Re-identification risk for common privacy preserving patient matching strategies when shared with de-identified demographics.

Objective: Privacy preserving record linkage (PPRL) refers to techniques used to identify which records refer to the same person across disparate datasets while safeguarding their identities. PPRL is increasingly relied upon to facilitate biomedical research. A common strategy encodes personally identifying information for comparison without disclosing underlying identifiers. As the scale of research datasets expands, it becomes crucial to reassess the privacy risks associated with these encodings. This paper highlights the potential re-identification risks of some of these encodings, demonstrating an attack that exploits encoding repetition across patients.

Materials and methods: The attack leverages repeated PPRL encoding values combined with common demographics shared during PPRL in the clear (e.g., 3-digit ZIP code) to distinguish encodings from one another and ultimately link them to identities in a reference dataset. Using US Census statistics and voter registries, we empirically estimate encodings' re-identification risk against such an attack, while varying multiple factors that influence the risk.

Results: Re-identification risk for PPRL encodings increases with population size, number of distinct encodings per patient, and amount of demographic information available. Commonly used encodings typically grow from <1% re-identification rate for datasets under one million individuals to 10%-20% for 250 million individuals.

Discussion and conclusion: Re-identification risk often remains low in smaller populations, but increases significantly at the larger scales increasingly encountered today. These risks are common in many PPRL implementations, although, as our work shows, they are avoidable. Choosing better tokens or matching tokens through a third party without the underlying demographics effectively eliminates these risks.