How Autism is Diagnosed
Autism diagnosis requires a comprehensive evaluation by trained professionals using standardized diagnostic tools, preceded by routine screening at 18 and 24 months of age, with confirmation based on DSM-5 criteria through direct behavioral observation and structured caregiver interviews—not through any blood test or biomarker. 1, 2, 3
Screening Process
- All children should be screened at 18 and 24 months during routine well-child visits using validated tools like the Modified Checklist for Autism in Toddlers (M-CHAT), even without specific parental concerns. 2, 4, 5
- Key early warning signs between 12-24 months include: no response to name when called, reduced eye contact and social smiling, lack of pointing to show interest, absence of conventional gestures, no imaginative or pretend play, and repetitive behaviors. 1, 2, 6, 3
- If screening indicates significant ASD symptomatology, proceed immediately to comprehensive diagnostic evaluation. 1
Diagnostic Confirmation
The diagnosis must be made by professionals specifically trained in autism diagnosis (developmental pediatricians, child psychiatrists, neurologists, or psychologists with autism expertise) using objective criteria and standardized tools. 1
Core Diagnostic Components
- Standardized diagnostic instruments are essential and include:
- Direct observation focusing specifically on social interaction patterns (eye contact, joint attention, response to social overtures) and restricted/repetitive behaviors (stereotyped movements, rigid routines, intense circumscribed interests). 1, 4
- Detailed developmental history including timing of symptom onset, any regression (loss of previously acquired skills), response to previous interventions, and family psychiatric history. 1, 4
Differential Diagnosis Requirements
The diagnosis is confirmed only when DSM-5 criteria are met and other conditions are adequately ruled out. 1 Critical differentials include:
- Hearing impairment: Every child must have a formal audiogram before finalizing ASD diagnosis, as hearing loss can mimic autism symptoms. 1, 7, 4
- Developmental language disorders: Distinguished by preserved pointing for interest and use of conventional gestures at 20-42 months, which are impaired in autism. 1
- Intellectual disability alone: Differentiated by examining social communication relative to overall developmental level; at 24 months, directing attention (showing) and attention to voice distinguish autism. 1
- Reactive attachment disorder: Improves substantially with adequate caretaking, unlike autism. 1
- Anxiety disorders: Lack the prominent social-communication impairments and have developed social insight not seen in autism. 1
Essential Medical Workup
Mandatory Initial Testing
- Physical examination with specific attention to dysmorphic features and Wood's lamp examination for tuberous sclerosis signs. 1, 4
- Formal audiogram to rule out hearing loss (mandatory before proceeding). 1, 7, 4
- Cognitive testing to establish baseline developmental level and identify scatter in abilities or "splinter skills." 1, 7, 4
Genetic Evaluation (Tiered Approach)
A genetic consultation should be offered to all individuals with ASD, as thorough evaluation identifies an underlying etiology in 30-40% of cases. 7, 4
First-tier genetic tests (order initially):
- Chromosomal microarray (CMA): 10% diagnostic yield—now standard of care 1, 7, 4
- Fragile X DNA testing: 1-5% yield 7, 4
- High-resolution karyotype: 2.5% yield (if CMA not available) 1
Second-tier genetic tests (order based on specific features):
- MECP2 gene testing: 4% yield in females with ASD 1, 7, 4
- PTEN gene testing: 5% yield when head circumference >2.5 standard deviations above mean 1, 7, 4
Additional testing only when clinically indicated:
- EEG if history of staring spells, seizures, or marked aphasia (consider Landau-Kleffner syndrome) 1, 4
- Metabolic screening (urine mucopolysaccharides, organic acids, serum lactate, amino acids) only if clinical indicators present 1
- Brain MRI only for specific neurological signs, not routinely 1
Multidisciplinary Assessment Components
Coordinate evaluation across disciplines to avoid fragmented care: 1
- Communication assessment: Measure receptive/expressive vocabulary and pragmatic language use (social communication). 1
- Adaptive functioning assessment: Evaluate real-world skills relative to cognitive abilities. 1, 4
- Occupational/physical therapy evaluation: Assess sensory processing difficulties and motor skills when indicated. 1
Critical Pitfalls to Avoid
- Do not delay diagnosis waiting for the child to "grow out of it"—early intervention is most effective when started before age 5. 3, 5
- Do not skip the audiogram—hearing loss is a common mimic that must be ruled out. 1, 7, 4
- Do not order extensive genetic testing without clinical geneticist evaluation first—the stepwise approach is more cost-effective and better tolerated by families. 7, 4
- Do not overlook comorbidities: Screen for ADHD (occurs in ~75% of children with ASD), anxiety disorders (11% vs 5% in general population), depression (20% vs 7%), sleep difficulties (13% vs 5%), and epilepsy (21% with co-occurring intellectual disability). 1, 3, 5
- Avoid diagnostic overshadowing—the tendency to miss other comorbid conditions when autism is the more noticeable diagnosis. 1
Adult Diagnosis Considerations
The same diagnostic criteria and standardized tools (ADOS-2, ADI-R) apply to adults, though obtaining detailed developmental history may be more challenging. 3, 8 Many high-functioning adults receive delayed diagnosis because milder social impairments were overlooked in childhood. 1, 8
Genetic Counseling and Family Planning
Provide genetic counseling to all families regardless of whether a specific etiology is identified: 7, 4