About the Classification of IEMs

About the Classification of IEMs

Inborn errors of metabolism (IEMs) represent a large class of rare genetic disorders. The first IEM was described in 1902 by Sir Archibald Garrod. Since then, many more have been described, variously estimated as >600 or >700. We initially established a proposed nosology of IEMs, by first defining specific criteria to include a disorder as an IEM, in an attempt to create a self-consistent schema. Our initial approach identified 1015 well-characterized inborn errors of metabolism described in the literature. In addition, there were 111 less well-characterized conditions that may be IEMs but did not meet strict criteria for inclusion in the nosology. This nosology was later supplanted by the International Classification of Inherited Metabolic Disorders (ICIMD), including 1450 disorders.

By necessity, the classification is subject to constant revision as a result of the rapid pace of progress in the field of genetics. Thus, as new disorders are discovered, and our understanding of the pathophysiology of existing disorders continues to improve, frequent updates to the current classification will be critical to maintain its usefulness.


Carlos R Ferreira, Shamima Rahman, Markus Keller, Johannes Zschocke, ICIMD Advisory Group.
An international classification of inherited metabolic disorders (ICIMD).
J Inherit Metab Dis. 2021 Jan;44(1):164-177.doi: 10.1002/jimd.12348.