Evaluating current policy for detecting mosaicism in amniotic fluid cultures: implications for current cell counting practices.

Chromosomal mosaicism is one of the most vexing problems for clinical cytogenetic laboratories and personnel time used for analysis at the microscope is one of the principle costs in cytogenetic laboratories. We use data collected from 26 cytogenetic laboratories to evaluate whether the American College of Medical Genetics guidelines for minimum number of cells to count to exclude mosaicism in amniotic fluid specimens is appropriate. An accurate estimate of the number of mosaics that are missed by current cell counting practices is an important step in this process.