Predictive validity of developmental milestones for detecting limited intellectual functioning

Developmental milestones are commonly used in child health care, although from many milestones the predictive validity has not been adequately assessed. We aimed to determine the predictive validity of 75 developmental milestones for detecting limited intellectual functioning that can be obtained before the age of 4 years. We performed a case-control study with 148 children aged 5-10 years with limited intellectual functioning (IQ 50-69), who were in special education (cases) and a random sample of 300 children aged 5-10 years who were in regular elementary education (controls). Developmental milestones scores were retrieved from Child Healthcare files. We calculated sensitivity, specificity, positive likelihood ratios (LR+) and diagnostic odds ratios (DOR) for limited intellectual functioning. The LR+ determines whether a test result changes the probability that a condition exists. Given the prevalence of intellectual disability (1-3%), we considered that an LR+ > 10 would be clinically useful, as it increases the a priori probability of limited intellectual functioning from 2% to a posteriori probability of at least 17%. Out of 75 assessed milestones, 50 were included in the analysis. We found nine milestones to have a significant adjusted (for socioeconomic status and prematurity) DOR > 1 and a significant LR+ > 10 (assessment age in months between brackets): 'says dada-baba''' (9), 'balances head well while sitting' (9), 'sits on buttocks while legs stretched' (9), 'babbles while playing' (12), 'sits in stable position without support' (12), 'walks well alone' (24), 'says sentences of 3 or more words' (36), 'places 3 forms in form-box' (36) and 'copies circle' (48). Sensitivities of these 9 milestones varied from 8-54%, specificities of these 9 milestones varied from 95-100%. Combining these milestones at 9,12, and 36 months respectively resulted in sensitivities of 27-60% and specificities of 94-99%. These nine developmental milestones have substantial predictive validity for limited intellectual functioning.