Doctors usually assess antibiotic resistance by testing bacteria from a specimen taken from the suspected infection. A laboratory may grow and identify the organism, then measure which antibiotics inhibit it. Faster molecular tests can detect selected resistance markers, but they do not necessarily show how the bacteria respond to every drug. Clinicians use the results alongside the infection site, its severity, and the patient’s circumstances to choose or adjust treatment. The process described here reflects primarily U.S. clinical and laboratory practice; specific tests and services vary by setting.
How does the testing process start?
Choose a specimen for the suspected infection
A clinician first evaluates symptoms and the likely site of infection, then orders a specimen suited to that site. For a suspected bloodstream infection, conventional diagnosis uses blood cultures. A culture can reveal bacterial growth and allow the laboratory to recover bacteria for identification and susceptibility testing. A blood draw or swab is not, by itself, a general test for resistance: the result depends on what is tested and which method is used.
Identify the organism—and decide whether it represents infection
The clinical microbiology laboratory detects growth and identifies the organism. Finding bacteria or a resistance marker does not always establish that bacteria are causing active illness; clinicians interpret the result in the context of the specimen and the patient. Some rapid methods identify organisms from a positive culture and look for a limited set of resistance markers, potentially providing useful information before conventional susceptibility testing is complete.
What do the different tests tell doctors?
| Approach | What it can show | Scope and timing | Important limitation |
|---|---|---|---|
| Culture plus phenotypic antimicrobial susceptibility testing (AST) | The laboratory recovers and identifies bacteria, then measures their growth response to tested antibiotics. | Can provide a drug-specific susceptibility profile for the drugs and methods tested. It may take additional time for bacteria to grow and for testing to be completed; reference broth microdilution takes multiple days. | Laboratory conditions do not perfectly reproduce the conditions at an infection site in the body. |
| Rapid molecular or resistance-marker testing | Whether selected organisms or resistance genes or markers included in the assay are detected. | Can provide targeted information sooner. A cited American Society for Microbiology review reports that some assays run from positive blood cultures return results in under two hours. | The assay only checks its targets. A negative marker result does not prove susceptibility to every antibiotic, and a marker result is not necessarily a complete susceptibility profile or proof of active infection. |
These methods can complement one another: targeted results may help clinicians act sooner, while culture-based AST supplies measured responses to the drugs tested. The under-two-hour timing applies to the specific rapid assays described in the ASM review after a blood culture is positive—not to every molecular test or the full process from specimen collection to final susceptibility results.
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How phenotypic AST measures response
Phenotypic AST exposes recovered bacteria to antimicrobial agents and measures whether they grow. Methods described by the Centers for Disease Control and Prevention (CDC) include broth microdilution, Kirby-Bauer disk testing, and gradient diffusion strips. Broth microdilution is a reference method, but conventional AST takes time and its in-vitro results cannot fully reproduce what happens in a patient.
What molecular tests add
Molecular tests detect only the organisms or resistance markers their assays are designed to find. They can help identify selected resistance mechanisms earlier, but they do not automatically replace phenotypic testing or establish how the bacteria will respond to every available drug.
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How should a susceptibility report be read?
The laboratory compares measured results with clinical breakpoints: organism- and drug-specific thresholds used to assign categories such as susceptible, intermediate, susceptible-dose dependent, or resistant, as applicable under the relevant standard. These categories are not a universal ranking of antibiotics. Their meaning depends on the organism, the drug, the method and breakpoint system used, and the report date; standards can be updated as evidence changes.
Laboratory recommendations also evolve. A 2026 ASM review reports that 2025 CLSI recommendations called for carbapenemase testing in Enterobacterales resistant to at least one tested carbapenem, in addition to standard phenotypic AST. That is technical guidance for laboratories, not an instruction for patients; laboratories follow applicable standards and local protocols.
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How do doctors use results to choose or change treatment?
A susceptibility result is an important input, not a prescription by itself. Clinicians consider whether the organism is causing infection, where the infection is, how severe it is, the test findings, allergies, toxicity risks, and other practical clinical factors. They may start with broad coverage when appropriate, then narrow treatment as results arrive, change a drug that appears ineffective, or seek specialist advice. The right regimen is specific to the infection and the patient; there is no single treatment for all antibiotic-resistant bacteria.
When might a difficult case receive additional testing?
In the United States, CDC’s Antibiotic Resistance Laboratory Network (AR Lab Network) provides confirmatory or expanded testing for selected resistant organisms and hard-to-treat infections. Eligibility and services are defined, and this network is not a substitute for routine clinical care.
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- Selected carbapenem-resistant isolates: CDC describes testing that can include organism confirmation by MALDI-TOF, carbapenemase tests, PCR for selected carbapenemase genes, and broader AST.
- PCR colonization screening: CDC’s AR Lab Network testing-services page states that results for its described PCR screening service are available within two business days after the specimen is received. This is a specific screening-service timeline, not a routine patient AST turnaround.
- Expanded AST for selected hard-to-treat infections: A 2021 CDC document reports that some results are available within three business days. Because that timing comes from an older document, current criteria and turnaround should be confirmed with the service.
What can the tests not establish on their own?
- A detected bacterium or resistance marker does not, by itself, prove that the organism is causing active infection.
- A rapid assay’s result is limited to its targets; it may not describe susceptibility to all relevant antibiotics.
- A laboratory susceptibility category is based on testing and breakpoints, not a guarantee of how treatment will work in a particular patient.
- Turnaround varies with the specimen, organism, laboratory method, and need for additional testing; there is no single timeline for every resistance test.
Early research should also be interpreted in context. A 2024 ASM clinical pilot of GoPhAST-R reported 95% overall agreement with standard AST across 42 positive blood cultures. The study authors said larger and more diverse validation is essential, so this pilot result does not establish the approach as a general replacement for standard testing.
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