NDT Advance Access originally published online on August 22, 2008
Nephrology Dialysis Transplantation 2008 23(11):3501-3508; doi:10.1093/ndt/gfn476
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Can we really predict the change in serum sodium levels? An analysis of currently proposed formulae in hypernatraemic patients
1 Intensive Care Unit 13H1, Clinic for Internal Medicine III 2 Department of Nephrology and Dialysis, for Internal Medicine III, Medical University of Vienna, Vienna 3 Department of Nephrology, Krankenhaus der Elisabethinen, Linz, Austria
Correspondence and offprint requests to: Gregor Lindner, Department of Nephrology and Dialysis, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Wien, Austria. Tel: +43-650-412-74-22; Fax: +43-1-40400-4386; E-mail: lindner.gregor{at}gmail.com
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Abstract |
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Background. Hypernatraemia is common in intensive care patients and may present an independent risk factor of mortality. Several formulae have been proposed to guide infusion therapy for correction of serum sodium. Unfortunately, these formulae have never been validated comparatively. We assessed the predictive potential of four different formulae (Adrogué–Madias, Barsoum–Levine, Kurtz–Nguyen and a simple formula based on electrolyte-free water clearance) in correction and maintenance of serum sodium in 66 hyper- and normonatraemic ICU patients.
Methods. With daily measurements of sodium/potassium and fluid/electrolyte balances, a day-to-day prediction of serum sodium levels was calculated using the four formulae. This was compared to the measured changes in serum sodium.
Results. Six hundred and eighty-one patient-days (194 hypernatraemic) in 66 patients were available for calculations. Prediction of serum sodium levels using all four formulae correlated significantly (P < 0.05) with measured changes in serum sodium. Individual variations were extreme, and the mean differences (±SD) for predicted versus measured serum sodium were within the range of 3.4–4.5 (±4.4–4.7) mmol/l similar for the Adrogué–Madias, Barsoum–Levine and Nguyen–Kurtz formulae. In comparison, our proposed formula underestimated the changes of serum sodium (mean ± SD –1.5 ± 5.3). During hypernatraemia, the differences between predicted and measured values were even greater (mean ± SD 5.0–6.7 ± 3.9–4.3) using the published formulae compared to our formula (mean ± SD 0.2 ± 4.0).
Conclusions. Currently available formulae to guide infusion therapy in hyper- and normonatraemic states do not accurately predict changes of serum sodium in the individual ICU patient. In clinical practice, infusion therapy should be based on the reasons for hypernatraemia and serial measurements of serum sodium to avoid evolution of derangements.
Keywords: correction; formula; hypernatraemia; prediction; sodium
Abbreviations: Na1, serum sodium concentration day 1 • Na2, serum sodium concentration day 2 • TBW, total body water: TBW calculated on the day of admission with correction based on the total daily water balance • Nainf and Kinf, sodium and potassium concentration of the infused fluids • Volinf, volume infused in ml • Nainput and Kinput, sodium and potassium concentration of all applicated fluids (oral, intravenous) • Volinput, total volume input in ml (oral, intravenous) • Naurine and Kurine, sodium and potassium concentration of the urine • Volurine, volume urine in ml • 
Vol, difference in volume between measured inputs and outputs • Volout, total volume output: urine + extrarenal fluid loss via tubes (nasogastric suction and wound drains) • (Na+K)out, sodium and potassium concentration of extrarenal losses, calculated as a hypotonic fluid with a fixed value of 110 mmol/l
* Both authors contributed equally to this study.
Received for publication: 12.12.07
Accepted in revised form: 26. 6.08