A method for measuring the concentration of at least two analytes in a biological liquid sample by using a set of ion selective electrodes is disclosed. The method obtains from the ion selective electrodes first voltage values representative of the concentration of sodium and potassium respectively in those biological samples, and obtains from the ion selective electrodes second voltage values representative of the concentration of sodium and potassium respectively in calibration standards. The method verifies by a predetermined procedure whether each of the second voltage values obtained by measuring the calibration standards has an abnormal value caused by a disturbance of the measurement conditions in one of the ion selective electrodes, and if this is the case, marks with a flag as doubtful the measurement results derived from the first voltage values obtained for the corresponding sample which was measured before measuring the calibration standards for sodium and potassium.