What is an antibiogram?

An antibiogram (or cumulative antibiogram) is a graphical report (usually a table) that lists the percentage of isolates of various bacteria that are susceptible to various antibiotics over a defined period of time. Most often this is for a hospital or hospital ward and reported over a period of one year. An antibiogram provides two important functions: it guides clinicians in selection of empiric antibiotics and it helps an institution monitor the development of antibiotic resistance over time. The Clinical Laboratory and Standards Institute (CLSI) publishes guidelines (known as M39-A4) on how to develop and present antibiograms. When looking at an antibiogram, there are some important things to note about how CLSI’s guidelines impact what you see:

• Only susceptible organisms are counted toward the reported percentage in the antibiogram. Intermediate and resistant organisms are not reported. The only exception is viridans streptococcus against penicillin where intermediate susceptibility can be included (although this is listed separately than susceptible isolates).

• Some susceptibilities are dependent on the site the culture was obtained from (urine, blood, central nervous system, etc). Rarely the susceptibilities are dose dependent for the particular antibiotic.

• When an organism is reported to be sensitive or resistant, that doesn’t mean that all organisms actually underwent testing. Some labs use “cascading” rules. For example, if an E. coli isolate is susceptible to cefazolin, it will also be susceptible to all other cephalosporins, so they will automatically be reported as susceptible at that point.

• Only clinical cultures are included. No surveillance cultures or rapid tests.

• It doesn’t matter what tissue or site the culture was obtained from. All can be included. Some institutions make a site-specific antibiogram such as “urine cultures only.”

• If duplicate cultures are obtained from the same patient (same organism, no mater the site of culture), only the first one is included. This decreases the chance of over-estimating resistance.

• If there are less than 30 isolates of an organism, that organism should be excluded from the antibiogram report. Sometimes these are included with a footnote explaining that susceptibility estimates may not be precise.

• Methacillin sensitive and methacillin resistant staphylococcus aureus are to be reported separately.

So how do I use an antibiogram?

When empirically treating a patient’s bacterial infection, you have to consider the likely pathogens first. You can then find the predicted susceptibility on the antibiogram. Each row represents the organism; each column represents an antibiotic. The table cell where the column and row meet is the percent susceptible during the period of time that the antibiogram represents.

Not all antibiotics will be reported on the antibiogram. Only antibiotics that are routinely tested by a microbiology lab included. Many of these are intravenous antibiotics which can be particularly problematic for outpatient clinicians. Many antibiotics in the same class can be used as a surrogate estimate for susceptibility. We have a list of some common antibiotic interchanges.

There is also a total number of isolates included in the report for each organism. Usually this is reported next to the name of the organism. This will give you an idea of how frequently that organism is isolated clinically by culture.

Remember that an antibiogram is useful for predicting susceptibilities for empiric treatment. If you have cultures and sensitivities available from the patient already, that information is more important and the cumulative antibiogram should be disregarded.

What is different about antibiogramDSM?

Because antibiogramDSM is a combined community antibiogram, there are a few additional issues with interpretation versus typical antibiograms. When you hover over the percent susceptibility in any cell, the interactive Tableau features will show you an estimated number of isolates and number of institutions contributing to that cell’s value. You also have the ability to view data from years prior, but for clinical decisions, you should only consider the most recent year of data.

AntibiogramDSM represents data from four different hospital systems in the Des Moines metro area. Because bacterial resistance varies from hospital to hospital and region to region, we do not know how useful this tool is outside of central Iowa.

The data used to build antibiogramDSM is a mix of inpatient and outpatient data. This means that resistance in certain organisms might be over-estimated as bacterial resistance (particularly among Gram negatives) can be more problematic in hospitals compared to outpatient settings.