A method for determining a body fluid glucose level of a patient from a continuous signal of a glucose sensor element of a continuous body fluid glucose monitoring device in a data processing unit, comprising receiving measurement data representing a continuous sensor signal provided by a glucose sensor element of a continuous body fluid glucose monitoring device in the data processing unit, receiving calibration data representing a time-dependent zero-signal level of the glucose sensor element in the data processing unit, determining a body fluid glucose level by processing at least the measurement data and the calibration data in the data processing unit, the processing comprising subtraction of the time dependent zero- signal level from the continuous sensor signal, and providing result data indicative of the continuous body fluid glucose level in the processing unit； and a system for determining a body fluid glucose level of a patient.
