Probability tables for exclusion of mosaicism in prenatal diagnosis.

The decision concerning the number of metaphases that need to be analysed to detect mosaicism of a certain degree depends mainly, for the same confidence levels, on the culture method used (in situ or flask methods). Several probability tables, designed for either the in situ or the flask method, have been reported and can be used to assist laboratories in making the decisions referred to above. However, there are instances where part of the analysis is done using the in situ and flask methods. In such situations, the previously published tables are of limited use. We have generated a new table that can be used in such situations, as well as in cases where only the flask method is used.