Automated detection of rare fetal cells in maternal blood: eliminating the false-positive XY signals in XX pregnancies

Abstract

Objective The purpose of this study was to develop a new method to help differentiate XX from XY signals in maternal blood from women carrying XY fetuses. Study design We have developed a system to scan automatically for cells that bear X and Y fluorescence in situ hybridization signals. These XY target cells are identified by scans at low (×20) magnification, and all identified targets are revisited and verified at high (×100) magnification. The viewer software component of the system displays ×20 images of all cells and intracellular fluorescence in situ hybridization signals that are present in each of the 4000 optical fields per slide, along with ×100 images of automatically detected target cells. Results We initially examined 36,000 fields from 18 slides in 12 pregnancies (6 male and 6 female) using our system that is based on fluorescence in situ hybridization with a single probe for the X-chromosome and a single probe for the Y-chromosome and found XY nuclei in all samples, regardless of fetal gender. In the second phase of the study, a refinement of the approach that incorporated 2 independent probes for the Y-chromosome resulted in a false-positive rate for detection of XY nuclei in XX cases <0.00005%. Conclusion Our data suggest that this system may allow for excellent “signal to noise” separation, which is required absolutely for fetal cell methods to differentiate aneuploid from normal pregnancies. Quantitation of fetal cells in the maternal circulation and standardization of processes that have been developed for their enrichment are crucial to moving fetal cell assessment from esoteric basic science to applied new technology.