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Phenotypic resistance testing explained

Phenotypic resistance testing is a laboratory method that directly measures the actual susceptibility of pathogens to medications, enabling targeted and personalised treatment decisions.

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Things worth knowing about "Phenotypic resistance testing"

Phenotypic resistance testing is a laboratory method that directly measures the actual susceptibility of pathogens to medications, enabling targeted and personalised treatment decisions.

What is phenotypic resistance testing?

Phenotypic resistance testing is a laboratory diagnostic procedure in which the actual susceptibility or resistance of a pathogen to one or more therapeutic agents is measured directly in vitro. Unlike genotypic resistance testing, which searches for known resistance-associated gene mutations, phenotypic testing observes the biological behaviour – the phenotype – of the pathogen when exposed to a drug. This approach provides immediate, functional information about whether a given substance is capable of inhibiting the growth or replication of the pathogen.

Areas of application

Phenotypic resistance testing is used across several medical specialties:

Infectious diseases and microbiology: Routinely applied in bacterial infections to determine antibiotic susceptibility via antibiograms.
HIV treatment: Used to assess the susceptibility of HIV variants to antiretroviral agents, particularly in cases of treatment failure or suspected resistance.
Oncology: Increasingly applied in cancer medicine to test the sensitivity of tumour cells to chemotherapy agents or targeted therapies.
Mycology and parasitology: For fungal infections (e.g., Candida, Aspergillus) and parasitic diseases to evaluate the efficacy of antifungal or antiparasitic agents.

Methods of phenotypic resistance testing

Minimum inhibitory concentration (MIC)

The most widely used method is the determination of the minimum inhibitory concentration (MIC). The pathogen is exposed in vitro to serial dilutions of a drug. The lowest concentration at which no visible growth of the pathogen is observed is defined as the MIC. Based on established clinical breakpoints, the organism is then classified as susceptible, intermediate, or resistant.

Disk diffusion test (Kirby-Bauer method)

In the classic disk diffusion test, an agar plate inoculated with the pathogen is incubated with antibiotic-impregnated discs. The drug diffuses into the agar and inhibits pathogen growth within a certain radius. The size of the inhibition zone reflects the degree of susceptibility.

Broth microdilution

Broth microdilution involves incubating the pathogen in liquid growth medium containing graded concentrations of the drug. This method is considered the gold standard for MIC determination and is frequently used in automated diagnostic systems.

Phenotypic HIV resistance testing

In HIV medicine, specialised cell-based assays are used. Recombinant viruses carrying the gene sequences from the patient's own virus are introduced into cell cultures. The replication capacity of these viruses at various drug concentrations is then measured and compared to a reference value to determine the fold change in susceptibility, indicating the degree of resistance.

Advantages and limitations

Advantages

Provides a direct, functional measurement of a drug's efficacy against the specific pathogen of the individual patient.
Can detect previously unknown or complex resistance mechanisms that genotypic tests may not capture.
Delivers clinically actionable results for treatment selection.

Limitations

Often more time-consuming than genotypic methods, as pathogens must be cultured.
Not feasible for all pathogens (e.g., slow-growing organisms such as Mycobacterium tuberculosis).
Laboratory conditions may not fully reflect the in vivo environment.
Specialised phenotypic assays (e.g., for HIV) are costly and available only at specialist centres.

Clinical relevance

Phenotypic resistance testing is a cornerstone of personalised medicine. It enables clinicians to select the most appropriate therapy for the individual patient, avoid unnecessary side effects from ineffective medications, and contribute to the global fight against antimicrobial resistance. In HIV care, it is particularly valuable for treatment-experienced patients or those with suspected multi-drug resistance, helping to identify agents that retain activity against the virus.

References

European Committee on Antimicrobial Susceptibility Testing (EUCAST): Breakpoint tables for interpretation of MICs and zone diameters. Version 14.0. EUCAST, 2024. Available at: https://www.eucast.org
Richman DD. - Antiviral Drug Resistance. In: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier, 2019.
World Health Organization (WHO): Global Action Plan on Antimicrobial Resistance. WHO Press, Geneva, 2015. Available at: https://www.who.int/antimicrobial-resistance/global-action-plan/en/

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