Antibiotic sensitivity, resistance and clinical response
Why an Antibiotic May Still Work Despite Reported Resistance
An antibiotic sensitivity report is extremely important, but it is not interpreted in isolation. Drug concentration, infection site, dosing, the reliability of the sample, source control and the patient’s clinical response can all influence whether treatment is likely to succeed.
- Antibiotic resistance
- Susceptibility testing
- EUCAST
- Drug exposure
- Colonisation
- Antimicrobial stewardship
Example susceptibility report
Clinical response matters
Culture results should be interpreted alongside whether the patient is improving, the severity of infection and whether the organism is genuinely causing disease.
A true “R” result still matters
Under EUCAST, resistant means there is a high likelihood of treatment failure even with increased antibiotic exposure.
What does an antibiotic resistance result actually mean?
The laboratory grows or detects the organism and assesses its response to antimicrobial drugs using standardised methods. Results are then interpreted against clinical breakpoints.
Those breakpoints combine microbiological information with pharmacokinetics, pharmacodynamics, dosing and clinical outcome data.
Resistance should not simply be ignored
If an organism is genuinely responsible for the infection and is reported resistant to the antibiotic being used, treatment failure becomes more likely. Continuing treatment therefore requires a clear clinical or microbiological rationale.
Reading the microbiology report
What do S, I and R mean on a sensitivity report?
| Result | Current EUCAST meaning | Clinical implication |
|---|---|---|
| S | Susceptible, standard dosing regimen | High likelihood of therapeutic success using a standard dosing regimen |
| I | Susceptible, increased exposure | High likelihood of success if drug exposure is increased by dose, dosing interval, infusion strategy or concentration at the infection site |
| R | Resistant | High likelihood of therapeutic failure even with increased exposure |
“I” does not mean the same thing as “R”
Under current EUCAST terminology, both S and I are susceptible categories. An I result may be treatable when the correct increased-exposure regimen is used.
Antibiotic concentration at the infection site matters
Antibiotics do not distribute equally throughout the body. Some reach high levels in urine, some penetrate lung tissue well, while others may have limited penetration into the brain, bone, prostate, abscess cavities or bloodstream.
Urinary concentration
Some antibiotics become highly concentrated in urine and can therefore be effective for infection confined to the lower urinary tract.
Tissue penetration
The same drug may be unsuitable when infection is located in kidney tissue, lung tissue, blood, bone or another compartment where adequate concentrations are not achieved.
Dose and administration
Exposure may sometimes be increased by altering dose, frequency or infusion duration where the susceptibility category and drug allow this.
Nitrofurantoin is a useful example
Nitrofurantoin achieves useful urinary concentrations and is commonly used for uncomplicated lower urinary-tract infection. It is not appropriate for pyelonephritis or bloodstream infection because adequate tissue and serum exposure is not achieved.
Why antibiotic breakpoints are context-dependent
A clinical breakpoint is not simply a laboratory number. It is intended to predict treatment success using a particular antimicrobial exposure against a particular organism.
Breakpoint systems therefore take into account drug dose, administration schedule, pharmacokinetics, pharmacodynamics and clinical evidence.
Standard exposure
Some organisms can be treated reliably with conventional dosing.
Increased exposure
Other organism–drug combinations may require higher exposure, such as an adjusted dose or prolonged infusion.
Infection-site considerations
Drug concentration at the site of infection may influence how susceptibility is interpreted for specific clinical settings.
This is different from overriding genuine resistance
Using an evidence-based increased-exposure regimen for an EUCAST I result is not the same as deliberately treating a genuinely resistant organism with an ineffective drug.
Why laboratory testing cannot reproduce the whole patient
Immune response
The patient’s immune system may substantially contribute to clearing infection.
Drug penetration
Laboratory media cannot fully reproduce the antibiotic concentration reached in every tissue or body fluid.
Biofilms
Bacteria growing on prosthetic material, catheters or damaged airways may behave differently from organisms tested in a laboratory.
Abscesses
Pus, necrotic tissue and poor blood supply may reduce effective antibiotic delivery.
Tissue environment
Oxygen concentration, pH and local tissue conditions can affect bacterial behaviour and antimicrobial activity.
Whether the organism matters
A culture may grow an organism that is present but not actually causing the illness.
Interpreting the culture correctly
The cultured bacterium may not be causing the infection
Microorganisms can live on or within the human body without causing invasive disease. This is called colonisation.
A culture result should therefore always be interpreted alongside the type and quality of sample and the clinical picture.
Colonisation
A microorganism can be recovered from a sample without being the principal cause of the patient’s current illness.
Active infection
The importance of a resistance result increases substantially when the cultured organism fits the clinical syndrome and is judged to be the true pathogen.
Why a doctor may not change treatment
If a patient is recovering and the resistant organism is thought to represent colonisation rather than the cause of the illness, switching to a broader antibiotic could expose the patient to harm without providing benefit.
This issue is particularly relevant in chronic respiratory disease. Patients with recurrent or persistent infection can be assessed through the London Chest Specialist chest infection service .
Treating the source as well as the organism
Source control can be as important as the antibiotic
Antibiotics work poorly when infected material remains trapped or a continuously infected device remains in place.
Find the source
Look for a collection, infected device, obstruction, empyema, necrotic tissue or another structural problem maintaining infection.
Drain, remove or relieve
Physical treatment of the infectious focus may be required before antimicrobial therapy can work reliably.
Lower the bacterial burden
Removing infected material can substantially reduce the amount of infection that the immune system and antibiotic must control.
Use appropriate antibiotic therapy
Antibiotic choice, dose, route and duration can then be optimised for the organism, infection site and clinical response.
When might the current antibiotic reasonably be continued?
Reasons continuation may be reasonable
- the patient is clearly improving;
- the illness is mild and low risk;
- the organism is likely to represent colonisation;
- the result is actually EUCAST I rather than R;
- the correct increased-exposure regimen is being used;
- unusually high concentrations are achieved at the relevant infection site;
- adequate source control has been achieved.
Questions the clinician should ask
- Is this definitely the causative organism?
- Is this genuinely R, or susceptible with increased exposure?
- Does the drug reach the infected compartment?
- Is the dose appropriate?
- Is the patient continuing to improve?
- Is there an untreated source of infection?
- Would switching increase toxicity or unnecessary broad-spectrum exposure?
Situations where reliable activity is critical
When should a resistant result usually prompt a change?
Serious or worsening infection changes the balance
When the cultured organism is clearly the pathogen and the infection is invasive, severe or failing to respond, reliable antimicrobial activity becomes especially important.
Sepsis
Deteriorating systemic infection requires rapid reassessment of antimicrobial coverage and source control.
Bacteraemia
A bloodstream isolate generally carries much greater clinical weight than a potentially colonised superficial sample.
Pneumonia
Failure to improve with a resistant respiratory pathogen may require a change in treatment and reassessment for complications.
Meningitis
Antibiotic penetration into cerebrospinal fluid and reliable susceptibility are critical in central nervous system infection.
Endocarditis
Infection of the heart valves requires dependable, bactericidal therapy and specialist management.
Deep infection or immune suppression
Bone, joint, deep-tissue infection or serious infection in an immunocompromised patient usually warrants a particularly low tolerance for unreliable therapy.
The antimicrobial review checkpoint
Why antibiotics are often reviewed after 48–72 hours
Empirical antibiotics are often started before the exact organism or susceptibility pattern is known. Once cultures, imaging and the early clinical response are available, treatment can be refined.
Stop
Antibiotics may be stopped when infection is no longer the most likely diagnosis.
Continue
Treatment may continue when the diagnosis, drug and duration remain appropriate.
Narrow treatment
Broad empirical therapy can often be replaced with a more targeted antibiotic once the causative organism is known.
Change
Microbiology or lack of clinical response may indicate a different antimicrobial is required.
Switch intravenous to oral
Clinically stable patients may sometimes continue treatment with an appropriate oral antibiotic.
Reassess the diagnosis
Failure to improve should prompt reconsideration of whether the patient has the right diagnosis at all.
A structured clinical review
What should be checked when an antibiotic seems not to work?
Choosing the narrowest reliably effective treatment
Why not simply switch to the newest or broadest antibiotic?
Newer antibiotics can be extremely valuable when multidrug-resistant infections leave few alternatives, but newer does not automatically mean better for every patient.
The stewardship principle
Antimicrobial stewardship means using an effective antibiotic while avoiding unnecessarily broad, toxic or prolonged treatment.
The goal is not to use the strongest-sounding antibiotic. The goal is to use the right drug, at the right exposure, for the right infection and for an appropriate duration.
Putting the result into clinical context
The safest general rule for interpreting resistance
A true resistant result usually means the antibiotic should be avoided
Continuing despite resistance is most defensible only when there is a clear explanation: the culture does not represent the infection, the result has been misinterpreted, the clinical context changes its relevance or specialist pharmacological and microbiological advice supports the decision.
More reassuring
- the patient is clearly improving;
- infection is clinically low risk;
- the cultured organism probably represents colonisation;
- the report is I rather than R;
- exposure can be increased appropriately;
- source control is complete.
More concerning
- the patient is deteriorating;
- there is sepsis or organ dysfunction;
- the infection is invasive or deep-seated;
- the organism is clearly the pathogen;
- susceptibility is genuinely R;
- there has been no meaningful clinical response.
Frequently asked questions
If the laboratory says resistant, should the antibiotic always be stopped?
A genuine resistant result usually argues against using that antibiotic, especially for serious infection. However, the clinician first needs to confirm that the cultured organism is actually causing the illness and that the result applies to the clinical context.
What does “I” mean on an EUCAST report?
It means “susceptible, increased exposure”. There is a high likelihood of treatment success when sufficient exposure is achieved through the appropriate dose, dosing interval, infusion method or drug concentration at the infection site.
Is “I” the same as intermediate resistance?
Not under current EUCAST terminology. I is a susceptible category and should not be grouped together with R.
Can a resistant antibiotic still appear to work?
Yes, for several reasons. The organism may not be causing the illness, the infection may have improved because of source control or immune response, or the laboratory result may not fully represent the conditions at the infection site. Apparent improvement does not make genuine resistance irrelevant.
Why does the infection site affect antibiotic choice?
Antibiotics reach different concentrations in different tissues and body fluids. A drug that works well in urine may not reach adequate concentrations in kidney tissue, blood, cerebrospinal fluid or an abscess.
Why might a sputum culture show a resistant bacterium when I am getting better?
In chronic lung disease, bacteria can sometimes colonise the airways without being the main cause of the current illness. The culture must therefore be interpreted alongside symptoms, imaging, inflammatory markers and clinical response.
Why are antibiotics reviewed after 48–72 hours?
By that point, clinicians often have more microbiology, imaging and clinical-response information. Treatment can then be stopped, narrowed, changed, continued or switched from intravenous to oral where appropriate.
Are broader antibiotics always more effective?
No. Broader antibiotics may increase toxicity, Clostridioides difficile risk and future resistance. The preferred treatment is usually the narrowest drug that is reliably effective for the infection.
What is source control?
Source control means physically removing or correcting the focus of infection, such as draining an abscess, removing an infected catheter or draining an empyema.
When is antibiotic resistance especially concerning?
It is particularly concerning when the patient is deteriorating, has sepsis, bacteraemia, meningitis, endocarditis, severe pneumonia, deep infection, significant immune suppression or no meaningful response to treatment.
The bottom line
Antibiotic susceptibility reports are essential to modern infection care, but they are not interpreted in isolation.
Drug concentration at the infection site, the distinction between EUCAST S, I and R categories, the reliability of the specimen, colonisation, source control, dosing and the patient’s clinical response can all affect the final treatment decision.
A true resistant result should still be taken seriously. Under current EUCAST definitions, R means there is a high likelihood of therapeutic failure even when antibiotic exposure is increased.
In contrast, an I result means susceptible with increased exposure and may be successfully treated using an appropriate regimen.
This is why clinicians treat the patient rather than simply reacting to one line on a laboratory report—but good clinical judgement includes taking antimicrobial resistance seriously, reassessing response and using microbiology appropriately.
References and further information
- European Committee on Antimicrobial Susceptibility Testing. Definitions of S, I and R. View EUCAST definitions
- Clinical and Laboratory Standards Institute. M100: Performance Standards for Antimicrobial Susceptibility Testing. View CLSI M100 information
- National Institute for Health and Care Excellence. Antimicrobial stewardship: systems and processes for effective antimicrobial medicine use. NICE guideline NG15. View NICE antimicrobial stewardship guidance
- London Chest Specialist. Chest infection diagnosis and treatment. Explore the chest infection service
- London Chest Specialist. Bronchiectasis diagnosis and treatment. Explore the bronchiectasis service
Recurrent or Difficult-to-Treat Chest Infections?
A specialist respiratory review can interpret sputum cultures, resistance patterns, imaging, bronchiectasis risk and recurrent infection history to help identify the most appropriate treatment strategy.
