An antibiogram is a summary of how often each common organism in your hospital is susceptible to each antibiotic, usually shown as the percentage of isolates tested that were susceptible. It answers the question every prescriber faces before culture results arrive: which antibiotic is most likely to work for this patient, here, now?
Built well, it is the foundation of empirical therapy guidelines and antimicrobial stewardship. Built badly, with duplicate isolates, tiny sample sizes or out-of-date breakpoints, it can steer prescribers towards the wrong drug. This guide walks through building a cumulative antibiogram that follows CLSI M39, the international standard for analysing and presenting cumulative susceptibility data.
Why a local antibiogram matters
Resistance varies widely between countries, between hospitals in the same city and between units in the same hospital. National surveillance data is valuable for policy, but it cannot replace your own. An ICU with high carbapenem-resistant Klebsiella pneumoniae needs different empirical choices from a general medical ward a floor below.
Step 1: Set the scope and time period
CLSI M39 recommends analysing and presenting cumulative data at least once a year, usually covering the previous 12 months. Agree in advance which organisms, specimen types and locations will be included, and who will sign off the final report. This is usually the clinical microbiologist with the antimicrobial stewardship (AMS) committee.
Step 2: Extract the right data
- Use final, verified results only.
- Include diagnostic isolates only. Exclude surveillance and screening cultures, such as rectal swabs screened for CRE or nasal swabs screened for MRSA, because they bias the results.
- For each isolate, capture the patient identifier, collection date, organism to species level, specimen type, ward or unit, and the interpretation (S, I/SDD or R) for each drug tested.
Step 3: Apply the first-isolate rule
Count only the first isolate of a given species per patient per analysis period, regardless of its susceptibility profile. Without this rule, a single long-stay patient with repeated cultures can skew the results, usually towards higher resistance.
Step 4: Check sample sizes
CLSI M39 advises reporting percentage susceptible only for organisms with at least 30 isolates in the period. For less common organisms, combine two or more years of data, or present the figure with a clear footnote warning that it is statistically unreliable.
Step 5: Calculate percentage susceptible correctly
- Report % susceptible. Do not count intermediate or susceptible-dose-dependent results as susceptible.
- State which breakpoints were applied (CLSI or EUCAST, and which version). If breakpoints changed during the period, for example CLSI's 2022 revision of piperacillin-tazobactam breakpoints for Enterobacterales, note it, because year-on-year comparisons may otherwise mislead.
- Report only drugs that were tested routinely across the organism group. A drug tested only on resistant isolates will look worse than it is.
Step 6: Present only clinically relevant drugs
Follow your laboratory's selective (cascade) reporting rules. Do not show agents that are inappropriate for an organism or site, such as nitrofurantoin for anything other than urinary isolates. A cluttered antibiogram invites inappropriate choices.
Step 7: Stratify where it changes decisions
A single hospital-wide table hides the differences that matter most. Common and useful stratifications include:
- ICU versus non-ICU locations.
- Specimen type, especially urinary E. coli compared with bloodstream isolates.
- Inpatient versus outpatient or emergency department isolates.
- High-risk units such as oncology, burns and neonatal care, where numbers allow.
Step 8: Go further with combination and syndromic antibiograms
Prescribers often start two drugs together, or treat a syndrome rather than a known organism. A combination antibiogram shows how often at least one drug in a regimen is active. A weighted-incidence syndromic combination antibiogram (WISCA) estimates the likelihood that a regimen covers all likely pathogens for a syndrome such as hospital-acquired pneumonia. Both need more data and statistical care, but they map more closely to real empirical decisions.
Step 9: Put it where decisions are made
- Publish it as a pocket card, an intranet page and, ideally, inside the prescribing workflow.
- Use it to update empirical therapy guidelines for your top syndromes.
- Review it with the AMS committee and present the year-on-year trends to the Hospital Infection Control Committee.
- Date it clearly, and replace it on schedule.
Common antibiogram mistakes
- Including duplicate isolates, or screening swabs.
- Reporting organisms with fewer than 30 isolates without a warning.
- Mixing results interpreted with old and new breakpoints.
- Publishing a single hospital-wide table that ICU teams cannot use.
- Producing it once a year as a PDF that nobody opens.
From an annual report to a live tool
For most hospitals the hardest part is not the method but the data. It means extracting results from the laboratory system, de-duplicating isolates and rebuilding spreadsheets every year. That is why many antibiograms are out of date by the time they are printed. For more on making lab data work harder, see How Hospitals Can Turn Microbiology Reports into Actionable Infection Intelligence.
Why hospitals choose Ibhar
Ibhar gives hospitals a current, CLSI-aligned antibiogram without the annual spreadsheet exercise, so empirical therapy reflects local resistance. Ibhar is the infection intelligence platform for infection prevention and control (IPC), antimicrobial stewardship (AMS) and antimicrobial resistance (AMR). Hospitals from India to the Middle East use it, and it works alongside Epic, Oracle Health (Cerner) and InterSystems.
- Technology partner to the JIPMER-led national antibiogram study across 75+ hospitals.
- Around 500,000 positive culture records analysed.
- 800+ hospitals with Ibhar implementation experience.
To see what Ibhar could do for your hospital, talk to our team.
Frequently asked questions
How often should a hospital antibiogram be updated?
CLSI M39 recommends at least once a year, usually covering the previous 12 months.
How many isolates are needed for an antibiogram?
CLSI M39 advises at least 30 isolates of an organism in the period. With fewer, combine years of data or flag the result as statistically unreliable.
Should repeat cultures from the same patient be included?
No. Count only the first isolate of each species per patient per analysis period, whatever its susceptibility profile.