Variability of adjustments to indices in determining patient risk in biochemical screening

It has long been appreciated that the measurement of biochemical parameters for prenatal screening for neural tube defects, and later aneuploidy, is n ot as simple as measuring hemoglobin or hematocrit. Early in the game, it w as recognized that there are gestational age curves, and that since a-fetop rotein (AFP), for example, is a fetal product, its distribution varies as a function of maternal plasma volume, and therefore the weight of the mother . A number of different adjustment factors have been used for AFP and other parameters for years, with varying degrees of consistency and reliability. Here we review a number of adjustments that have been used, and try to giv e priority to those that have been most effective. Furthermore, laboratorie s and programs need to be cognizant that with newer pa ra meters bei ng add ed, the specifics of requirements will vary on a case-by-case parameter bas is, and optimal screening can only be achieved with the appropriate adjustm ents.