Rare Disease - Definition

Definition

There is no single, widely accepted definition for rare diseases. Some definitions rely solely on the number of people living with a disease, and other definitions include other factors, such as the existence of adequate treatments or the severity of the disease.

In the United States, the Rare Disease Act of 2002 defines rare disease strictly according to prevalence, specifically "any disease or condition that affects less than 200,000 persons in the United States," or about 1 in 1,500 people. This definition is essentially like that of the Orphan Drug Act of 1983, a federal law that was written to encourage research into rare diseases and possible cures.

In Japan, the legal definition of a rare disease is one that affects fewer than 50,000 patients in Japan, or about 1 in 2,500 people.

However, the European Commission on Public Health defines rare diseases as "life-threatening or chronically debilitating diseases which are of such low prevalence that special combined efforts are needed to address them." The term low prevalence is later defined as generally meaning fewer than 1 in 2,000 people. Diseases that are statistically rare, but not also life-threatening, chronically debilitating, or inadequately treated, are excluded from their definition.

The definitions used in the medical literature and by national health plans are similarly divided, with definitions ranging from 1/1,000 to 1/200,000.