Antibiotic Susceptibility Testing: A Complete Guide

A patient with a serious bacterial infection is started on antibiotics before anyone knows which antibiotic will work. Antibiotic susceptibility testing is what closes that gap. It tells the clinician which drugs the organism responds to, and it is the single test that converts a best guess into a targeted prescription.

It is also slow. Conventional antibiotic susceptibility testing adds roughly a day to a diagnostic pathway that has already spent a day growing the organism, and almost every commercial opportunity in this part of the market comes back to that delay. This guide covers how antibiotic susceptibility testing works, the main methods and their trade-offs, how results are interpreted under EUCAST, why turnaround time is the central problem, and what a UK laboratory weighs before it adopts a new AST method.

What Is Antibiotic Susceptibility Testing?

Antibiotic susceptibility testing, usually shortened to AST and sometimes called antimicrobial susceptibility testing or sensitivity testing, measures whether a bacterial isolate will be inhibited by a given antibiotic at a concentration achievable in the patient.

The sequence in a clinical laboratory is consistent. A sample arrives (blood, urine, sputum, a wound swab, cerebrospinal fluid). The organism is cultured and identified. Only then is susceptibility tested, because AST is performed on a pure isolate of a clinically significant organism rather than on the original sample. The result is reported back as a category the prescriber can act on.

That ordering matters commercially. AST sits downstream of culture and identification, so any product promising a faster susceptibility result has to address the whole upstream pathway, or explain convincingly why its step alone is the constraint worth solving.

The Main AST Methods

Four approaches account for most routine testing. They differ in what they measure, how precisely, and how much hands-on work they need.

Disk Diffusion

Paper disks impregnated with individual antibiotics are placed on an agar plate seeded with the organism. Where the drug is effective, a clear zone of inhibition forms, and the zone diameter is measured and compared against published breakpoints. It is inexpensive, flexible in the choice of antibiotics tested, and remains widely used. It is also labour intensive and gives a category rather than a precise concentration.

Broth Microdilution And The MIC

The organism is exposed to a series of doubling antibiotic concentrations to establish the minimum inhibitory concentration, the lowest concentration that prevents visible growth. Broth microdilution is the reference method against which others are judged, and the MIC is the underlying quantitative measurement that breakpoints are derived from. It is more informative and more standardised, and correspondingly more work.

Gradient Diffusion Strips

A plastic strip carrying a continuous antibiotic gradient is applied to a seeded plate, giving an MIC from an agar based workflow. It is a practical middle ground, commonly used for specific organism and drug combinations where a number is needed rather than a category, though the per test consumable cost is higher.

Automated Systems

Commercial instruments incubate miniaturised broth panels and read growth optically at intervals, generating MICs with expert software interpretation and a direct link into the laboratory information system. Most UK hospital microbiology departments run automated AST for routine workload, reserving manual methods for organisms and drugs the panels do not cover well.

How AST Results Are Interpreted

A raw zone diameter or MIC means nothing without a breakpoint to interpret it against. In the UK and across Europe, breakpoints come from EUCAST, the European Committee on Antimicrobial Susceptibility Testing.

EUCAST revised its reporting categories in 2019, and the change is still misread. The categories are now:

• S, Susceptible, standard dosing regimen. The organism is likely to respond at the normal dose.
• I, Susceptible, increased exposure. Likely to respond where higher drug exposure is achieved at the site of infection, through a higher dose, prolonged infusion or natural concentration at that site.
• R, Resistant. Treatment failure is likely even with increased exposure.

The important point is that "I" no longer means intermediate or uncertain. It is a form of susceptibility that carries a dosing instruction. EUCAST introduced the change because the old intermediate category left clinicians unsure whether the drug could be used at all, and laboratories were expected to implement the revised definitions by the end of 2020.

Why Turnaround Time Is The Central Problem

Conventional susceptibility testing depends on growing bacteria, and bacteria grow at their own pace. Assessing and determining bacterial susceptibility by standard phenotypic methods generally takes 16 to 24 hours once an isolate is available, as set out in a review of current and emerging AST methods published in Diagnostics by Khan and colleagues. Add the culture step before it and a definitive susceptibility result commonly lands 48 hours or more after the sample was taken.

During that window the patient is on empirical therapy chosen from local guidelines and clinical judgement. That gap produces three problems at once, and each is a distinct commercial argument.

• Clinical Risk. In serious infection, particularly bloodstream infection and sepsis, time spent on an antibiotic the organism resists is time the patient does not have.
• Stewardship Cost. Empirical therapy is usually broad spectrum by design. Every hour before de-escalation is an hour of selection pressure driving further resistance.
• Resource Use. Patients stay on broader, more expensive agents, often intravenously, longer than the eventual result shows was necessary.

This is why "rapid" is the most contested word in AMR diagnostics, and why buyers have become sceptical of it. A laboratory director will want to know exactly which step a product removes and what the result replaces.

Rapid AST And What Is Actually Changing

Two distinct approaches are compressing the timeline, and they answer different questions.

Faster phenotypic testing keeps measuring growth inhibition but does it sooner, working directly from a positive blood culture rather than waiting for a subculture, or reading early growth signals by optical, microscopic or microfluidic means. The advantage is that the answer is still a genuine susceptibility measurement, so it maps onto existing breakpoints and existing clinical reasoning.

Molecular resistance detection looks for known resistance genes or mutations instead, and can return an answer in an hour or two. The trade-off is real: it detects the mechanisms it is designed to detect, so absence of a target is not proof of susceptibility, and gene presence does not always translate into phenotypic resistance. It answers "is this specific mechanism present" rather than "will this drug work".

Most laboratories treat these as complementary rather than competing. A molecular result can guide the first 24 hours while phenotypic AST provides the definitive answer. Vendors who position a molecular assay as a replacement for phenotypic AST tend to meet resistance from microbiologists for exactly this reason.

This guide covers susceptibility testing methods themselves. The broader question of resistance detection strategy, including how molecular and phenotypic approaches are combined across an infection service, is a separate subject that deserves its own treatment.

What UK Laboratories Weigh Before Adopting A New AST Method

Susceptibility testing is a regulated, accredited activity, so changing method is never only a purchasing decision. If you are selling into UK microbiology, these are the questions that decide whether a trial becomes a contract.

• EUCAST Alignment. Does the system report current breakpoints and the increased exposure category correctly, and how fast are annual updates delivered?
• Verification Burden. UK laboratories operate to ISO 15189 under UKAS accreditation. A new method must be verified locally against the existing one before clinical use, and that work falls on staff who are already stretched. Suppliers who arrive with a verification protocol and reference data remove a genuine obstacle.
• Organism And Drug Coverage. Does the panel cover the local formulary and the organisms this laboratory actually sees, or will manual testing still be needed alongside it?
• LIMS Integration. Results have to flow into the laboratory information system and out to the ward without transcription. Integration effort is frequently the reason a technically sound product stalls.
• Quality Control And EQA. Routine QC organisms, participation in external quality assessment, and clear behaviour when a control fails.
• Workflow Fit. Bench space, staffing pattern and whether a faster result actually reaches a clinician who can act on it. A result delivered at 2am to a laboratory with no overnight clinical microbiology cover changes nothing.
• Stewardship Sponsorship. The strongest business cases are co-owned by the antimicrobial stewardship team, because that is where the argument about de-escalation and length of stay is made.

The last point is the one commercial teams most often miss. Microbiology holds the technical decision, but the funding case for faster AST is usually built on stewardship and bed day savings, which means a second set of stakeholders has to be persuaded in their own language.

The Commercial Picture For AMR Diagnostics

The policy backdrop in the UK is favourable and specific. UKHSA reported in its English surveillance programme for antimicrobial utilisation and resistance report for 2024 to 2025 that antibiotic resistant infections rose by 13.1 per cent over the previous five years, while antibiotic use fell by 2 per cent against pre-pandemic levels. The UK National Action Plan published in 2024 set a five year strategy including an ambition to reduce total human antimicrobial use by 5 per cent by 2029 compared with 2019.

Rising resistance combined with a formal commitment to reduce consumption is precisely the position that makes better susceptibility testing a policy priority rather than a discretionary purchase. Prescribing less while treating effectively requires knowing sooner what will work.

For diagnostics companies the practical implication is about how the case is framed. A faster AST product sold on speed alone competes on a claim buyers have heard many times. The same product sold on de-escalation rates, reduced broad spectrum days, or measurable alignment with National Action Plan targets is speaking the language the people holding the budget are already using.

Where FutureDx Summit Fits For AMR Diagnostics Companies

AMR sits at the centre of the FutureDx Summit 2026 programme rather than at its edge. The summit runs on 27 and 28 October 2026 at the Copthorne Tara Hotel London Kensington. Day 1 covers sepsis diagnostics and speed of clinical impact, followed by drug resistant tuberculosis, both of which are susceptibility testing problems before they are anything else. The agenda sets out the full two days.

The audience is the reason this matters for companies in this field. FutureDx is built around 150+ senior professionals from NHS trusts, public health bodies, industry and academia, with a curated delegate list rather than open registration. The 23 confirmed speakers include Peter Gayo Munthali, Antimicrobial Stewardship Lead at University Hospital of Coventry and Warwickshire, Suparna Mitra, Microbiome and AMR Research Lead at the University of Leeds, Linda Oyama of Queen's University Belfast, and Jean Lutamyo Mbisa and Richard Vipond, Deputy Directors at the UK Health Security Agency. The summit is officially supported by BIVDA, the British In Vitro Diagnostics Association.

Sponsorship routes are organised by what a commercial team is trying to do. The AMR diagnostics sponsorship route aligns a company with the resistance sessions, the sepsis diagnostics route with the speed to result argument, and there are dedicated routes for reaching consultant microbiologists and NHS laboratory directors. Senior packages include a speaking slot in the main programme, which for an AST product is often more useful than a stand, since the evidence discussion is the sale.

To be clear about what a conference can and cannot do: it provides structured access to the people who evaluate and specify susceptibility testing, and a credible setting to present data to them. It does not shorten NHS procurement or produce a guaranteed commercial outcome.

Frequently Asked Questions

What To Take From This

Antibiotic susceptibility testing is a mature, standardised process whose limitation is biological rather than technical: bacteria take time to grow, and a category result is only as useful as the moment it arrives. Every serious commercial opportunity in this area is a claim about compressing that interval without losing the interpretive rigour that EUCAST breakpoints and UKAS accreditation demand.

If you are bringing an AST product to the UK market, the decision in front of you is not really about the assay. It is whether you can evidence a clinical or stewardship outcome, carry a laboratory through local verification, integrate cleanly, and reach the microbiologists and stewardship leads who will decide together. The last of those is usually the hardest, and it is the one a focused conference is genuinely good at solving.