Distribution of Hemoglobin F, A, S, C, E, and D Quan- Tities in 4 Million Newborn Screening Specimens, John

serum assay seems to be more restrictive than the urinary assay for detecting products derived from the telopeptide region of collagen type I. The decrease in bone remodeling that occurs in older girls with respect to younger girls is a generally accepted fact. In accordance with the results found in this work in b-CTX and bAP concentrations, it would be possible suggest that serum b-CTX assay (Serum CrossLaps) reflects better than urinary b-CTX assay (CrossLaps) the changes in bone remodeling that take place in young girls. It is possible that, independent of the differences attributable to renal clearance, the serum assay can be more specific for bone than the urinary assay because of the different antigenicity of the assays. More work is necessary to investigate this point, but the observed differences between the two b-CTX methods constitute an interesting point to consider when selecting a serum or a urinary assay to perform a particular study.