DNA damage index measured by comet assay associated with occupational exposure to cancer chemotherapy agents: mean comet score. In addition, Mader et al. (2009) reported that a trend analysis for the entire observation period revealed a significantly greater (p=0.02) genotoxicity in the alkaline comet assay compared to unexposed subjects; no means or standard deviations were provided. The mean comet scores were calculated by binning the cells per category of DNA damage and multiplying the number of cells per category by a number per the level of DNA damage, and summing up the numbers across the categories. Specifically, Izdez et al. (2009) evaluated 50 cells per subject, classified them into 3 categories, and presented the data as a total score based on the following calculation: 0 x number of cells with no tail migration + 1 x number of cells with low tail migration + 2 x number of cells with high tail migration. Two studies of nurses and physicians in Brazil evaluated 50 cells (Maluf et al., 2000) or 100 cells per subjects (Rombaldi et al., 2009), classified them into 5 categories, and calculated a DNA damage index defined as: 0 x number of cells with no damage (<5% tail migration) + 1 x number of cells with 5-20% tail migration, 2 x number of cells with [>]20-40% tail migration, 3 x number of cells with [>]40-95% tail migration, and 4 x number of cells with >95% tail migration. A longitudinal cohort study of 15 nurses from Austria reported on a DNA damage index of 1,000 cells per subject that were classified into 5 categories, and presented as a DNA damage index (which they called a tail factor), but calculated the DNA damage index (which they called a tail factor) by the sum of the number of cells with: >5% tail migration multiplied by 2.5, 5-20% tail migration multiplied by 12.5, >20-40% tail migration multiplied by 30, >40-95% tail migration multiplied by 67.5 and >95% tail migration multiplied by 97.5) divided by 1,000 cells scored (Mader et al., 2009).