OBJECTIVES: The Delphi method is widely used for reaching consensus in . However, there are no standard criteria for the definition of panel members, what constitutes a sufficient number of participants to ensure stability of results, or how to maintain a high response rate and interest, particularly in rare diseases. We reviewed scientific literature and outline personal experience to provide insight into good practices for defining and selecting experts on rare diseases, and to increase the response rate at first and subsequent Delphi rounds.

METHODS: EMBASE and Medline were searched on January 5^th, 2024. Out of 237 records, ten articles of interest assessed the application of the Delphi method in rare disease research, paying attention to expert definition, selection, recruiting, panel size, and retention management.

RESULTS: Most often cited expert selection criteria are membership in professional boards, at least two years of clinical experience, drafting guidelines on disease management and recent publishing in high-ranking journals. Conditions and panel sizes vary depending on the disease. To ensure sufficient professional heterogeneity, the literature recommends that experts are recruited through purposive sampling and not snowballing. Effective strategies to reduce attrition, thus increasing the validity of results, include online modified Delphi panels, obtaining participants’ written consent to participate in subsequent rounds, attending to survey layout, sending out reminder e-mails between rounds, and maintaining a short between-round timeframe. Inviting panel members who missed a round to a subsequent round is a robust approach to the nonrandom loss of opinions that could lead to false consensus.

CONCLUSIONS: Many challenges are raised when using the Delphi technique. Researchers need to adapt the method to suit their disease field and study aim. Online modified Delphi panels are increasingly used by researchers worldwide and represent a promising and rigorous research technique in healthcare.