection. Antimicrobial resistance in Helicobacter pylori is increasing. Disease caused by Helicobacter pylori resistant to clarithromycin or metronidazole is associated with a greater incidence of treatment failure than disease caused by a susceptible strain.
The Clinical and Laboratory Standards Institute (CLSI) recommends agar dilution for Helicobacter pylori antimicrobial susceptibility testing. Amoxicillin, ciprofloxacin, clarithromycin, metronidazole and tetracycline are routinely tested. The only antimicrobial for which interpretive breakpoints have been defined by the CLSI is clarithromycin.
Recovery of Helicobacter pylori from gastric specimens for antimicrobial susceptibility testing of the organism (amoxicillin, ciprofloxacin, clarithromycin, metronidazole and tetracycline are routinely tested)
A positive result provides definitive evidence of the presence of Helicobacter pylori.
Organisms may be detected in asymptomatic (colonized) individuals.
False-negative culture results may occur since the organism may die between biopsy collection and laboratory culture.
When antimicrobial susceptibilities are performed, definitions of susceptible, susceptible-dose dependent, intermediate, resistant, and nonsusceptible are as follows:
The "susceptible" category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the dosage recommended to treat the site of infection is used.
Susceptible-Dose Dependent (D)
The "susceptible-dose dependent" category 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 D category, it is necessary to use a dosing regimen (ie, higher doses, more frequent doses, or both) that results in higher drug exposure than the dose that was used to establish the susceptible breakpoint. Consideration should be given to the maximum approved dosage regimen, because higher exposure gives the highest probability of adequate coverage of a D isolate. The drug label should be consulted for recommended doses and adjustment for organ function. The D category may be assigned when doses well above those used to calculate the susceptible breakpoint are approved and used clinically.
The "intermediate" category includes isolates with antimicrobial agent minimum inhibitory concentrations (MIC) that approach usually attainable blood and tissue levels, and for which response rates may be lower than for susceptible isolates. 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.
The "resistant" category implies that the isolates 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 where specific microbial resistance mechanisms are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.
The "nonsusceptible" category is used for isolates for which only a susceptible interpretive criterion has been 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 is 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.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Culture-negative results may occur due to the fastidious nature of the organism. Delays in specimen transportation will decrease recovery of the organism. Culture should be set up as soon as possible following specimen collection. Antimicrobial therapy may render the culture negative.
Due to Helicobacter pylori’s fastidious nature and slow-growth, it may take 7 days to recover the organism and up to an additional 21 days to perform antimicrobial susceptibility testing.
The only antimicrobial for which interpretive breakpoints have been defined by the Clinical and Laboratory Standards Institute is clarithromycin.
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 after 7 days
Susceptibility results are reported as MIC in mcg/mL
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Theel ES. Helicobacter pylori Infection: Test Utilization Strategies for Diagnosis. Mayo Medical Laboratories Communique 2013;38(6):1-8