Suprathermal ions in fusion plasmas have different confinement properties in tokamak and reversed field pinch (RFP) configurations. These differences are explained in terms of guiding centre drifts by an analytical approach. Magnetic field perturbations and passing fast ions can satisfy resonance conditions which do not hold for thermal or low energy particles. Such a resonance is controlled by a generalization for passing fast ions of the safety factor parameter. Applications to experimental situations show good agreement in predicting the radial position of the resonances in the fast ion phase space. Numerical simulations in RFP devices are qualitatively consistent with the longer confinement times of fast ions, especially in helical regimes.