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All About Antibiograms

Sanford Guide's Stewardship Alerts are educational resources developed by our Antimicrobial Stewardship Program Manager, Ann Lloyd, Pharm.D., BCPS, BCIDP. The content of our Stewardship Alerts may not reflect the views of the Sanford Guide editorial board and intended for educational use only. A printer friendly version of this information (formatted slightly differently) is available for download here.

Stewardship Alert:

An Introductory Guide to Antibiograms

The primary goal of antimicrobial stewardship (AMS) programs is to optimize antibiotic use. Interventions such as prospective audit and feedback and preauthorization are the primary methods used to achieve this goal. However, tracking and reporting antibiotic use and outcomes are also part of the Centers for Disease Control and Prevention (CDC) Core Elements of Hospital Antibiotic Stewardship Programs.1 The CDC Core Elements recommend that microbiology and stewardship personnel work together to present data from lab reports, such as antibiograms, in a way that encourages optimal antimicrobial use.

The Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America implementation guidelines for AMS programs offer further suggestions about incorporating antibiograms into the AMS program.2 The guidelines emphasize creation of stratified antibiograms to assist programs with development of empiric therapy guidelines. They also recommend that programs work with the microbiology laboratory to determine the impact of different susceptibility data reporting strategies on the construction of the antibiogram.

Many major guidelines recommend using antibiograms to inform treatment selection and improve antibiotic use. However, individual clinicians may not be familiar with antibiograms and their importance in antibiotic prescribing and antimicrobial stewardship. One study found that while residents were aware of institutional antibiograms, only 70% felt comfortable using them, and less than half knew how to access them.3

The following information is meant to introduce antibiograms and their use in antibiotic decision-making. Users seeking more advanced information are encouraged to review the references at the end of this page for further reading.

What is an antibiogram?4-5

An antibiogram is a report generated from antimicrobial or antifungal susceptibility test results that lists the percent of organisms isolated that are susceptible to the antimicrobial agents tested. The antibiogram typically includes data from a single health care facility or health system and is from a defined time frame. Simply put, it is a display of a facility’s pathogens and susceptibilities. An antibiogram is different from the antimicrobial susceptibility profile of an organism reported with an individual patient’s microbiology culture results.

What are the different types of antibiograms?4,6-7

Routine or Single Facility Antibiogram

The most common type of antibiogram is the routine or single facility antibiogram. This is a report comprised of aggregated cumulative antimicrobial susceptibility testing data from a single health care facility.

Abbreviated Antibiogram

An abbreviated antibiogram can be constructed from a traditional antibiogram and may be limited to the most common organisms encountered along with antimicrobials used to treat them.

Enhanced Antibiogram

Enhanced antibiograms, also known as customized or specialty antibiograms, are antibiograms where the data is stratified to guide empiric therapy in a specific population. An example of an enhanced antibiogram is a syndromic antibiogram. This is an antibiogram specific to a type of infection such as urinary tract infection and would only include pathogens reported in urine cultures.

Rolling or Real-Time Antibiogram

A rolling antibiogram, also known as a real-time antibiogram, is an antibiogram constructed with a shifting time frame or longer time frame. This type of antibiogram provides users with current data while maintaining sufficient isolates to provide statistical validity (minimum of 30 isolates).

Advanced Antibiograms

Two types of advanced antibiograms, combination and escalation have recently become more common:

  • A combination antibiogram is used to show the likelihood that at least one drug in a combination will provide coverage for a given pathogen.
  • An escalation antibiogram provides information about susceptibility rates to antibiotics based on their resistance to other agents.

What are the components of an antibiogram?4,6

Most antibiograms include:

  • the time frame from which the data was collected and analyzed
  • name of the facility
  • comments on the methodology
  • list of organisms
  • number of isolates analyzed
  • list of antibiotics (or antifungals)
  • percent susceptibility (range 0-100%)

The following may be included using footnotes or as a supplement:

  • information about how the data were collected and/or reported
  • cost information
  • source information (blood, urine, etc.)
  • formulary status
  • other facility-specific information
  • graphs or charts with susceptibility trends over time
  • genetic markers of antimicrobial resistance

The Clinical and Laboratory Standards Institute (CLSI) recommends the following for antibiogram development, but there can be variability between facilities in adhering to the recommendations. Users should check with their laboratory department and/or AMS team and use caution when comparing antibiogram data from different facilities.

  • Data is analyzed and presented annually but could occur more often.
  • Surveillance isolates and results from screening cultures (examples: screening for methicillin-resistant Staphylococcus aureus or vancomycin-resistant Enterococcus species) are excluded because they misrepresent the types of isolates found in patients with suspected infection.
  • Only the first isolate, per patient, per time period is included regardless of source or susceptibility testing profile.
  • A minimum of 30 isolates of each organism is necessary to get a reliable percent susceptible statistic. Using a smaller sample size results in a wider confidence interval and a reduced predictive value of the antibiogram. Therefore, organisms with fewer than 30 isolates should not be reported.
  • Only includes results for antibiotics (or antifungals) that are routinely tested at the facility.
  • Does not include results for antibiotics (or antifungals) that are only tested on resistant isolates. They should be excluded because they are biased toward lower susceptibility since they were only tested against a less susceptible group of the isolates.
  • Results should be reported as percent susceptible.

What can an antibiogram be used for?4,6,8

Guiding Empiric Therapy

Antibiograms can be used by clinicians as a general guide to help with empiric antimicrobial therapy decisions while the results of definitive susceptibility testing are pending or not available.

Treatment Selection Before and Organism is Identified

The antibiogram can also be used when the organism is not yet known. Clinicians can look at the most common pathogens isolated at the facility and select therapy based on known causative pathogens for the suspected site of infection. For example, if the patient presents with signs of a urinary tract infection, the clinician could use the antibiogram to identify which Gram-negative pathogens typically associated with that infection are common at the facility (such as Escherichia coli).

Antibiotic Selection Based on Susceptibility Percentage

While there is limited data to support a specific percent susceptible needed for optimal coverage, if the risk of morbidity and/or mortality is high, agents with 90-95% susceptibility should be selected. Agents with 80-85% susceptibility may be acceptable for treating infections in patients without a risk for morbidity and/or mortality in the next 24-48 hours. However, other factors need to be considered in conjunction with the antibiogram.

Users must not simply look for the agent with the highest percent susceptible and instead should look at the data in the clinical context. The type of infection, severity of illness, hospital versus community onset, possible microbiological etiologies, previous treatments or infections, and local guideline recommendations should all be considered along with the antibiogram.

Multifacility and Regional Applications of Antibiograms

Some states, regions, or communities may aggregate antimicrobial susceptibility data in a multifacility antibiogram. These can be used to guide prescribing at a facility when local data is not available. They can also be used to identify antimicrobial resistance in a region and serve as a benchmark for individual facilities.

Where might I find an antibiogram?4

Antibiograms should be easily accessible to all stakeholders at a facility. Historically, antibiograms were printed and available in a pocket guide format. More recently, a downloadable file may be distributed via email, placed on the facility’s website, or linked in the electronic health record. Facilities may also incorporate their antibiograms in a mobile application for use by clinicians at the bedside. Community, state, and regional antibiograms may be found on local, regional, or state health department websites.

What are some limitations of the antibiogram?5-6,9

There are several limitations that should be considered when using antibiograms to determine antimicrobial regimens for patients.

Lack of Time Relativity

Antibiograms do not include the time of the sample collection relative to the hospital admission making it challenging to differentiate between community-onset and hospital-onset infections.

Culturing Practices Impact Antibiogram Data

The culturing practices at a facility impact the data available for the antibiogram (for example, not culturing uncomplicated infections can lead to biased results toward more resistant organisms) and there is no way to determine if the organisms included are pathogens or colonizers.

Not Helpful for Empiric Therapy in Recurrent or Recent Infection

The antibiogram is not helpful for empiric therapy decisions for patients with recurrent or recent infections. Instead, users should use the patient’s microbiology results and antimicrobial history to determine the best treatment.

Missing Pharmacokinetic Factors

Antibiograms do not include any pharmacokinetic factors such as site of infection, pharmacokinetic/pharmacodynamic parameters, contraindications, efficacy, safety, Clostridioides difficile risk, or other patient factors.

Antibiograms Represent All Culture Sources

Unless an antibiogram is a syndromic antibiogram, it includes data from all culture sources (blood, urine, etc.). This makes it more challenging to differentiate therapy for a specific type of infection. It also lacks information for managing infections with more than one pathogen.

What is the role of the antimicrobial stewardship program in antibiogram use?5-6,8-9

AMS programs may use antibiograms for a number of purposes, including:

  • recommending empiric therapy for individual patients
  • developing treatment guidelines and algorithms
  • making decisions about the formulary for antimicrobial agents

The antibiogram can also be used to monitor trends and changes over time, although this method is not without limitations. AMS teams can use antibiograms as an educational tool for discussions around local susceptibility patterns and empiric antibiotic decisions.

AMS programs can work with microbiology labs to develop stratified antibiograms to improve the predictive validity of antibiograms for individual patient cases. They can also work to get the antibiogram integrated in the electronic health record to provide information to clinicians in a timely manner.

References:

  1. CDC. Core Elements of Hospital Antibiotic Stewardship Programs. Atlanta, GA: US Department of Health and Human Services, CDC; 2019. Available at https://www.cdc.gov/antibiotic-use/core-elements/hospital.html. Accessed August 25, 2023.
  2. Barlam TF, Cosgrove SE, Abbo LM, et al. Implementing an antibiotic stewardship program: guidelines by the Infectious Diseases Society of American and the Society for Healthcare Epidemiology of America. Clin Infect Dis. 2016;62(10):e51-e77.https://doi.org/10.1093/cid/ciw118
  3. Tallman GB, Vilches-Tran RA, Elman MR, et al. Empiric antibiotic prescribing decisions among medical residents: the role of the antibiogram. Infect Control Hosp Epidemiol. 2018; 39(5):578-583. doi: 10.1017/ice.2018.28. Epub 2018 Mar 1. PMID: 29493481
  4. CLSI. Analysis and Presentation of Cumulative Antimicrobial Susceptibility Test Data. 5th ed. CLSI guideline M39. Clinical and Laboratory Standards Institute; 2022.
  5. Klinker KP, Hidayat LK, DeRyke CA, et al. Antimicrobial stewardship and antibiograms: importance of moving beyond traditional antibiograms. Ther Adv Infect Dis. 2021 May 5;8:20499361211011373. doi: 10.1177/20499361211011373. PMID: 33996074.
  6. Truong WR, Hidayat L, Bolaris MA, Nguyen L, Yamaki J. The antibiogram: key considerations for its development and utilization. JAC Antimicrob Resist. 2021 May 25;3(2):dlab060. doi: 10.1093/jacamr/dlab060. PMID: 34223122.
  7. Teitelbaum D, Elligsen M, Katz K, et al. Introducing the escalation antibiogram: a simple tool to inform changes in empiric antimicrobials in the nonresponding patient. Clin Infect Dis. 2022 Nov 14;75(10):1763-1771. doi: 10.1093/cid/ciac256. PMID: 35380628.
  8. Simner PJ, Hindler JA, Bhowmick T, et al. What's New in Antibiograms? Updating CLSI M39 Guidance with Current Trends. J Clin Microbiol. 2022;60(10):e0221021. doi:10.1128/jcm.02210-21. PMID: 35916520.
  9. MacDougall C. A cloudy crystal ball: critically assessing and rethinking the antibiogram. Clin Infect Dis. 2023; ciad468, https://doi.org/10.1093/cid/ciad468

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