Both the American Psychiatric Association (APA) and The American Association on Intellectual and Developmental Disabilities (AAIDD) use severity codes. These codes further refine diagnosis and categorize intellectual disabilities. However, the meanings of the severity codes between these two diagnostic systems are somewhat different. Both systems now use adaptive functioning to determine severity. Previously, the APA used IQ scores to determine severity. However, the APA severity is based on limitations. In contrast, the AAIDD is based on the degree of support needed. This is called support intensity.
The American Psychiatric Association and severity codes for intellectual disability
The DSM-5 (APA, 2013) severity codes indicate the diagnosing clinician's impression of the severity of adaptive functioning. Severity is assessed across three domains. These are conceptual, social, and practical life skills.
Mild intellectual disability includes about 85 percent of people with intellectual disabilities. Many individuals within this group can achieve some academic success. They usually meet elementary academic levels or beyond with sufficient supports. People with this degree of severity are mostly self-sufficient with sufficient supports. In many cases, they can live independently within their communities with a minimal level of additional supports. These supports might include assistance with life decisions. Additional time, instructions, and reminders may be needed for other life skills such as finances, nutrition, shopping, and transportation.
Moderate intellectual disability includes around 10 percent of the individuals with intellectual disabilities. People in this range have adequate communication skills but complexity is more limited. Social cues, social judgment, and social decisions (particularly romantic decisions) regularly need support. Most self-care activities can be performed but may require extended instruction and support. Independent employment can be achieved in positions that require limited conceptual or social skills. However, additional supports may be required. Likewise, independent living may be achieved with moderate supports such as those available in group homes.
Severe intellectual disability describes 3 to 4 percent of this population. Communication skills are very basic. Self-care activities require daily assistance. Many individuals in this category will require safety supervision and supportive assistance. Residence in supported housing is usually necessary.
Profound intellectual disability describes a very small portion of the persons with intellectual disabilities. Only 1 to 2 percent fall into this category. This person is dependent upon others for all aspects of daily care. Usually 24-hour care and support are needed. Communication skills are quite limited. People with profound intellectual disability usually have co-occurring sensory or physical limitations.
The American Association on Intellectual and Developmental Disabilities (AAIDD) severity codes for intellectual disabilities:
Like the DSM-5 (APA, 2013) the AAIDD has a categorical system for classifying intellectual disabilities. However, instead of classifying by the severity of functional limitations, the AAIDD assesses severity based on the intensity of supports that are needed. These needs are typically identified using a standardized support need instrument such as the Supports Intensity Scale (AAIDD, 2004). The AAIDD system evaluates a person's strengths and abilities, not just their limitations. It categorizes each person's level of functioning based on the level of support that person needs to function reasonably well in his or her preferred environment:
Intermittent support: Many people with intellectual disabilities do not require regular support or assistance. Instead, they may only require additional supports during times of transition, uncertainty, or stress. Usually people requiring this level of support would be categorized under the APA standards as mild intellectual disability.
Limited support: Some people with intellectual disabilities can learn to improve their adaptive behavior. With additional training, they can increase their conceptual skills, social skills, and practical skills. However, they may still require additional support to navigate everyday situations. People in this group would often be categorized by APA standards as moderate intellectual disability.
Extensive support: Other people with intellectual disability require support that is more intensive. These individuals have some basic communication skills and can complete some self-care tasks. However, they will usually require daily support. This level of support is usually associated with severe intellectual disability by APA criteria.
Pervasive support: Pervasive support describes the most intense level of support. Daily interventions are necessary to help the individual function. Supervision is necessary to ensure their health and safety. This lifelong support applies to nearly every aspect of the individual's routine. This classification is associated with those who have profound intellectual disability.
Problems with severity classification systems
The DSM-5 (APA, 2013) uses a categorical approach to the severity classification of intellectual disabilities (ID, formerly mental retardation). When a person is diagnosed with an ID, the severity of their disability is estimated. Severity falls into one of the four severity categories: mild, moderate, severe, or profound. However, there are several problems with this approach. These problems have led many professionals to lobby for a more useful method of classification.
A categorical approach to classification represents severity as four, separate, distinct categories. However, the severity of a disability may be more accurately described as a continuum. It could range from very mild to very severe with an infinite number of points in between these two extreme poles. Nonetheless, the question remains: Does a single dimension, called 'severity' communicate useful and practical information? Many professionals would argue it does not. For instance, what does it mean to have a "mild" intellectual disability? How does a person with a "mild" disability differ from someone with a "moderate" disability? Do two people with "mild" disabilities have the same needs and abilities? Do they each require the same type and intensity of support?
Because of these problems, some professionals argue for a classification according to the intensity of supports that are needed. This would be very similar to the method used by AAIDD. The AAIDD may indeed provide more useful and practical information. However, it still relies upon a categorical approach. Others have suggested a multi-dimensional approach to assess functional impairment along several dimensions. However, such an approach can become too complex for practical use. Therefore, while categorical classification lacks specificity, its advantage is simplicity.