EXCLUSION OF CHROMOSOMAL MOSAICISM IN AMNIOTIC-FLUID CULTURES - DETERMINATION OF NUMBER OF COLONIES NEEDED FOR ACCURATE ANALYSIS

Most laboratories use the in situ or flask culture method and a two-st age approach to mosaicism detection. Determination of the optimum numb er of metaphases to be counted depends largely on whether or not the c olonies that grow from the cells in the amniotic fluid can be consider ed independent. Previous statistical analysis of data from mixed male and female amniocyte cultures indicated that for mosaicism detection t hese colonies can be considered independent (Cheung et al., 1990). Thi s analysis was repeated with a set of mosaic cases from two independen t prenatal diagnosis programmes. The same degree of colony independenc e was found with this set of data. This result was used to calculate t he level of mosaicism that is excluded at a particular confidence leve l when set numbers of colonies are analysed at each stage. The tables generated apply directly to the in situ method and with modification t hey can be used with the flask method. The conclusions are (1) analysi s of cells from multiple colonies enhances the likelihood of excluding true mosaicism; (2) analysis of more than one metaphase per colony of fers little advantage in excluding mosaicism; and (3) the two-stage ap proach is the most efficient. These conclusions should be used togethe r with the expected clinical outcome of the actual cytogenetic abnorma lity, as discussed by Hsu et al. (1992).