To the Editor.
—The article by Ruff et al1 that addressed the association of lead levels and cognitive functions included the finding that ferritin levels were correlated with better initial cognitive function in the study group, an effect that has been previously documented.2To separate the effects of iron and lead in a simple way, Ruff et al could have presented the average and SD of lead levels, ferritin levels, and CIs separately for iron-deficient and iron-sufficient children at the beginning of the study and at the end when all children were iron sufficient (ferritin level, >16 μg/L). Readers could then judge if the suggested "small but significant" increase in CI for the whole study population was related to changes in ferritin levels (the mean increased from 9.8 to 19.2 μg/L) in the subgroup of iron-deficient children or to changes in lead levels in the whole population. Alternatively,
Schindler AM, Haecker T, Gould J, Turner E, Torchia M, Kaye R, Cockerill M, Spachman S. Declining Blood Lead Levels and Cognitive Change in Children. JAMA. 1993;270(7):828. doi:10.1001/jama.1993.03510070049020