Apparatus for monitoring a trans-membrane ionic current of a biological cell comprising: - a passage through which electromagnetic radiation passes to interact with said cell; the apparatus being characterised in that it further comprises: - electromagnetic radiation phase measuring means for receiving said electromagnetic radiation having passed through the cell and for providing temporal measurements of a phase of said electromagnetic radiation, and calculating means configured to determine the ionic current using the following formula: I t = V 0 ε d dt φ 0 φ t 1 / s − r wherein V 0 is an initial cell volume before ionic current; ε is a constant representing the volume variation associated with the net charge movement through the cell membrane expressed in millilitres per Coulomb; s is a parameter to take into account cell processes inducing volume variations leading to refractive index charges by mechanisms other than dilution or concentration of the intracellular content by ions and water fluxes, s being one of the two values 0 and 1, and s = 1 corresponding to a simple dilution or concentration of the cell content; φ ( t ) is the phase signal at the time t ; and r is a parameter related to the cell deformation along a direction of the probing beam, associated with the volume variation. - a passage through which electromagnetic radiation passes to interact with said cell; - electromagnetic radiation phase measuring means for receiving said electromagnetic radiation having passed through the cell and for providing temporal measurements of a phase of said electromagnetic radiation, and calculating means configured to determine the ionic current using the following formula: I t = V 0 ε d dt φ 0 φ t 1 / s − r wherein V 0 is an initial cell volume before ionic current; ε is a constant representing the volume variation associated with the net charge movement through the cell membrane expressed in millilitres per Coulomb; s is a parameter to take into account cell processes inducing volume variations leading to refractive index charges by mechanisms other than dilution or concentration of the intracellular content by ions and water fluxes, s being one of the two values 0 and 1, and s = 1 corresponding to a simple dilution or concentration of the cell content; φ ( t ) is the phase signal at the time t ; and r is a parameter related to the cell deformation along a direction of the probing beam, associated with the volume variation. V 0 is an initial cell volume before ionic current; ε is a constant representing the volume variation associated with the net charge movement through the cell membrane expressed in millilitres per Coulomb; s is a parameter to take into account cell processes inducing volume variations leading to refractive index charges by mechanisms other than dilution or concentration of the intracellular content by ions and water fluxes, s being one of the two values 0 and 1, and s = 1 corresponding to a simple dilution or concentration of the cell content; φ ( t ) is the phase signal at the time t ; and r is a parameter related to the cell deformation along a direction of the probing beam, associated with the volume variation. <EOS>