High Concentrations of Lithium Heparin Decrease Measured Serum Sodium in Some Analyzers

Abstract

While studying a handheld analyzer (i-STAT) in a neonatal intensive care unit (NICU), we found hyponatremia (<128 mmol/L sodium) in 25 of 137 newborn infants (postnatal age <6 days for 75% of the infants) with sodium values within the reference interval on a Vitros 750 analyzer (Johnson & Johnson). The difference in sodium values ranged from 5 to 19 mmol/L. Investigation into the clinical presentation and into the various treatments disclosed no common feature. We tested the hypothesis that the heparinized capillary tubes used in the NICU for sample collection may explain the observed discrepancies in sodium values. We used the i-STAT analyzer and adult venous blood samples to compare capillary tubes supplied either by Chiron (from Ortho Diagnostics, with 130–200 kIU lithium heparin/L) or by Radiometer (50 kIU electrolyte-balanced heparin/L). The sodium results were lower (mean difference, −2.14 mmol/L; range, 0 to −7 mmol/L) with the Chiron capillary tubes (Fig. 1⇓ A). The same discrepancy was …