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 not as simple as measuring hemoglobin or hematocrit. Early in the game, it was recognized that there are gestational age curves, and that since alpha-fetoprotein (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 give priority to those that have been most effective. Furthermore, laboratories and programs need to be cognizant that with newer parameters being added, the specifics of requirements will vary on a case-by-case parameter basis, and optimal screening can only be achieved with the appropriate adjustments.