Interference in the Vitros CREA method when measuring urine creatinine on samples acidified with acetic acid.

corresponding to 12.5 moles of KSCN, to 10 mL of ISE solution 1, which contains 115 mmol/L NaCl. Because of the high sensitivity of the chloride electrode for KSCN, this solution mimics a chloride concentration of 120 mmol/L. A loss of selectivity should lead to markedly increased values. After placement of a new electrode, results for the selectivity control solution gradually increased with a final peak at 2–6 weeks (Fig. 1). After replacement of the electrode, selectivity control solution results returned to the expected concentration range (Fig. 1). Results for control A showed this effect less well and could not demonstrate unambiguously deterioration of the electrode (Fig. 1). Control B varied statistically around the target value (Fig. 1) and therefore was not a suitable indicator. The time dependence of the deterioration was consistent with an “aging” of the electrode, which could be the reason for the emerging loss of selectivity. One possible explanation for this effect could be the coating of the membrane with protein, particularly fibrinogen. Daily preventive maintenance performed according to the manufacturer’s instructions did not improve our results sufficiently. After such a procedure the selectivity of a worsening electrode was reconstituted for some hours only. We believe that this short-lived reconstitution was attributable to partial peeling of the electrode surface. After the maintenance procedure, falsely increased results for patient samples were detected more frequently than in the first hours after reconstitution. Repeat analyses with the coulometric reference method revealed significantly lower chloride concentrations [mean (SD) difference, 3 (3.7) mmol/L] in most cases. In 3% of those patient samples, the differences were 10 mmol/L. Replacing the chloride electrode whenever the result for selectivity control solution exceeded its 3 SD limit reduced the rate of falsely high (difference for ISE minus coulometry 10 mmol/L) results to 0.3%. The mean difference between ISE and coulometry has been halved [mean (SD) difference, 1.7 (1.75) mmol/L]. Thus the introduction of this selectivity control solution offers a novel and inexpensive way to achieve a maximum useful lifetime of the chloride electrode.