DNA damage index measured by comet assay associated with occupational exposure to cancer chemotherapy agents: mean number of cells per DNA damage category. Undeger et al. (1999) evaluated 200 cells per subject, classified them into 4 categories (<5% (no damage), 5-20% (low), [>]20-40% (medium), and >40% (highly damaged)), and presented the data per each category as the number of cells out of 200 cells scored or the % of cells scored.