Natremia, tonicity, and conductivity measurements in hemodialyzed patients.

ABSTRACT

PURPOSE: Natremia is usually considered to reflect tonicity in non-hemodialyzed patients. Some hemodialysis monitors provide an online value (NaCond) of natremia calculated from conductivity measurements. This study compared the relation between tonicity and natremia (NaLab) measured at laboratory with the relation between tonicity and NaCond in hemodialysis patients. METHODS: Fifty-five hemodialysis sessions performed with a Fresenius 5008 dialysis monitor (Fresenius Medical Care, Bad Homburg, Germany) providing a value of NaCond were analyzed. Tonicity (calculated as "osmolality - urea"), NaLab and NaCond were measured at the beginning and end of sessions. RESULTS: The r2 correlation-coefficient between tonicity and NaLab is 0.48 (n = 110). The correlation between tonicity and NaCond is stronger (r2 =  0.71). CONCLUSIONS: Conductivity measurements provide a natremia value (NaCond) that is a better surrogate for tonicity than natremia measured at laboratory. Because NaCond is not obtained from sodium measurement, dialysis monitors should display a value for plasma conductivity (mS/cm) instead for natremia (mmol/l).