Early warning system for player recovery? A series of case studies illustrating the application of individualised adaptive reference ranges in the longitudinal blood monitoring of English Premier League soccer players

Blood biomarkers can provide objective insight into a player's physiological state of recovery. Individualised approaches to biomarker monitoring may be of higher potential value in assessing player health and recovery compared to population-based reference ranges. We aimed to explore the application of individualised adaptive reference ranges (IARR) in English Premier League (EPL) soccer players using a POC biomarker for C-Reactive Protein (CRP) as a marker of inflammation. Using historical data collected from players' CRP values during the 2019-2020 season, we evaluated the effectiveness of static and IARR in identifying abnormal values and reported sensitivity and specificity at a 5% significance level. Our analysis confirmed that monitoring with IARR is more effective in identifying true abnormalities compared to population-based static reference ranges, particularly when the intra-individual variability is considerably lower than inter-individual variability. The application of IARR for blood monitoring data could assist the practitioner in identifying periods where a player may require performance (e.g. workload management and recovery practices) or medical support from the multi-disciplinary team. However, IARR serve more as an early warning system than a diagnostic tool. Thus, significant care is needed to prevent misuse and misinterpretation when implementing this strategy in real-world settings.