Strategies for rare-event detection: An approach for automated fetal cell detection in maternal blood

This article explores the feasibility of the use of automated microscopy an d image analysis to detect the presence of rare fetal nucleated red blood c ells (NRBCs) circulating in maternal blood. The rationales for enrichment a nd for automated image analysis for "'rare-event" detection are reviewed. W e also describe the application of automated image analysis to 42 maternal blood samples, using a protocol consisting of one-step enrichment followed by immunocytochemical staining for fetal hemoglobin (HbF) and FISH for X- a nd Y-chromosomal sequences. Automated image analysis consisted of multimode microscopy and subsequent visual evaluation of image memories containing t he selected objects. The FISH results were compared with the results of con ventional karyotyping of the chorionic villi. By use of manual screening, 4 3% of the slides were found to be positive (greater than or equal to 1 NRBC ), with a mean number of 11 NRBCs (range 1-40). By automated microscopy, 52 % were positive, with on average 17 NRBCs (range 1-111). There was a good c orrelation between both manual and automated screening, but the NRBC yield from automated image analysis was found to be superior to that from manual screening (P = .0443), particularly when the NRBC count was >15. Seven (64% ) of 11 XY fetuses were correctly diagnosed by FISH analysis of automatical ly detected cells, and all discrepancies were restricted to the lower cell- count range. We believe that automated microscopy and image analysis reduce the screening workload, are more sensitive than manual evaluation, and can be used to detect rare HbF-containing NRBCs in maternal blood.