What is the diagnostic approach for intellectual disability, including the role of IQ testing?

Diagnostic Approach for Intellectual Disability

IQ testing is essential but insufficient for diagnosing intellectual disability—diagnosis requires both standardized IQ testing (IQ ≤70) AND documented deficits in adaptive functioning across conceptual, social, and practical domains, with the adaptive functioning profile determining severity classification, not the IQ score. 1, 2

Screening and Referral

• Screen systematically at 9-, 12-, 18-, 24-, and 30-month well-child visits using validated tools like the Ages and Stages Questionnaire (ASQ-III, sensitivity 0.83-0.89, specificity 0.80-0.92) 1, 2
• Refer for standardized IQ testing any child or adolescent with academic performance difficulties or behavioral challenges, even in later childhood when academic demands exceed capacities 1
• Use "Global Developmental Delay" for children under 5 years, as IQ measures are unreliable in this age group; diagnosis requires significant limitations in ≥2 developmental domains 1, 2

Diagnostic Evaluation Components

Intellectual Functioning Assessment

• Administer individually standardized IQ tests by qualified practitioners (psychologists) to document intellectual deficits in reasoning, problem solving, planning, abstract thinking, judgment, and learning from experience 1, 2
• Interpret the IQ subtest profile, not just the composite score—the profile reveals cognitive strengths and weaknesses requiring targeted supports and is more clinically useful than a single number 1, 3
• Apply clinical judgment when interpreting results, as multiple factors affect test reliability: motivation, cooperation, physical/mental health, test setting, examiner attitude, and communication/sensory/motor factors 1

Adaptive Functioning Assessment

• Assess adaptive functioning across three domains using both standardized scales AND clinical assessment: 1, 2

  • Conceptual domain: academic learning, reading, writing, money management, complex reasoning
  • Social domain: communication, social participation, recognizing others' thoughts/feelings, social judgment
  • Practical domain: self-care (hygiene, dressing), independent living, managing finances, complex life decisions

• Prioritize adaptive functioning over IQ for severity classification—DSM-5 explicitly states IQ scores are "approximations of conceptual functioning but may be insufficient to assess reasoning in real-life situations and mastery of practical skills" 1, 4, 2

Critical Diagnostic Criteria

Both criteria must be met: 1, 2

• Intellectual deficit: IQ ≤70 (approximately 2 SD below mean) confirmed by clinical assessment AND standardized testing
• Adaptive functioning deficit: Failure to meet standards for personal independence and social responsibility across multiple environments (home, school, work, community)
• Onset during developmental period: Before age 18

Special Populations and Pitfalls

Cultural and Linguistic Minorities

• Expect underestimation of abilities on standardized tests in children from cultural and linguistic minorities, as testing instruments lack sensitivity for these populations 1, 2
• Weight clinical assessment heavily in these cases rather than relying solely on numerical scores 1

Severe ID or Language Impairment

• Recognize limited reliability of IQ testing in individuals with more severe ID or language impairment, as fewer such individuals were included in establishing score ranges 1
• Use "Unspecified Intellectual Disability" for individuals ≥5 years when assessment is impossible due to sensory/physical impairments, communication difficulties, locomotor disability, or severe behavioral/psychiatric comorbidities 1, 2

Neuropsychological Profiles

• Examine individual cognitive profiles based on neuropsychological testing using the Cattell-Horn-Carroll (CHC) model, which considers both general intelligence factor (g) and specific factors 3
• Identify specific patterns that guide targeted interventions—for example, deletion 22q11.2 shows verbal IQ higher than performance IQ with visuospatial deficits 1

Ongoing Assessment Requirements

• Re-evaluate at least every 3 years in school-aged children as mandated by federal law 1
• Screen for psychiatric comorbidities using validated tools (Developmental Behaviour Checklist, Nisonger Child Behavior Rating Form, Reiss Screen), as psychiatric disorders occur at 3-fold higher rates than in typically developing children 2
• Monitor for common comorbidities: epilepsy, autism spectrum disorder, ADHD, anxiety disorders, oppositional defiant disorder 2

Etiologic Workup

After establishing the diagnosis of intellectual disability: 5

• Obtain 3-generation family history and perform dysmorphologic examination
• Order standard karyotype, Fragile X molecular genetic testing, and array comparative genomic hybridization (aCGH provides diagnosis in ~10% of cases)
• Consider neuroimaging based on clinical findings
• Recognize that history and examination by an expert clinician leads to diagnosis in 2 of 3 patients where etiology is identified, with laboratory studies providing diagnosis in the remaining one-third 5