The longitudinal monitoring of oxidative stress (OS) in athletes may enable the identification of fatigued states and underperformance. The application of OS biomarker monitoring programs in sport are hindered by reliability and repeatability of in-the-field testing tools, the turnaround of results, and the understanding of biological variation (BV). Knowledge of BV and critical difference values (CDV) may assist with data interpretation in the individual athlete. Methods: We aimed firstly to assess the repeatability of the clinical point of care redox test, Free Oxygen Radical Test (FORT) and the Free Oxygen Radical Defence (FORD) in trained participants and elite athletes and secondly to calculate the analytical, BV, CDV and index of individuality (II) for FORT, FORD, red blood cell glutathione, lutein, α and γ–tocopherol. Part 1: Fifteen elite athletes were sampled in duplicate for calculation of the repeatability of the FORT and FORD tests. Part 2: Twelve well-trained athletes had venous samples drawn every 2 hours from 0800 to 1800 for calculation of BV, CDV, II for FORT, FORD, RBC GSH, lutein, α-tocopherol and γ–tocopherol. Results: Repeatability of the FORT and FORD assay was 3.9% and 3.7% respectively. Biomarker CDV ranged from 12.8% to 37%, with a circadian effect for FORT, α-tocopherol and γ-tocopherol (p<0.01), with all biomarker indices of individuality < 0.8 arbitrary units. Conclusion: We report that the use of the novel redox test in athletes is practical, and the generation of BV and CDV for biomarkers of OS enhances the interpretation of physiologically meaningful changes in individuals above the use of clinical reference ranges alone.