The present invention discloses compositions and methods of fixing and staining rare cells. Farther, disclosed herein are methods of identifying circulating tumor cells (CTC). In some embodiments, themethod includes: imaging a cell sample to identity a cell of interest; determining a first pixel intensity of a stained nuclear area; determining a second pixel intensity of a background area; calculating a ploidy status of the cell of interest by subtracting the second pixel intensity from the first pixel intensify; and determining whether the cell of interest is a CTC based on the ploidy status. The method may be computer implemented, such, that the method uses a machine learning algorithm to identify a feature; process the feature to extract a parameter of interest; analyze the parameter of interest; and when the parameter of interest is greater than or less than a predetermined threshold, classify the cell of interest as a CTC.