Carina Ladeira, Stefano Bonassi, Cristian Del Bo, Peter Møller, Amaya Azqueta, Monica Neri, Gudrun Koppen, Elisa Boutet-Robinet, Goran Gajski, Andrew Collins, Patrizia Riso, Nurşen Başaran, Susan J. Duthie, Lisa Giovannelli, Universidad de Navarra [Pamplona] (UNAV), IdiSNA, Navarra Institute for Health Research, Escola Superior de Tecnologia da Saude de Lisboa, Escola Nacional de Saúde Pública (ENSP-NOVA), Universidade Nova de Lisboa = NOVA University Lisbon (NOVA), Università degli Studi di Firenze = University of Florence [Firenze] (UNIFI), Contaminants & Stress Cellulaire (ToxAlim-COMICS), ToxAlim (ToxAlim), Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Ecole Nationale Vétérinaire de Toulouse (ENVT), Institut National Polytechnique (Toulouse) (Toulouse INP), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut National Polytechnique (Toulouse) (Toulouse INP), Université Fédérale Toulouse Midi-Pyrénées-Ecole d'Ingénieurs de Purpan (INPT - EI Purpan), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), Istituto di Ricovero e Cura a Carattere Scientifico (IRCCS San Raffaele Pisana), Universita Vita Salute San Raffaele = Vita-Salute San Raffaele University [Milan, Italie] (UniSR), Institute for Medical Research and Occupational Health, Robert Gordon University (RGU), Università degli Studi di Milano [Milano] (UNIMI), Flemish Institute for Technological Research (VITO), Hacettepe University = Hacettepe Üniversitesi, University of Oslo (UiO), University of Copenhagen = Københavns Universitet (KU), and The authors are grateful to the COST Action CA15132, 'hCOMET', for support. Amaya Azqueta thanks the Ministry of Economy, Industry and Competitiveness ('Ramon y Cajal' programme, RYC-2013-14370) of the Spanish Government for personal support. The work of Stefano Bonassi and Monica Neri was supported by grants funded by the Italian Ministry of Health [Institutional Research 2017-2018], and by AIRC (Associazione Italiana per la Ricerca sul Cancro), IG2015/17564.
The comet assay is a well-accepted biomonitoring tool to examine the effect of dietary, lifestyle, environmental and occupational exposure on levels of DNA damage in human cells. With such a wide range of determinants for DNA damage levels, it becomes challenging to deal with confounding and certain factors are inter-related (e.g. poor nutritional intake may correlate with smoking status). This review describes the effect of intrinsic (i.e. sex, age, tobacco smoking, occupational exposure and obesity) and extrinsic (season, environmental exposures, diet, physical activity and alcohol consumption) factors on the level of DNA damage measured by the standard or enzyme-modified comet assay. Although each factor influences at least one comet assay endpoint, the collective evidence does not indicate single factors have a large impact. Thus, controlling for confounding may be necessary in a biomonitoring study, but none of the factors is strong enough to be regarded a priori as a confounder. Controlling for confounding in the comet assay requires a case-by-case approach. Inter-laboratory variation in levels of DNA damage and to some extent also reproducibility in biomonitoring studies are issues that have haunted the users of the comet assay for years. Procedures to collect specimens, and their storage, are not standardized. Likewise, statistical issues related to both sample-size calculation (before sampling of specimens) and statistical analysis of the results vary between studies. This review gives guidance to statistical analysis of the typically complex exposure, co-variate, and effect relationships in human biomonitoring studies.