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Five Important Things To Know About Hospital Antibiograms

What is a hospital antibiogram? What can a hospital antibiogram be used for? This article is meant to help readers understand the answers to these questions and provide basic information about antibiograms.

Authored By: Timothy P Gauthier, Pharm.D., BCPS-AQ ID & Julie Ann Justo, Pharm.D, MS, BCPS-AQ ID

[Last updated: 6 April 2018]

To determine whether a specific antibiotic may be effective versus a given bacterial isolate, susceptibility testing can be performed in a microbiology laboratory. If the antibiotic is found to work against the bacteria, then it is labeled as “S” for susceptible. If the antibiotic is found not to work against the bacteria, then it is labeled as “R” for resistant.

In some instances sensitivity testing results fall between susceptible and resistant. In these cases the result may be called “I” for intermediate or “S-DD” for susceptible, dose-dependent.

In performing sensitivity testing, microbiology staff determine a bacterial isolate’s minimum inhibitory concentration (MIC) to a particular antibiotic. An MIC is the lowest concentration of a drug that prevents visible growth of the bacteria. MIC testing is typically done using a limited, pre-specified range of drug concentrations and in a fashion of doubling dilutions. For example, testing may be done at drug concentrations of 1 mcg/mL, 2 mcg/mL, 4 mcg/mL, and 8 mcg/mL. If no visible growth is found at any of these concentrations, the MIC can be designated <1 mcg/mL. If growth is seen at 1 mcg/mL, but not at 2 mcg/mL, then the MIC can be labeled as 2 mcg/mL. If there is growth at all of the tested concentrations, the MIC can be labeled >8 mcg/mL.

Once an MIC is known, the lab worker or clinician can refer to the breakpoints for that specific combination of bacteria and antibiotic to determine if it should be labeled as S, I / S-DD, or R. Breakpoint recommendations are provided by organizations like the Clinical & Laboratory Standards Institute (CLSI) or European Committee on Antimicrobial Susceptibility Testing (EUCAST). You can access CLSI breakpoints here and the EUCAST breakpoints here.

Let’s use an example where the reference tells you a breakpoint for susceptible is <1 mcg/mL, intermediate is 2-4 mcg/mL and resistant is >8 mcg/mL. If the MIC testing shows the MIC is <1 mcg/mL it can be labeled as susceptible. If the MIC testing shows 2 or 4 mcg/mL it can be labeled as intermediate. If the MIC testing shows 8 mcg/mL or more it would be labeled as resistant.

If the microbiology lab has a specimen that is positive for bacterial growth, they can perform sensitivity testing on that isolate versus a variety of antibiotics. The results of these tests can then be reported to the clinician in what is called an antibiogram. This is also referred to as a susceptibility testing report. When they hear the term “antibiogram” most people will likely think of a hospital antibiogram. That is something different, discussed shortly.

An antibiogram that a clinician may receive for a single isolate may look something like this here in table 1. Of particular note, the drug with the lowest MIC is not necessarily always the best drug to treat for infection, so MIC results are sometimes hidden to help providers avoid making the mistake of picking a drug just because it has the lowest MIC.

Table 1. Example of an antibiogram for a Staphylococcus aureus isolate

Staphylococcus aureus

Drug

MIC Result

MIC Interpretation
Oxacillin

64

R
Clindamycin

16

R
Doxycycline

0.5

S
Sulfamethoxazole/trimethoprim

32

R
Vancomycin

0.25

S

Over the course of a year a hospital may report hundreds or thousands of susceptibility testing results. This data can be combined into a cumulative report, which is more commonly known as a hospital antibiogram or institutional antibiogram. Table 2 provides an example of what an abbreviated annual hospital antibiogram might look like.

Table 2. Example of an abbreviated annual hospital antibiogram

Organism

Percent Susceptible

Cefazolin

Cefepime

Meropenem
Escherichia coli

(300 isolates)

60

85

95
Klebsiella pneumoniae

(224 isolates)

35

84

100

In this instance, the susceptibility rate during the last year for cefazolin versus E. coli was 60%. To say it in reverse, the non-susceptible rate at this institution during the last year for cefazolin versus E. coli was 40% (i.e., 100% total – 60% susceptibility = 40% non-susceptible). Sometimes “non-susceptible” and “resistant” are used interchangeably in practice, however you should beware that using the term “resistant” in the above scenario may not account for isolates that have been found to be intermediate / dose-dependant susceptible.

In clinical practice, if a patient at this institution is suspected to have an infection due to an E. coli and the provider elects to employ cefazolin as empiric therapy (i.e., before knowing sensitivity results), based upon this hospital antibiogram there is an estimated 40% chance the organism will go untreated due to cefazolin resistance.

Now that you are oriented to what antibiograms are and how they are produced, here are five important things to know about hospital antibiograms. Note, a full review of the benefits and limitations of antibiograms is beyond the scope of this text and readers are directed to the suggested readings and resources for more in-depth information.

1. Antibiograms can be helpful for picking empiric antibiotic therapy

Empiric therapy is defined as the antibiotic regimen prescribed prior to knowing organism identification and sensitivities. This type of therapy is selected based upon case-specific factors, which help clinicians identify the most likely organisms to cause infection in a particular patient case scenario.

Similar to the case described in the introduction, there are many instances in which an antibiogram can be referenced to help choose what regimen to employ for empiric therapy. A general rule is if the resistance rate is above 20%, then that drug should be avoided as empiric therapy for serious infections. For critically ill patients, that threshold may become even stricter at above 5 or 10%.

If two drugs both have a resistance rate below 20% and are both being considered for part of empiric therapy in the management of a serious infection, then the clinician may opt to go with the antibiotic that is more likely to cover the bacteria as indicated by the local antibiogram.

In some institutions antibiograms are done for specific areas of a hospital, such as an intensive care unit. In this case, the critical care providers can refer to their more specific data to help guide antibiotic selection.

2. Antibiograms help track local antibiotic resistance trends

If you want to know the status of antibiotic resistance at your institution, have a look at the recent antibiograms. Has there been more or less MRSA over the last few years? Is cefepime and piperacillin-tazobactam resistance to Pseudomonas aeruginosa becoming more or less prevalent? How is meropenem holding up against Acinetobacter baumannii? These are questions an antibiogram can help you to answer.

Reviewing antibiogram data can help us to see the big picture. Having an idea of what types of antibiotic resistance to be on the lookout for at your institution can improve your ability to avoid suboptimal therapies and in turn provide better care. It may also assist in identifying overuse of specific antibiotics, as more antibiotic use leads to more antibiotic resistance.

3. Not all hospitals have an antibiogram

Creating an antibiogram is complex and not every hospital has the means to develop their own local institutional antibiogram. While having an antibiogram may be helpful, do not be surprised if your institution does not have one readily available for you to access or if the one that is available dates back several years and is in serious need of an update.

If your hospital does not have an antibiogram, maybe there is potential for you to take on a fun side-project! In addition, there is a growing interest in creating community-based antibiograms, which collate susceptibility data from outpatient settings (e.g. ambulatory care clinics and nursing homes). You may want to check with your public health department to see if there is a county, regional, and/or statewide antibiogram for your area.

4. Antibiograms can vary significantly between hospitals

You cannot take the antibiogram from one hospital and apply it to another hospital. Even hospitals in the same city located directly across the street from each other can have antibiograms that look completely different.

A big reason why antibiograms differ between hospitals is because each hospital serves specific patient populations and provides a unique set of clinical services. For those sites that frequently provide high acuity care to complex patient populations requiring extended durations of antibiotic treatments, it is somewhat inevitable that they will see higher resistance rates, as compared to smaller institutions who care for less complex patients.

If you want to know institutional resistance patterns, you need to acquire institutional data.

5. Antibiograms are not simple to create and methodology can vary between hospitals

Before CLSI published the M39-A in 2002, there were no guidelines or standard methods for composing an antibiogram. This document (now the M-39A4, which is not free) helps address questions such as:

  • Should surveillance cultures be included in an antibiogram?
  • Should multiple cultures from the same patient for the same organism be included in an antibiogram?
  • How many isolates are needed to make the data valid enough to include the organism in the antibiogram?
  • How frequently should an antibiogram be composed?
  • Which antibiotics and which bacteria should be reported within an antibiogram?

Even with the guidance of CLSI, adhering to a consistent standard and ensuring complete compliance is a major challenge. Do not be surprised if you speak to several people from different practice sites who each made an antibiogram for their institution, only to discover they have all partially deviated from CLSI recommendations.

Closing comments

Antibiograms are an important tool in the fight against antibiotic-resistant organisms. They can help identify problematic pathogens or help identify which antibiotics to give a sick patient.

While antibiograms are not without considerable limitations, there is no doubt they will be an important asset in the continued fight to encourage safe antibiotic use and curb the widespread development of antibiotic resistance.

Antibiogram Examples

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