Subclinical levels of lead and developmental deficit--a multivariate follow-up reassessment

Scores on the McCarthy Scales of Children's Abilities, school reading tests, teacher ratings, and several exploratory measures were obtained for urban black school-aged children, first studied five years previously. These were related, for 63 children, to preschool blood lead, school-age blood lead, and free erythrocyte protoporphyrin levels, and, for 34 children, to dentine lead. Most outcome variables were not significantly related to the lead variables. Preliminary analyses indicated that results of several of the McCarthy Scales, including the critical General Cognitive Index and Verbal Scales, and the reading test were significantly impaired in higher lead level groupings. However, incorporating a brief measure of parent IQ into the analyses decreased variance associated with lead and led to a strong suspicion of the remaining significant results. Few investigators reporting positive effects have considered parent intelligence, which is known to be a major determinant of developmental status. For this and other admittedly difficult methodologic reasons, conclusions from prior studies are questioned.