A method for measuring the concentration of at least two analytes in a biological liquid sample by means of a set of ion selective electrodes each of which is suitable for measuring one of those analytes, one of those analytes being sodium and another of those analytes being potassium, said method comprising:
- measuring a series of different biological samples of the same kind with said set of ion selective electrodes;
- obtaining from the ion selective electrodes first voltage values in millivolt which are representative of the concentration of sodium and potassium respectively in each of those biological samples;
- storing the first voltage values in a suitable form for data processing;
- measuring a calibration standard for sodium and potassium with the respective ion selective electrodes after measuring each of said biological samples;
- obtaining from the ion selective electrodes second voltage values in millivolt which are representative of the concentration of sodium and potassium respectively in each of those calibration standards;
- storing the second voltage values in a suitable form for data processing;
- and
- verifying whether each of said second voltage values has an abnormal value caused by a disturbance in the operation of one of the ion selective electrodes, said verifying comprises processing measured second voltages values obtained for successive measurement results corresponding to different samples (N and N−1), and said processing comprises:
- (i) calculating and storing the absolute value (ΔCSN=|CSN−CSN-1|) of the difference of the second voltage values measured by the electrode for sodium which correspond to a sample (N) and to the immediately preceding sample (N−1),
- (ii) calculating and storing the absolute value (ΔCPN=|CPN−CPN-1|) of the difference of the second voltage values measured by the electrode for potassium which correspond to a sample (N) and to the immediately preceding sample (N−1),
- (iii) verifying whether each of the calculated and stored absolute values of the changes calculated in steps (i) and (ii) is larger than a first predetermined threshold value, and if the result of this verification is positive, then said processing further comprises:
- (iv) verifying whether the calculated and stored absolute values obtained in steps (i) and (ii) differ from each other by an amount which is smaller than a second predetermined threshold value, and if the result of this verification is positive, then said processing further comprises:
- (v) verifying whether the calculated absolute values obtained in step (i) for a sample (N) and for the immediately preceding sample (N−1) differ from each other by an amount which is larger than a third predetermined threshold value, and if the result of this verification is positive,
- (vi) generating a signal indicating that the measurement results of the sample (N) are doubtful,
- then said method further comprises:
- marking 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.