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O Direct ISE O No dilution of sample prior to analysis O Fixed electrodes with semi-permeable membranes requiring more frequent replacement (except VITROS slides with disposable electrodes) O Direct methods measure only the free ion giving a more physiologically correct result. O Indirect ISE O Pre-dilution of sample prior to analysis O Artifacts or erroneous results in samples containing high lipids and proteins O In Direct method measure not only the free but also the bound & complexed ions. Methods of Electrolyte Measurement