A computer-implemented method for determining a measurable parameter indicating the function of an organ in a subject, comprising the steps of: a) providing a first concentration-time curve obtained by transcutaneously measuring in a body fluid at a first position (i) background fluorescence in at least one first time point and (ii) fluorescence of an indicator compound in at least a second, a third, a fourth, a fifth, and a sixth time point, b) providing a second concentration-time curve obtained by transcutaneously measuring in a body fluid at a second position (i) background fluorescence in at least one seventh time point and (ii) fluorescence of an indicator compound in at least a eighth, a ninth, a tenth, an eleventh, and a twelfth time point, c) fitting the first and the second concentration-time curve into a kinetic model representing at least four diffusion compartments, and d) thereby determining an organ function in a subject, wherein the diffusion compartments represent an application compartment, plasma, interstitium, and a local interstitium (I,L), assigning parameters being either global or local, wherein the first position is different from the second position, wherein measuring fluorescence of an indicator compound at a first and a second position is performed simultaneously, and wherein the concentration of said indicator compound at time t (c M (t)) in said kinetic model representing four diffusion compartments is described by equation (0.57): c M t = D * ⋅ [ λ 2 − λ 3 ⋅ λ 1 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 1 ⋅ t − λ 1 − λ 3 ⋅ λ 2 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 2 ⋅ t − λ 2 − λ 1 ⋅ λ 3 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 3 ⋅ t] + E * ⋅ λ 2 − λ 3 ⋅ λ 1 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 1 + R 3, L V I, L ⋅ e λ 1 ⋅ t − e − R 3, L V I, L ⋅ t − λ 1 − λ 3 ⋅ λ 2 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 2 + R 3, L V I, L ⋅ e λ 2 ⋅ t − e − R 3, L V I, L ⋅ t − λ 2 − λ 1 ⋅ λ 3 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 3 + R 3, L V I, L ⋅ e λ 3 ⋅ t − e − R 3, L V I, L ⋅ t with λ 1...3 and V p /R 1 being global parameters, and R 3,L /V I,L, D*, and E* being local parameters, and R1 being the indicator of the function of an organ. a) providing a first concentration-time curve obtained by transcutaneously measuring in a body fluid at a first position (i) background fluorescence in at least one first time point and (ii) fluorescence of an indicator compound in at least a second, a third, a fourth, a fifth, and a sixth time point, b) providing a second concentration-time curve obtained by transcutaneously measuring in a body fluid at a second position (i) background fluorescence in at least one seventh time point and (ii) fluorescence of an indicator compound in at least a eighth, a ninth, a tenth, an eleventh, and a twelfth time point, c) fitting the first and the second concentration-time curve into a kinetic model representing at least four diffusion compartments, and d) thereby determining an organ function in a subject, wherein the diffusion compartments represent an application compartment, plasma, interstitium, and a local interstitium (I,L), assigning parameters being either global or local, wherein the first position is different from the second position, wherein measuring fluorescence of an indicator compound at a first and a second position is performed simultaneously, and wherein the concentration of said indicator compound at time t (c M (t)) in said kinetic model representing four diffusion compartments is described by equation (0.57): c M t = D * ⋅ [ λ 2 − λ 3 ⋅ λ 1 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 1 ⋅ t − λ 1 − λ 3 ⋅ λ 2 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 2 ⋅ t − λ 2 − λ 1 ⋅ λ 3 + V P R 1 ⋅ λ 1 ⋅ λ 2 ⋅ e λ 3 ⋅ t] + E * ⋅ λ 2 − λ 3 ⋅ λ 1 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 1 + R 3, L V I, L ⋅ e λ 1 ⋅ t − e − R 3, L V I, L ⋅ t − λ 1 − λ 3 ⋅ λ 2 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 2 + R 3, L V I, L ⋅ e λ 2 ⋅ t − e − R 3, L V I, L ⋅ t − λ 2 − λ 1 ⋅ λ 3 + V P R 1 ⋅ λ 1 ⋅ λ 2 λ 3 + R 3, L V I, L ⋅ e λ 3 ⋅ t − e − R 3, L V I, L ⋅ t with λ 1...3 and V p /R 1 being global parameters, and R 3,L /V I,L, D*, and E* being local parameters, and R1 being the indicator of the function of an organ.