There is increasing use of circulating cell death biomarkers in patients and clinical trials. Knowledge of the potential noise and confounders in assays are vital for biomarker interpretation. The daily and diurnal variability and effect of menstruation and exercise on nucleosomal DNA (nDNA), total cytokeratin 18 (tK18) and apoptotic specific cytokeratin 18 (cK18) were assessed in 3 cohorts of healthy volunteers; 12 pre-menopausal women to establish the effect of menstruation, 12 men to perform exercise and 12 post-menopausal women. All 36 subjects were evaluated to establish daily and diurnal variability. Estimates of variability were derived in a linear mixed effects model and presented as the back transformed coefficient of variation (%CV). Minimal variation was seen in cK18 (11%CV) and tK18 (11%CV) but higher variability was seen in nDNA (85%CV). K18 results appeared stable throughout the day but a possible peak in nDNA was seen at 15:00. Menstruation had minimal effects but exercise led to immediate short-lived elevations in cell death biomarkers. There is no evidence of significant daily variability in K18 assays. We recommend subjects should not exercise for 6h before blood sampling.