On the 1st of June 2026, the medical community received the much-anticipated results of the NHS-Galleri trial. Presented at the American Society of Clinical Oncology’s annual meeting and reported in The BMJ, the data confirmed that the highly publicised multi-cancer early detection blood test failed to meet its primary endpoint. In a cohort of 142,000 adults, the test did not produce a statistically significant reduction in stage III or IV diagnoses for twelve prespecified cancers, yielding an incidence rate ratio of 1.03.
While the headline is undoubtedly disappointing for oncological research, it provides a profoundly useful case study for candidates preparing for the Applied Knowledge Test (AKT) and Medical Licensing Assessment (MLA). It serves as a stark reminder of the foundational epidemiological principles that govern safe, effective clinical practice.
Back to Basics: Wilson and Jungner
The political and financial momentum behind the Galleri test - manifested in a £150 million NHS trial - reflects a common desire within healthcare to catch disease as early as possible. However, enthusiasm for new technology must always be anchored by the cornerstone of public health screening: the Wilson and Jungner criteria.
A central tenet of these criteria is that any implemented screening programme must utilise a test that is suitable, acceptable, and possesses a rigorously validated balance of sensitivity and specificity. The Galleri trial highlights exactly why these criteria are not merely academic hurdles, but vital safeguards. When a diagnostic tool is applied to an asymptomatic population before its operating characteristics are fully understood, the clinical fallout can be substantial.
The Trap of Low Prevalence and PPV
For medical students and developing clinicians, this trial is a perfect real-world application of Positive Predictive Value (PPV). We know that the PPV of a test is inextricably linked to the prevalence of the disease in the tested population.
When you apply a test with imperfect specificity to a massive, low-prevalence population - such as 142,000 generally healthy adults - the mathematical reality is that the PPV plummets. The test inevitably generates a high absolute number of false positives. In clinical practice, a "positive" screening result cannot simply be ignored; it mandates action.
Real-World Consequences: Resources and Iatrogenic Harm
The cascade of secondary investigations triggered by false positives is where theoretical epidemiology meets ward-level reality. Patients flagged by a multi-cancer blood test must undergo a battery of confirmatory diagnostics - such as urgent CT, MRI, or invasive biopsies - to locate a malignancy that, in many cases, does not exist.
This creates two immediate, pressing issues that are highly relevant to the AKT curriculum:
Resource Allocation: Managing a high volume of false positives directly diverts finite NHS diagnostic resources. Every scan or biopsy allocated to a healthy, false-positive patient is a scan delayed for a symptomatic patient with a demonstrable clinical need.
Non-Maleficence: The ethical duty to do no harm applies heavily to diagnostics. The psychological morbidity inflicted upon an asymptomatic individual told they may harbour a developing cancer is profound. Furthermore, invasive confirmatory tests carry their own independent risks of procedural complications.
Conclusion
The NHS-Galleri trial serves as a sobering corrective to the assumption that more data invariably leads to better patient outcomes. For the clinician navigating the MLA curriculum, the takeaway is clear: true clinical excellence relies on evidence-led practice rather than the uncritical adoption of heavily marketed innovations. Mastering the principles of screening means recognising that knowing when not to test is often just as critical as the diagnostic capability itself.
References
Iacobucci, G. (2026). Galleri cancer test: NHS trial of controversial early detection tool fails to meet main aim, results confirm. BMJ, 393, e091078.
- Wilson, J. M. G., & Jungner, G. (1968). Principles and practice of screening for disease. World Health Organization.