How should I assess an 18‑month‑old child for autism spectrum disorder?

Assessment of Autism in an 18-Month-Old Child

Use the Modified Checklist for Autism in Toddlers-Revised with Follow-Up (M-CHAT-R/F) as your primary screening tool at 18 months, and if positive on the initial questionnaire, complete the structured follow-up interview before referring for full diagnostic evaluation. 1, 2

Screening Approach at 18 Months

Primary Screening Tool: M-CHAT-R/F

The M-CHAT-R/F is the most extensively studied and recommended screening instrument for 18-month-olds 1, 2. This is a two-stage process:

Stage 1: Parent completes the M-CHAT-R questionnaire (20 items assessing communication skills, joint attention, repetitive movements, and pretend play)

Stage 2: If screen-positive, conduct the structured follow-up interview before making referral decisions

• Without the follow-up interview, the positive predictive value (PPV) is only 11% in low-risk populations 1
• With the follow-up interview, PPV improves to 43-65% 1, 3
• Sensitivity ranges from 72-83% across age groups 1
• Specificity is 55-77% 1

Important Caveats About 18-Month Screening

Screening at 18 months has inherent limitations compared to 24-month screening 1, 3:

• Higher false-positive rates than screening at ≥24 months 1
• The M-CHAT-R/F shows lower sensitivity at 18 months compared to 24 months 3
• Many children will be missed at 18 months and detected only at 24-month rescreening 4

However, 18-month screening remains valuable because:

• Early signs are already apparent to caregivers at this age 5
• Children identified at 18 months tend to have milder symptoms and may be more amenable to early intervention 6
• Symptoms can be reliably detected as early as 14 months 6

Clinical Assessment Components

When conducting the evaluation, specifically assess:

• Social communication: Response to name, eye contact, social smiling, showing/sharing interest with others
• Joint attention: Pointing to show interest (protodeclarative pointing), following a point, bringing objects to show parents
• Repetitive behaviors: Hand flapping, toe walking, unusual sensory interests, repetitive play patterns
• Language development: Babbling, single words, gestures for communication
• Play skills: Pretend play (feeding a doll, pretending to talk on phone), functional play with toys

Action Based on Screening Results

If M-CHAT-R is positive:

1. Complete the follow-up interview immediately
2. If follow-up interview confirms concerns, refer for comprehensive diagnostic evaluation with ADOS-2 (Autism Diagnostic Observation Schedule-Second Edition) 5, 3
3. Do not delay referral waiting for 24-month visit

If M-CHAT-R/F is negative:

• Rescreen at 24 months - this is critical as repeated screening detects children missed at 18 months who may be more severely affected 6, 4
• Children detected at 24 months after negative 18-month screening show more impairment in cognitive, adaptive, and social functioning 6

Alternative Considerations

The Q-CHAT-10 with ordinal scoring (Q-CHAT-10-O) shows some advantages over M-CHAT-R/F at 18 months, including higher sensitivity and no requirement for follow-up interview 3. However, it has lower specificity and is not as widely validated in clinical practice.

For high-risk populations (siblings of children with autism), targeted screening yields a 22.7% detection rate and should be implemented routinely 5. In these populations, the M-CHAT-R/F demonstrates sensitivity of 0.72-0.83 but specificity of only 0.55-0.77 5.

Critical Practice Points

• Do not use the CHAT (Checklist for Autism in Toddlers) - it has unacceptably low sensitivity of 18% 1
• The two-stage M-CHAT-R/F process is essential; skipping the follow-up interview results in excessive false positives 1
• Universal screening at 18 months identifies children when symptoms are milder, but rescreening at 24 months is mandatory to capture those with later-emerging or more severe presentations 6, 4
• Positive screening requires prompt referral for diagnostic evaluation - do not adopt a "wait and see" approach 2

Regarding USPSTF Guidance

The USPSTF gives an "I" statement (insufficient evidence) for universal screening in children without parental or clinician concerns 2. However, this does not prohibit screening - it means clinicians should use clinical judgment 2. The Pediatrics guideline provides stronger support for ASD-specific screening at 18 and 24 months 1, and given the availability of effective early interventions (particularly applied behavior analysis-based approaches) and minimal harms from screening 2, screening at 18 months is justified in clinical practice.