The present invention relates to a detector, a patient interface and a patient interface system comprising this detector, and a method for preventing the formation of red marks due to the wearing of a patient interface (12) that is pressing against the skin of the patient (18) for a longer time. With the detector and via the method, the blood flow and/or the occlusion of the blood vessels of the patient (18) are monitored, and the assessed degrees are used as a basis to adjust the patient interface (12) in order to reduce the force pressing against the skin of the patient (18), such that the formation of red marks is prevented.