OBJECTIVES: Rare diseases (RDs) are chronic, life-threatening medical conditions individually affecting a low percentage of the population compared to other diseases. A medication used for diagnosis, prevention, or treatment of a rare disease may be known as an Orphan Drug (OD). There is no universal definition for RDs or ODs. This systematic review (SR) identified the quantitative and qualitative criteria used to define RDs, and ODs as well as the underlying rationale, internationally. Keywords: Orphan Drugs; Rare Diseases; Systematic Review; Criteria; Definition.

METHODS: A SR was conducted to identify the criteria used to define RDs and ODs internationally. using different databases PubMed, Medline, EMBASE, Scopus, Web of Science, GreyLit and OpenGrey. Included articles were screened by two independent reviewers using the following criteria: no geographical or timing limitation, all articles being written in English. Rare cancers, infectious rare diseases, or rare diseases due to poisoning were excluded. Extracted data were analysed using thematic and content analyses for qualitative descriptors, and quantitative data were analysed descriptively and reported in the form of frequencies and percentages.

RESULTS: A total of 2712 articles were identified, with 701 articles screened by title and abstract according to inclusion criteria after duplicates were removed. After exclusion and full text screening, only 98 articles were taken forward for data extraction and analysis.

CONCLUSIONS: There is a significant knowledge gap and lack of consensus defining RDs and ODs. This review provided details on international approaches, facilitating development of national policy and collection of harmonized data.