Diagnostic classification of the aetiology of mental retardation in children.

Children with mental retardation are often subjected to a routine battery of tests, a practice that has been criticised.' Investigations often include a blood count, a biochemical profile, urinary screening for metabolites including amino acids, and radiography of the skull and wrist for bone age and may be more extensive. Such routine testing of all children with mental retardation is done in the hope of classifying cases in which there are no specific clinical clues to aetiology. Several series have been published, however, in which no unsuspected diagnoses were made in this way.24 Our aim is to provide a basis of rational investigation planned individually for each patient. Smith and Simons have suggested primary categorisation according to the time of onset of evident mental retardation.4 This, however, leaves 41% of patients unclassified. Our method is based