Most individuals who have multiple disabilities also fit the criteria for severe disabilities, while not all with severe disabilities have multiple disabilities. The primary measures used to diagnose these individuals are individual intelligence tests and tests of adaptive behavior. Early assessment of movement limitations, muscle tone and flexibility, seizure activity, breathing control, sucking and swallowing, vision and hearing, and genetic makeup are also, and prenatal assessment of genetic material or physical identification of deformities via sonograms may be conducted.
Accurate psychological testing of these individuals is challenging due to their frequent limitations in controlled movement, vision, hearing, communication, or cooperative behavior. Thus, interviews with family members and educators regarding the person's adaptive behavior skills (i.e., communication, self-care, home living, social skills, community use, self-direction, health and safely, functional academics, leisure, and work) may be more informative and reliable than a norm-based IQ or achievement score.