It sounds simple, you look at the antimicrobial culture and sensitivity report and if it is susceptible, you can use it! If you’ve been in practice for any amount of time you know that is both true and false. You must possess a core understanding to correctly interpret and apply antimicrobial C&S reports to patient care.
In this unit we will look beyond the reported susceptible (S) or resistant (R) values reported. The goal is to show you how to look at your available options of susceptibility and make the best clinical decision for your patient.
Antibiotic Stewardship
The goal with antimicrobial C&S reports is to analyze the results in such a way that you select the most narrow and specific coverage while considering costs, site of infection, drug interactions and disease state interactions.
A “susceptible” results means that there is a high likelihood of therapeutic success with appropriate dosing of that antibiotic. We will start with how the reported antibiotics are chosen.
Selection of Antibiotics Reported on Antimicrobial C&S
Have you ever wondered why some drugs are left off the susceptibility report even though they are likely susceptible?
Each lab will have a predefined list of antibiotics that they will test against a specific pathogen. However, not all the antibiotics tested will be provided on the antimicrobial C&S report. We refer to this as selective antibiotic reporting or cascade reporting.
Of all the antibiotics that are tested in the lab, what is eventually reported is filtered based on the degree of susceptibility, site of infection and ideally formulary.
1. Susceptibility
If a pathogen is highly susceptible the lab will only report narrow spectrum antibiotics.
If resistance is detected, lab will release additional, more broad-spectrum antibiotics.
For instance, do not report meropenem for highly susceptible MSSA. Do not report a 3rd generation cephalosporin if there is susceptibility to 1st and 2nd generation cephalosporins.
These policies will vary by institution. Some might restrict the reporting of linezolid for vancomycin susceptible MRSA, reserving its use for VRE. While some might argue that knowing the susceptibility is valuable for switching to oral treatment which facilitates quicker discharge.
2. Site of Infection
If a patient has a sample from the lungs that is susceptible to daptomycin in vitro (in lab) that should not be reported because daptomycin cannot penetrate the lungs and would therefore be ineffective for treating pneumonia.
In the United States, we use fosfomycin only for treating urinary tract infections.
If a bug shows susceptibility in a sample from any other site other than urinary, those results should be suppressed.
3. Formulary
To further complicate things, your best option may not be one of the drugs tested or reported. Ideally your facility will test only against drugs on your formulary.
If you notice that this not true for the antimicrobial C&S results at your facility, perhaps this is an opportunity to work with your microbiology department to optimize reporting. Narrowing down what is reported is an important first step in antibiotic stewardship. It is referred to as diagnostic stewardship.
Organization like the CLSI (Clinical and Laboratory Standards Institute) and EUCAST (European Society of Clinical Microbiology and Infectious Diseases) and the FDA (US Food and Drug Administration) have a wealth of resources to guide policies and procedures for antimicrobial testing and reporting.
Selection from Antimicrobial C&S
1. Choose the Narrowest Spectrum of Coverage
A simple way to visual the spectrum of antibiotic classes is to fit them into concentric circles with the narrowest option at the center. This will make is easier to choose the most appropriate options from the various antibiotics listed on the antimicrobial C&S report.
Of all the antibiotics reported as susceptible, chose the option that is closest to the center of the circles.
2. Your Best Choice May Not be Reported
Keep in mind that your best option may not be listed on the report. A good example is amoxicillin. Oral amoxicillin is still a very good, cost-effective option for penicillin susceptible staph infections. Ampicillin and nafcillin are other viable options that must be extrapolated from the report of penicillin susceptibility.
3. Use Gram Stain & PCR Testing
PCR testing uses the detection of genetic material specific to certain strains of bacteria. These will result faster (often within 24 hours) than the finalized antimicrobial C&S.
For instance, the detection of mecA/C suggests the presence of methicillin resistance, and you likely have MRSA.
The presence of vanA/B suggest the presence of vancomycin resistance; you likely have VRE.
The presence of NDM suggests the presence of carbapenem resistance.
4. IV to PO Conversions
Use your C&S results to transition to oral antibiotics that are able to penetrate the site of infection. There are numerous benefits associated with PO conversions: cost savings, length of stays reductions and less risk of infection to name a few.
Common Pitfalls of Interpretation of Antimicrobial C&S
1. Consider Time of Draw
A common mistake I’ve seen in my 12+ year career is escalation to more aggressive antibiotics if an initial culture shows sensitivity to one of the empiric antibiotics. Whether out of haste or misunderstanding, this is interpreted as “a positive culture despite treatment with a susceptible agent” instead of assurance that the patient is being appropriately treated.
This may seem silly to some, but it is a misconception that does occur. So always look at the timing of the culture. Was it ideally drawn prior to initiation of therapy or was it a culture drawn after antibiotics were initiated for some time.
2. Consider Location of Infection
As discussed above, ideally your lab will suppress antibiotics that do not have reliable penetration to a specific site of infection. If they do not, you need to remember to apply your own filters.
3. The Role of Antibiograms
Antibiograms are valuable for helping you select empiric antibiotics before you know susceptibilities. Once you have C&S results, antibiograms should not guide your decisions.
Interpretation of culture and sensitivity results can be daunting, making it the prefect opportunity for you to add value to your team and deeply impact patient care.
If you’ve found this unit helpful, I would love to hear from you! Leave a question or comment below.
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The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.