The comet assay as a tool in human biomonitoring studies: Effects of confounding factors

The comet assay is widely used in human biomonitoring studies of environmental and occupational exposures. However, it is clear from multiple studies that various types of confounding factors might affect the direct relationship between exposure and DNA damage in the comet assay. In addition to common confounders such as age, sex, and smoking, other factors considered to be important determinants for background levels of DNA damage in the comet assay include exhaustive physical exercise, chronic diseases, medical treatment, and diet. These are typically controlled in biomonitoring studies by restriction or matching of subjects. Period effects (or seasonal variation) have been observed in a relatively large number of studies. There are various putative factors, which may cause period effects, including temporal variation in solar radiation, temperature, and air pollution. The present review describes the effects of these confounding factors in measurements of DNA strand breaks by the comet assay. In general, the literature does not indicate that any confounding factor is consistently associated with an increased level of DNA damage, measured by the comet assay. In this respect, it is important to balance the need to control for confounding with the risk of introducing in the statistical analysis a variable, which is influenced by exposure and outcome (i.e. collider bias). In addition, it is important that investigators describe procedures for controlling the effect of confounding factors in the selection of subjects and statistical analysis. Care should be taken in study design and statistical analysis to avoid unwanted effects of confounding factors.