Abstract. Elevated serum sodium and potassium levels were recently observed when sampled through a heparinbonded umbilical catheter and measured with certain ionselective electrodes. The cationic surfactant, benzalkonium chloride (BZC), is known to falsely elevate those cations in serum. Inasmuch as most heparin-bonded umbilical catheters use BZC during the bonding process, an in vitro study was performed to estimate the quantity of BZC released and the duration of sodium and potassium elevations during pooled sera infusion. Three heparinbonded umbilical catheters and 3 silastic umbilical catheters were first flushed with 0.3 mL of normal saline and then perfused with pooled sera (sodium, 142 mEq/L; potassium, 4.6 mEq/L) at 2.5 micro L/h. Effluent samples were collected from 0 to 8 hours and analyzed by ionselective electrodes. Elevated serum sodium concentrations from 160 to is greater than or equal to 250 mEq/L and potassium concentrations from 6.0 to is greater than or equal to 9.6 mEq/L were observed. The BZC concentration in the catheter effluent was measured by mass spectrometry, with peak values of 10 micro g/mL detected by this method. When varying concentrations of BZC (1 to 100 micro g/mL) were added directly to pooled serum, a dose-dependent increase in serum sodium was observed. These data demonstrate that BZC released from heparin-bonded umbilical catheters elevates serum electrolyte values measured with some ion-selective electrodes. The observed increase in sodium and potassium concentrations may lead to clinical errors in management. Benzalkonium chloride is associated with myriad of clinical symptomatology. Whether this amount of BZC is toxic in the small premature neonate is presently unknown. [ABSTRACT FROM AUTHOR]