Abstract: A potentiometric method for determining low concentration of an analyte of interest present in a fluid sample in a concentration optionally below its linear range of detection in a presence of at least one interfering substance of known concentration the method is effected by following the steps of (a) adding to the sample a predetermined amount of the analyte of interest, such that a total concentration of the analyte of interest present in the sample is within the linear range of its detection; (b) providing an analyte selective electrode being selective to the analyte of interest and a reference electrode both electrodes being equilibrated with a solution including the at least one interfering substance in concentration similar to said known concentration; (c) contacting the analyte selective electrode and the reference electrode with the sample; (d) measuring a signal representative of a potential drop between the analyte selective electrode and the reference electrode; and (e) using the signal for determ