rare conditions is generated through systematic disease tracking systems.4 The Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute (NCI) collects data on a number of cancers, including some that are relatively uncommon. At the Centers for Disease Control and Prevention (CDC), programs on infectious diseases and birth defects track and report data on several rare conditions. The Agency for Toxic Substances and Disease Registry (ATSDR) tracks data on exposures to toxic substances with a focus on hazardous waste sites. The American Association of Poison Control Centers aggregates surveillance data from regional poison control centers, which report information on a broad range of poisonings, including those resulting from prescription and over-the-counter drugs, household products, and insect bites.

As newborn screening programs become more consistent in the United States, they may provide firmer data on the birth incidence of a number of genetic conditions. Work is continuing to develop a standard framework for reporting the results of newborn screening tests as part of electronic health records and also for analysis of trends by public health agencies (see description at http://newbornscreeningcodes.nlm.nih.gov).

For many rare conditions, one difficulty confronting epidemiologic studies involves the lack of condition-specific codes in the World Health Organization’s (WHO) International Classification of Diseases (ICD). The ICD provides the international standard diagnostic classification that is used for epidemiologic studies as well as for key health system management functions. To cite an example of the problem with lack of specific codes, a single ICD code (E75.2) covers Fabry disease, Gaucher disease, Krabbe disease, Niemann-Pick disease, Farber’s syndrome, metachromatic leukodystrophy, and sulfatase deficiency. (Codes for endocrine, nutritional, and metabolic diseases can be viewed at http://thcc.or.th/ICD-10TM/ge70.htm.) At the urging of a European rare diseases task force, WHO has created an advisory group to make recommendations about coding improvements for rare diseases. That group has been circulating draft materials for comment, which will be followed by field testing; implementation of coding changes is not expected until after 2015 (Aymé, 2009; Tejada, 2009). This project is complex, but its recommendations, if implemented, should strengthen the foundation for epidemiologic and other research on rare diseases. Much of the preparatory work on rare disease coding has been conducted by Orphanet, a European information consortium (originally established by the French Ministry of Health) (Aymé, 2009). Orphanet is also the source of the prevalence data discussed below.

4

International efforts are also important. For example, the International Network of Paediatric Surveillance Units, which does not include the United States, has supported studies that have described the molecular epidemiology and genotype-phenotype correlations for Rett syndrome, Prader-Willi syndrome, and Smith-Lemli-Opitz syndrome (Grenier et al., 2007).



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