The between-day reliability of fasted circulating irisin concentrations: a cohort study

Irisin blood concentrations are investigated in many applied physiology studies, however between-day reliability is required for accurate interpretation, particularly in longitudinal interventions. Between-day reliability of irisin was calculated, and the influence of participant characteristics explored. Venous blood was sampled 24 h apart in 32 rested and fasted healthy adults (males n = 15, 19–80 years). Plasma irisin concentrations were measured using MILLIPLEX® xMAP immunoassay analysis (Merck KGaA, Darmstadt, Germany). Irisin reliability was analysed using coefficient of variation (CV) and intraclass correlation coefficients (ICCs). Regression analysis was used to explore the influence of participant characteristics. Overall, irisin was reliable between days (CV 12.7%, ICC r = 0.963, p < 0.001), with no influence of participant characteristics, when pre-sampling controls (intake, activity) were applied.