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.