Assessment of reference values for DNA damage detected by the comet assay in human blood cell DNA.

Genotoxicity measured by the comet assay is expressed by different researchers using parameters that are not easy to conceptualize, except for percent tail DNA (%T) or visual score (arbitrary units). A total of 125 publications have reported genotoxicity as DNA damage (representing strand breaks, alkaline labile sites, and transient repair sites), endonuclease III (ENDOIII), or formamidopyrimidine DNA glycosylase (FPG) sensitive sites. I have recalculated the visual score so that it is expressed in the range of 0-100, similar to that of %T. Similar values were obtained for DNA damage and ENDOIII sites, regardless of whether of the data were reported as %T or visual score. Thus, these endpoints can be used interchangeably, assuming that the visual score is expressed in the 0-100 range. Pooled analysis of %T and visual score data showed that the median (25-75%) values of DNA damage, ENDOIII, and FPG sites were 8.6 (4.4-14.5), 11.0 (4.2-19.5), 7.6 (3.2-14.2), respectively. The duration of alkaline treatment and electrophoresis had no significant effect on the level of DNA damage. There was a positive correlation between age and the level of DNA damage. A sub-analysis of DNA damage obtained from European countries showed a negative correlation with latitude. In conclusion, reference values for DNA lesions measured by the comet assay are around 7-11 %T or arbitrary units.