Introduction: Early diagnosis of Autism Spectrum Disorder (ASD) implies earlier intervention and improved prognosis. In India use of international tools is limited by expense, cultural adaptability and  validation issues. ‘Indian Scale for Assessment of Autism (ISAA)’ diagnoses and classifies ASD, enables disability certification and is valid. concerns regarding applicability in young children have emerged.

Objectives: To determine diagnostic accuracy of ISAA in 2-9 year olds with ASD and level of agreement with Childhood Autism Rating Scale (CARS)

Methodology: In a Hospital based observational study 2-9 year olds with age inappropriate developmental skills and/ or behaviour underwent Expert comprehensive assessment resulting in diagnoses of ASD, Global Developmental Delay (GDD) / ID and others.  ISAA was administered to those with ASD and GDD/ ID (study group). Both interviewers were blinded. Statistical analysis was by SPSS 19.

Results: Study group comprised of 90 children (mean age 4.5 years). Validation against Expert diagnosis revealed sensitivity and specificity of ISAA of 92.3% and 97.4% respectively. Positive and Negative Likelihood Ratios and predictive values were found acceptable. Level of agreement with CARS was poor. Only the cut off score differentiating between No Autism and Autism was valid.

Conclusion: Although ISAA is a free, easily available diagnostic tool for ASD with proven validity and reliability in older individuals, use in 3 – 9 year olds is limited to identification of those ‘with’ and ‘without’ ASD. Sub-categorization is not satisfactory. Suitability in 2-3 year olds was inconclusive due to inadequate sample in that range.