BACKGROUND: The neutral comet assay was devised to measure double-stranded DNA 
breaks, but it has also been used to measure apoptosis based on its 
characteristic DNA fragmentation patterns. There is still uncertainty about the 
reliability of this method. By comparing the comet assay with a flow cytometry 
method that uses Annexin V binding to apoptotic cells, we have provided further 
evidence for evaluating the usefulness of the comet assay for detecting 
apoptosis.
METHODS: Apoptosis was induced in human peripheral blood mononuclear cells 
(PBMC) by ionizing radiation and measured using the comet assay and a flow 
cytometry method that measures Annexin V and propidium iodide (PI) staining.
RESULTS: The Annexin V flow cytometry assay distinguished among early apoptosis, 
late apoptosis, and an apoptotic or necrotic phase in which the cells were 
labeled with both Annexin V and PI. The comet assay detected only the latter two 
phases of apoptosis.
CONCLUSIONS: The comet assay is a useful tool for measuring the late stages of 
apoptosis whereas the Annexin V assay measures higher amounts of apoptosis 
because it can detect cells in an earlier stage of the apoptotic pathway.