Method for the determination of a flow rate of a substitution solution, for which a clearance for medium-molecular substances of a tangential flow filter of a blood treatment unit is achieved, comprising the following steps: a) Selection of the predetermined range in the form of an operating mode selected from Optimum, Efficiency or Economy; b) Measurement of a signal, which is generated by at least one medium-molecular substance in the dialysate flow of the blood treatment unit; c) Variation of the flow rate of the substitution solution during the measurement according to b); d) Transmission of the flow rates of the substitution solution and the corresponding measuring values of the signal of the at least one medium-molecular substance in the dialysate flow to a central processing unit; e) Correlation of the flow rates of the substitution solution with the signal of the at least one medium-molecular substance, wherein the correlation includes: Determination of the clearance for the at least one medium-molecular substance before change of the substituate flow; Determination of the clearance for the at least one medium-molecular substance after change of the substituate flow; Determination of the change of the substituate flow in ml/min (ΔS); Calculation of the change of the clearance for the at least one-medium-molecular substance in dependence of the change of the substituate flow (ΔS) in accordance with: ΔK 0.1 = K 1 − K 0 P = ΔK 0.1 / ΔS * 100 % wherein it is determined in the operating mode Optimum, if an increase of the substituate flow still induces at least a rise of 10% < P < 40%, wherein it is determined in the operating mode Efficiency, if an increase of the substituate flow still induces at least a rise of 40 ≤ P < 70%, wherein it is determined in the operating mode Economy, if an increase of the substituate flow still induces at least a rise of P ≥ 70%, f) Output of the values determined in e) according to the range determined for the operating mode. a) Selection of the predetermined range in the form of an operating mode selected from Optimum, Efficiency or Economy; b) Measurement of a signal, which is generated by at least one medium-molecular substance in the dialysate flow of the blood treatment unit; c) Variation of the flow rate of the substitution solution during the measurement according to b); d) Transmission of the flow rates of the substitution solution and the corresponding measuring values of the signal of the at least one medium-molecular substance in the dialysate flow to a central processing unit; e) Correlation of the flow rates of the substitution solution with the signal of the at least one medium-molecular substance, wherein the correlation includes: Determination of the clearance for the at least one medium-molecular substance before change of the substituate flow; Determination of the clearance for the at least one medium-molecular substance after change of the substituate flow; Determination of the change of the substituate flow in ml/min (ΔS); Calculation of the change of the clearance for the at least one-medium-molecular substance in dependence of the change of the substituate flow (ΔS) in accordance with: ΔK 0.1 = K 1 − K 0 P = ΔK 0.1 / ΔS * 100 % wherein it is determined in the operating mode Optimum, if an increase of the substituate flow still induces at least a rise of 10% < P < 40%, wherein it is determined in the operating mode Efficiency, if an increase of the substituate flow still induces at least a rise of 40 ≤ P < 70%, wherein it is determined in the operating mode Economy, if an increase of the substituate flow still induces at least a rise of P ≥ 70%, Determination of the clearance for the at least one medium-molecular substance before change of the substituate flow; Determination of the clearance for the at least one medium-molecular substance after change of the substituate flow; Determination of the change of the substituate flow in ml/min (ΔS); Calculation of the change of the clearance for the at least one-medium-molecular substance in dependence of the change of the substituate flow (ΔS) in accordance with: ΔK 0.1 = K 1 − K 0 P = ΔK 0.1 / ΔS * 100 % wherein it is determined in the operating mode Optimum, if an increase of the substituate flow still induces at least a rise of 10% < P < 40%, wherein it is determined in the operating mode Efficiency, if an increase of the substituate flow still induces at least a rise of 40 ≤ P < 70%, wherein it is determined in the operating mode Economy, if an increase of the substituate flow still induces at least a rise of P ≥ 70%, f) Output of the values determined in e) according to the range determined for the operating mode.