CDBC/BCAAN Education Bite, Originally emailed October 2, 2205

The DSM-5 allows for the diagnosis of intellectual developmental disorder (IDD) under certain circumstances when an individual has substantial intellectual deficits but an IQ score above 70. These situations are uncommon and often complicated, so psychologists should rely on careful clinical judgment and review multiple sources of information before deciding whether a diagnosis of IDD is appropriate. 

Diagnosis of IDD requires standardized testing, with IQ at least approximately 2 standard deviations below the mean 

IDD requires deficits in “intellectual functions…confirmed by both clinical assessment and individualized, standardized intelligence testing” (p.33, Criteria A). In the Diagnostic Features section, the DSM goes on to clarify that individuals with IDD have scores of “approximately two standard deviations or more below the population mean, including a margin for measurement error (generally +/- 5 points, p.37).” 

IQ scores may not fully reflect an individual’s functioning
The DSM-5 states, “IQ scores are approximations of conceptual functioning but may be insufficient to assess reasoning in real-life situations and mastery of practical tasks. For example, a person with deficits in intellectual functioning whose IQ score is somewhat above 65-75 may nevertheless have such substantive adaptive behavior problems in social judgment or other areas of adaptive functioning that the person’s actual functioning is clinically comparable to that of individuals with a lower IQ score (p.42).” 

Substantial impairment in intellectual functioning is required 

The DSM-5-TR emphasizes that an IQ “somewhat” above the typical IDD range may still warrant consideration of a diagnosis of IDD, but only when significant evidence of intellectual impairment is present. 

By adding the word “somewhat,” the DSM-5-TR clarifies that this is not intended to bypass requiring the presence of substantial impairment in measured IQ. Low adaptive functioning is necessary but not sufficient for a diagnosis of IDD. 

Psychologists must use clinical judgment to consider the various factors that can explain low adaptive functioning 

Adaptive scores may be explained by factors other than intellectual functioning. Standardized measures of adaptive functioning provide valuable information about the level of support needed in daily living domains, but they do not identify the underlying reasons for the support needs. Clinicians must therefore carefully consider co-morbid conditions, cultural influences, learning opportunities, and contextual factors when interpreting results. 

When co-morbid conditions are present, it may be useful to focus on daily functioning tasks less directly impacted by the condition. Although disentangling the effects of a co-morbid condition can be difficult, clinicians should consider whether the individual demonstrates impairments beyond what would be expected for the co-morbid condition. 

Cultural and contextual factors also play a critical role. Adaptive functioning expectations are shaped by cultural norms and opportunities for independence. Clinicians should ask: 

• Do socio-economic or cultural factors limit the validity of standardized assessment tools? 
• Are adaptive deficits evident across multiple environments (home, school, work, community), or do they reflect situational influences? 

Psychologists must use clinical judgment in interpreting the results of IQ tests 

In addition to IQ scores, clinical judgment regarding intellectual functioning should be based on file review, interviews, and observations, and should consider quantitative and qualitative evidence, including:  

1. Early developmental history (Criterion C requires onset within the developmental period) 
2. Developmental trajectory, including score patterns across multiple assessments 
3. Educational and vocational history 
4. Underlying developmental, psychological, medical, or neurological conditions affecting development 
5. Cultural and language bias in IQ testing and culturally appropriate developmental expectations 
6. Motivation and performance validity 

 Summary 

Diagnosing IDD when IQ is above 70 (+/- 5) means moving beyond test scores and weighing the broader picture of an individual’s functioning. Psychologists should ask: Are the observed adaptive deficits consistent, pervasive, and greater than expected for co-morbidities or environmental factors? Do they reflect longstanding developmental limitations rather than acquired impairments? Grounding the decision in developmental history, cultural context, and multiple sources of evidence helps ensure that the diagnosis is applied accurately, allowing for access to appropriate supports and services. Clear documentation of the clinical reasoning process is essential so that stakeholders understand how the decision was made. 

Note

Community Living BC (CLBC) requires a registered psychologist (not a school psychologist) to make a diagnosis of IDD with FSIQ > 75 in order to access CLBC services.