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Term of the Day

Natural history study

A natural history study is a preplanned observational study intended to track the course of a disease over time, identifying demographic, genetic, environmental and other variables that correlate with its development and outcomes in the absence of intervention, or under standard of care. Designs may be retrospective (chart review of existing records) or prospective (longitudinal follow-up of a cohort or registry).

Natural history data is particularly important in rare and paediatric diseases, where randomised placebo-controlled trials may be infeasible or unethical. The FDA (guidance on rare disease natural history studies, 2019) and the EMA accept well-designed natural history studies to define endpoints and biomarkers, identify patient subgroups, estimate sample sizes and, in some cases, serve as external or historical control arms for single-arm trials supporting orphan products.

Because they are non-interventional, natural history studies fall outside the CTR and are governed by national law (for example France's MR-003 or MR-004 reference methodologies) and by the GDPR. They typically involve secondary use of medical records, long-term follow-up, genetic data and small populations in which anonymisation is rarely achievable, so pseudonymisation, a DPIA and a robust research legal basis under Art. 9(2)(j) are essential. Registries maintained by patient organisations or academic consortia raise additional questions of joint controllership and data access governance.

R

Re-identification risk (motivated intruder test)

Re-identification risk is the possibility that a person can be identified from data intended to obscure their identity, including by combining it with other information. The assessment depends on the data, recipient and release context. Removing direct identifiers is therefore only one part of evaluating whether information is anonymous.

Key Takeaways

  • Assess combinations of characteristics and available external information, not just names and identification numbers.
  • A motivated intruder test explores realistic identification attempts; it is not a guarantee against every future attack.
  • The EDPB’s July 2026 anonymisation guidelines remain a consultation version on the source checked for this entry.

How Is the Risk Assessed?

ElementMeaning
DataRare combinations, detail and linkability
ContextRecipients, access and auxiliary information
TestingPlausible attempts and residual weaknesses

The ICO describes a contextual assessment of identification risk, including a motivated intruder approach. This asks what a reasonably competent person might achieve using available means, rather than imagining unlimited resources. ONS guidance treats intruder testing as a practical technique that complements other disclosure-control assessment. The EDPB’s Guidelines 02/2026 discuss anonymisation in a consultation text, not a final binding rule. Document which framework and version inform the assessment, especially when working across EU and UK requirements.

Primary guidance: ICO anonymisation effectiveness; ONS intruder testing guidance; EDPB anonymisation consultation.

Why It Matters for Data Protection

For a hypothetical public rare-disease dataset, we recommend examining whether age, treatment dates and a specialist centre could be combined with publicly available patient accounts. Record the assumed attacker, accessible information, attempted matches and limitations of the test. Choose transformations or access restrictions that address the observed risks while preserving the research purpose. Reassess when the dataset, release audience or available information changes. A failed identification attempt is useful evidence, but should not be treated as proof that identification is impossible in every context.

Frequently Asked Questions

Does removing names eliminate the risk?

No. Indirect identifiers and combinations may still make identification possible.

Is a motivated intruder test sufficient alone?

Not necessarily. Combine practical testing with the wider risk assessment.

Are the 2026 EDPB guidelines final?

The source checked on 27 October 2026 identifies them as a public consultation version.

Related reading: anonymization; pseudonymisation; EDPB anonymisation guidance and clinical trials.

Explore iliomad’s dpia support for your organisation.

Updated: 27 October 2026