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. In the context of elite football, the utilisation of a point-of-care (POC) test for measuring inflammation via capillary sampling, facilitates frequent data acquisition in order to assess recovery status within game-to-game micro-cycles. The smaller within-subject variability compared to the variability between subjects limits the practical use of population-based reference ranges. Individualised approaches to biomarker monitoring may therefore be of higher potential value in assessing player health and recovery. 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 individualised monitoring with personalised adaptive reference ranges is more effective in identifying true abnormalities compared to PBRR (87.8% vs 86.2% for pre-match CRPs and 85.2% vs 81.5% for post-match CRPs), 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.