Diurnal variation of 8-hydroxy-2'-deoxyguanosine in continuous time series of two breast cancer survivors

8-hydroxy-2'deoxyguanosine (8-OHdG) is an oxidative product removed from DNA following radical oxygen species-induced damage. Being water-soluble, it can be measured non-invasively in the urine and has thus been established as a marker for 'whole-body' oxidative stress. Its validity and reliability as an oxidative stress marker in various chronic diseases and early carcinogenesis screening in clinical diagnosis and research are widely debated. To determine optimal measurement timing and duration, it is essential to establish the circadian profile of 8-OHdG under everyday life conditions and use reliable sampling methods. Here, we show the presence of day-night differences for 8-OHdG normalized by creatinine or urine volume in continuous time series of two breast cancer survivors who participated in integrative single-case studies and sampled their urine in 12-h-pooled collections over one month. These findings support the importance of appropriately considering the dynamic characteristics of stress indicators to reduce the risk of inconsistent or false results in clinical diagnostics. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any external funding. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee of Leibniz University Hannover gave ethical approval for this work. Ethic committee of University of Freiburg gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.