What are the three diagnostic criteria for intellectual disability?

Definition

Three criteria must be met to establish the diagnosis of intellectual disability: significantly subaverage intellectual function, significant impairments in adaptive function, and onset before 18 years of age. The three diagnostic formulations—Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR), American Association on Intellectual and Developmental Disabilities (AAIDD), and Individuals with Disabilities Education Act (IDEA)—agree on the 3 criteria but define them differently.

Significantly subaverage general intellectual function refers to performance on an individually administered test of intelligence that is approximately two standard deviations (SD) below the mean. For a test that has a mean of 100 and SD of 15, IQ scores below 70 would meet these criteria. If the standard error of measurement is considered, the upper limits of subaverage intellectual function may extend to an IQ of 75. Using a score of 75 to delineate intellectual disability might double the number of children with intellectual disability, but the requirement for impairment of adaptive skills limits the false positives. Children with intellectual disability often show a variable pattern of strengths and weaknesses. Not all of their partial scores on IQ tests fall into the significantly subaverage range.

Significant impairment in adaptive behavior reflects the degree that the cognitive dysfunction impairs daily function. Adaptive behavior refers to the skills that are required for people to function in their everyday lives. Adaptive behavior may be assessed by three different constructs: the classification of DSM-IV-TR, the classification of AAIDD, and the IDEA.

The DSM-IV-TR classification of adaptive behavior addresses 10 domains: communication, self-care, home living, social and interpersonal skills, use of community resources, self-direction, functional academics, work, leisure, and health and safety. For a deficit in adaptive behavior to be present, a significant delay in 2 of the 10 areas must be present.

The AAIDD classification of adaptive behavior addresses 3 broad sets of skills: conceptual, social, and practical. Conceptual skills include language, reading and writing, money concepts, and self-direction. Social skills include interpersonal skills, personal responsibility, self-esteem, gullibility, naiveté, and ability to follow rules, obey laws, and avoid victimization. Representative practical skills are performance of activities of daily living (dressing, feeding, toileting and bathing, mobility), instrumental activities of daily living (housework, managing money, taking medication, shopping, preparing meals, using the telephone, etc) occupational skills, and the maintenance of a safe environment. For a deficit in adaptive behavior to be present, a significant delay in 1 of the 3 areas must be present. The rationale for requiring only 1 of the 3 areas is the empirically derived finding that people with intellectual disability can have varying patterns of ability and may not have deficits in all 3 areas.

The IDEA requires that the cognitive dysfunction affect school performance.

The requirement for adaptive behavior deficits is the most controversial aspect of the diagnostic formulation. The controversy centers on 2 broad areas: whether impairments in adaptive behavior are necessary for the construct of intellectual disability and what to measure. The adaptive behavior criterion may be irrelevant for many children; adaptive behavior is impaired in virtually all children who have IQ scores <50. The major utility of the adaptive behavior criterion is to confirm intellectual disability in children with IQ scores in the 65-75 range. It should be noted that deficits in adaptive behavior are often found in disorders such as Asperger syndrome (Chapter 28) and ADHD (Chapter 30) in the presence of typical intellectual function.

The issues of measurement are important as well. The independence of the 3 domains of the AAIDD and the 10 domains of the DSM-IV-TR has not been validated with research. The relationship between adaptive behavior and IQ performance is insufficiently explored. Most adults with mild intellectual disability do not have significant impairments in practical skills. It should be noted that adaptive behavior deficits must be distinguished from maladaptive behavior (e.g., aggression, inappropriate sexual contact).

Onset before age 18 yr distinguishes dysfunctions that originate during the developmental period. The diagnosis of intellectual disability may be made after 18 years of age, but the cognitive and adaptive dysfunction must have been manifested before age 18. The IDEA, because of its focus on school-aged children, does not require a limit of 18 years but refers to the “developmental period.”

The most commonly used medical diagnostic criteria for intellectual disability are those contained in the DSM-IV-TR (Table 33-1). The classification of intellectual disability that results from these definitions has been criticized for depending on IQ test performance rather than adaptive behavior, not taking the standard error of measurement into account, and not being predictive of outcomes for individuals. A new edition is currently being prepared that might address these issues. The AAIDD has proposed a different classification system. Instead of defining degrees of deficit (mild to profound), the AAIDD definition substitutes levels of support required in areas of adaptive function (intermittent, limited, extensive, or pervasive). The reliability of this approach has been challenged, and it blurs the distinction between intellectual and other developmental disabilities (communication disorder, autism, specific learning disabilities).

Table 33-1DIAGNOSTIC CRITERIA FOR INTELLECTUAL DISABILITY

B Concurrent deficits or impairments in present adaptive functioning (i.e., the person’s effectiveness in meeting the standards expected for his or her age by his or her cultural group) in at least two of the following areas: communication, self-care, home living, social and interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.

Code based on degree of severity reflecting level of intellectual impairment:

From American Psychiatric Association: Diagnostic and statistical manual of mental disorders, fourth edition, text revision, Washington, DC, 2000, American Psychiatric Association, p 49, reprinted by permission.

The term mental retardation should be cast aside because it is stigmatizing, has been used to limit the achievements of the individual, and has not met its initial objective of providing assistance to people with the disorder. The term intellectual disability is increasingly used in its place but has not been adopted universally; existing laws and their attendant entitlements still use the term mental retardation. In Europe, the term learning disability is often used to describe intellectual disability. Global developmental delay is a term often used to describe young children whose limitations have not yet resulted in a formal diagnosis of intellectual disability; it is often inappropriately used beyond the point when it is clear the child has intellectual disability, usually age 3 years.

What are the 3 diagnostic criteria for intellectual disability?

What are the key diagnostic criteria for intellectual development disorder?

What is DSM IV diagnostic criteria for intellectual disability?

What is the diagnosis of intellectual disability?