A two-part model for reference curve estimation subject to a limit of detection.

Reference curves are commonly used to identify individuals with extreme values of clinically relevant variables or stages of progression which depend naturally on age or maturation. Estimation of reference curves can be complicated by a technical limit of detection (LOD) that censors the measurement from the left, as is the case in our study of reproductive hormone levels in boys around the time of the onset of puberty. We discuss issues with common approaches to the LOD problem in the context of our pubertal hormone study, and propose a two-part model that addresses these issues. One part of the proposed model specifies the probability of a measurement exceeding the LOD as a function of age. The other part of the model specifies the conditional distribution of a measurement given that it exceeds the LOD, again as a function of age. Information from the two parts can be combined to estimate the identifiable portion (i.e. above the LOD) of a reference curve and to calculate the relative standing of a given measurement above the LOD. Unlike some common approaches to LOD problems, the two-part model is free of untestable assumptions involving unobservable quantities, flexible for modeling the observable data, and easy to implement with existing software. The method is illustrated with hormone data from the Third National Health and Nutrition Examination Survey.

Concurrent validity of the parent-completed Ages and Stages Questionnaires, 2nd Ed. with the Bayley Scales of Infant Development II in a low-risk sample.

Abstract Background This study assessed the concurrent validity of the Ages and Stages Questionnaire (ASQ) compared with Bayley Scales of Infant Development II (BSID II) amongst children aged 24 months. Methods Data were collected from 53 infants and mothers who participated in the New York State Angler Cohort Child Development Study. Parents completed the 24-month ASQ to assess communication, personal-social, problem-solving ability, and fine and gross motor control. The BSID II was administered by a clinical psychologist at the 24-month home visit for cognitive and psychomotor assessment. The ASQ was scored using age-specific norms of <2 SDs below any domain mean to define failure. A BSID II score of <85 indicated mild or severe delay, while a score of <70 suggested a severe delay. Results Scores on the ASQ communication and personal-social domains were moderately correlated with the BSID II Mental Scale (R= 0.52, P < 0.001; R= 0.45, P < 0.01) and ASQ gross motor with the BSID II Motor Scale (R= 0.46, P < 0.01), whereas ASQ problem-solving and fine motor domains were not significantly correlated with BSID II scores. The ASQ had a sensitivity of 100% and specificity of 87% at 24 months (n= 40) for severely delayed status. Conclusions Results suggest the ASQs provide a simple, valid, and cost-effective method for clinicians and field-based researchers to reduce the number of standardized assessments required to identify developmentally delayed infants at age 24 months. Future studies should further assess the validity of the ASQs in larger, more diverse populations of infants.