Objective: To define the relationship between current Bayley Scales of Infant and Toddler Development, Third edition (Bayley-III) scores and the Bayley Scales of Infant Development, second edition Mental Development Index (MDI) to aid the comparison of population outcomes.
Study design: MDI and Bayley-III cognitive/language scales were administered concurrently in 185 extremely preterm children (≤26 weeks) at 29-41 months of age. Cognitive and language scores were combined (combined Bayley-III score [CB-III scores]) for comparison with MDI scores.
Results: Bayley-III cognitive and language scores were 10 and 3 points higher than MDI scores, respectively; CB-III scores were 7 points higher. The relationship between CB-III and MDI scores was not a simple offset: CB-III values were increasingly higher than MDI at lower scores. Bayley-III scores underidentified MDI scores <70 (sensitivity 58%; specificity 100%). An algorithm for converting Bayley-III scores into MDI scores improved predictive value (sensitivity 95%; specificity 97%). Bayley-III scores <80 were similarly predictive (sensitivity 89%; specificity 99%).
Conclusions: We recommend caution in the interpretation of Bayley-III scores in population studies as the correlation with the previous edition appears worse at lower test score values and the predictive value for IQ is as yet unclear.
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