For detection of apoptosis with Annexin/PI, you cannot fix the cells as the assay only works if the cells are alive. Fixing the cells would compromise the membrane integrity, thus not allowing to discriminate early apoptosis from the other stages.

You could instead use sub-G1 analysis (which requires to fix the cells and usese PI staining) or other alternative apoptosis detection which allows fixation ( ie. TUNEL assay).

I agree with above answer. AnnexinV/PI to monitor apoptosis requires live cells. Other methods of measuring cell death can be used with fixed/dead cells (ICS for caspases or cyt c, immunoblot for apoptotic proteins, TUNEL, etc) as well as fixable viability dyes that maintain the distinction between viable vs. dead cells in your sample after fixation.

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