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.