In this paper, we focus on the ability of Bluetooth to support Quality of Service (QoS) requirements expressed by QoS sensitive applications. We focus on the quantitative QoS guarantee that can be granted to an uplink or downlink flow in terms of end-to-end delay. Two approaches are presented. The deterministic one is based on a worst case analysis and leads to a deterministic bound which is unfrequently reached. The probabilistic approach, based on the probability density function of the response time, is then introduced to evaluate the probability of missing a given deadline. This study shows that delays much smaller than the deterministic bound can be guaranteed with probabilities close to one. An admission control derived from these results is then proposed, providing a probabilistic QoS guarantee to uplink or downlink flows in a Bluetooth piconet.