1. A computer program product for identifying candidate target cells within a biological fluid specimen, the computer program product tangibly embodied in a non-transitory computer readable medium, comprising instructions to cause a processor to receive a digital image of the biological fluid specimen, the digital image having a plurality of color channels, identify first connected regions of pixels of a minimum first intensity in a first channel of the plurality of color channels, identify second connected regions of pixels of a minimum second intensity in a second channel of the plurality of color channels, determine first connected regions and second connected regions that spatially overlap, determine an aspect ratio of the spatially overlapping first connected regions and second connected regions based on a color channel of the plurality of color channels, identify spatially overlapping first connected regions and second connected regions for which the aspect ratio meets an aspect ratio threshold; and provide a portion of the image corresponding to at least some of the identified spatially overlapping first connected region and second connected region to a classifier as a candidate for classification.