Reading a Phagogram: What Phage Susceptibility Testing Can and Cannot Tell a UK Clinical Team

Antimicrobial resistance has a way of converting unusual questions into routine ones. A decade ago, few UK diagnostic laboratories expected to be asked whether a bacteriophage preparation might be active against a carbapenem-resistant organism recovered from a patient with a chronic prosthetic infection. In 2026, that enquiry is no longer exotic. It arrives from clinical teams, from patients who have read widely, and occasionally from colleagues who assume the laboratory already has a validated assay sitting on a shelf.

It does not. Phage susceptibility testing, where it happens at all in the UK, tends to sit at the boundary between research and service. That makes it an area where clear communication about what a result means is at least as important as the method used to generate it.

Why the question is arriving now

Surveillance reports from the UK's antimicrobial resistance programmes continue to describe organisms for which the reliable options are few, expensive or toxic. Against that backdrop, interest in narrow-spectrum alternatives is rational. Phages replicate at the site of infection, can be selected against a specific isolate, and appear, in published case series, to have helped some patients who had exhausted conventional treatment. Case series are not trials, and enthusiasm should be tempered accordingly, but the direction of travel is clear.

What a phagogram actually is

A phagogram is a laboratory assessment of whether one or more phage preparations produce measurable activity against a given bacterial isolate. The classic approach is a plaque assay: a bacterial lawn is exposed to serial dilutions of phage on an agar surface, and zones of clearing indicate productive infection. Simpler spot tests provide a yes-or-no signal; more quantitative formats estimate efficiency of plating relative to a reference host.

The output is not a minimum inhibitory concentration. It is closer to a host-range profile: which phages, at what approximate titre, produce how much killing against this isolate under these conditions. That distinction needs to be stated plainly whenever results are shared.

Method questions that decide the answer

Because no single reference method is universally adopted for clinical phagograms, the details carry unusual weight.

• Inoculum. Bacterial density, growth phase and mucoid phenotype all influence plaque formation. A standardised suspension, typically in the region of a 0.5 McFarland equivalent, is a reasonable starting point, but mucoid and small-colony variants may need separate handling.
• Phage titre and multiplicity. Reporting a result without stating the phage concentration used is close to meaningless. Titres should be determined in the same laboratory, on the same day, using a defined host.
• Media and ions. Calcium and magnesium availability affects adsorption for many phages, and agar depth influences plaque size. Consistency matters more than any particular recipe.
• Controls. Every run needs a no-phage growth control, a phage-only sterility control and, ideally, a reference host with known susceptibility. Duplicate plates and blinded reading reduce the temptation to see the plaque one expects.
• Format. Planktonic assays say little about biofilm-associated organisms, which is precisely the context in which many requests arise. Static biofilm models exist but are not standardised for this purpose.
• Validation. Laboratories offering the assay as part of a diagnostic service should be able to demonstrate verification or validation under ISO 15189, with documented uncertainty and limitations. External quality assessment provision in this field remains thin, which places greater weight on internal controls.

The interpretation gap

In vitro activity is a necessary but insufficient condition for clinical benefit. Phage pharmacokinetics differ fundamentally from antibiotics: phages are cleared by the reticuloendothelial system, may be neutralised by pre-existing antibodies, and can be eliminated rapidly from blood while persisting at a local site. Some phages penetrate biofilms well; others do not. Resistance can emerge through receptor mutation or CRISPR-based mechanisms, which is one argument for cocktails and for combining phage with antibiotics where synergy has been demonstrated in the laboratory.

The honest summary for a clinical team is that a positive phagogram indicates plausible activity against that isolate, using those phages, under those conditions. It does not predict outcome, and it should never be presented as though it does.

Regulation, access and the UK route

In the UK, phages are regulated as biological medicinal products. A licensed product does not currently exist for routine clinical use, which means access generally runs through clinical trials, through unlicensed supply routes involving a manufacturer holding the appropriate licence, or through importation for a named patient under MHRA oversight. Each route involves pharmacy, governance, indemnity and, usually, a good deal of paperwork.

The practical consequence is that a laboratory result is one input into a decision made by clinicians, pharmacists and regulators, not a prescription. Laboratories should be clear about this before accepting a request, and should be equally clear that sharing an isolate with an external phage group involves data protection, consent and material transfer arrangements that take time.

Communicating with clinicians and patients

Language matters enormously here. 'The phage works' is a sentence that will be remembered and repeated, and it overstates what any in vitro assay can show. A more accurate formulation is that a defined panel of phages showed activity against the patient's isolate in a laboratory model, that clinical use would require regulatory and pharmacy processes, and that outcomes in published reports are heterogeneous.

Patients who have researched phage therapy often arrive with detailed knowledge and considerable hope. A short, honest explanation of timelines, uncertainties and the difference between susceptibility and treatment is a clinical courtesy, not a bureaucratic hurdle.

Practical steps for a laboratory considering this work

Define scope before capacity. Decide whether the activity is research, a reference service for a small number of clinical partners, or something else entirely. Establish a written agreement with the phage provider covering panel version, titre data, biosafety information and any restrictions on use. Confirm containment requirements, waste routes and staff training. Build quality control into the workflow from the first run rather than retrofitting it at accreditation. And appoint a named contact who can speak to clinicians without overpromising.

None of this argues against phage susceptibility testing. It argues for doing it with the same discipline the laboratory applies to any other diagnostic method, and for describing the result in language that survives the journey from bench to bedside.