A method for the machine-side detection of a recirculation of blood or a change of a recirculation during the operation of a blood treatment machine, wherein during the operation, successive measurement values are determined via a sensor at the drain of a dialyzer of the blood treatment machine, and wherein a threshold value is predetermined for a respective measurement value generated by the sensor, and wherein when the measurement value reaches or exceeds the threshold value, an action such as the generation of an optical and/or acoustic warning notice is initiated, wherein a current detector value determined by the sensor is converted into an absorbance value and a target course for the absorbance and also for the course of the detector signal is calculated and monitored in order to detect a sudden recirculation or a sudden change in recirculation due to a sudden change in the absorbance or the course of the detector signal; wherein at least the following procedure is used to calculate the target course of the determined absorbance and/or of the target course of the detector signal: the target course of the detector signal or of the absorbance is calculated by linearization of the course of the detector signal, in that the slope of the course of the detector signal is determined at the beginning of the machine operation or, if applicable, also after a change in blood flow, and this slope is used as a target value for the further course; and wherein the respective threshold values for the respective measurement values are calculated in the same manner as the target course, by adding a percentage or fixed value to the respective or a previous or an expected measurement value, and/or the threshold value is calculated as a percentage of the target value at each point in time, and/or the threshold value is calculated via an offset value (offset) relative to the target course at each time, and/or the course of the theoretical measurement values without recirculation is calculated via a prediction and for this purpose a maximum permissible upper limit is determined as threshold value for the deviation, and/or the course of the detector signals is calculated on the basis of the target course of the measurement values, and on this basis a maximum permissible deviation for a successive measurement value is calculated as a threshold value. wherein during the operation, successive measurement values are determined via a sensor at the drain of a dialyzer of the blood treatment machine, and wherein a threshold value is predetermined for a respective measurement value generated by the sensor, and wherein when the measurement value reaches or exceeds the threshold value, an action such as the generation of an optical and/or acoustic warning notice is initiated, wherein a current detector value determined by the sensor is converted into an absorbance value and a target course for the absorbance and also for the course of the detector signal is calculated and monitored in order to detect a sudden recirculation or a sudden change in recirculation due to a sudden change in the absorbance or the course of the detector signal; wherein at least the following procedure is used to calculate the target course of the determined absorbance and/or of the target course of the detector signal: the target course of the detector signal or of the absorbance is calculated by linearization of the course of the detector signal, in that the slope of the course of the detector signal is determined at the beginning of the machine operation or, if applicable, also after a change in blood flow, and this slope is used as a target value for the further course; and wherein the respective threshold values for the respective measurement values are calculated in the same manner as the target course, by adding a percentage or fixed value to the respective or a previous or an expected measurement value, and/or the threshold value is calculated as a percentage of the target value at each point in time, and/or the threshold value is calculated via an offset value (offset) relative to the target course at each time, and/or the course of the theoretical measurement values without recirculation is calculated via a prediction and for this purpose a maximum permissible upper limit is determined as threshold value for the deviation, and/or the course of the detector signals is calculated on the basis of the target course of the measurement values, and on this basis a maximum permissible deviation for a successive measurement value is calculated as a threshold value. adding a percentage or fixed value to the respective or a previous or an expected measurement value, and/or the threshold value is calculated as a percentage of the target value at each point in time, and/or the threshold value is calculated via an offset value (offset) relative to the target course at each time, and/or the course of the theoretical measurement values without recirculation is calculated via a prediction and for this purpose a maximum permissible upper limit is determined as threshold value for the deviation, and/or the course of the detector signals is calculated on the basis of the target course of the measurement values, and on this basis a maximum permissible deviation for a successive measurement value is calculated as a threshold value.