Infusate Formula | Fluid-Loss Formula |
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Projects the effect of gaining 1 L of any infusate (inf) on the patient’s [Na^{+}]_{s} | Projects the effect of losing 1 L of any fluid (fl) on the patient’s [Na^{+}]_{s} |

Derivation | Formulas are based on the Edelman equation.^{23} Note that in the infusate formula, the patient’s [Na^{+}]_{s} is subtracted from the electrolyte composition of the infusate and 1 L is added to TBW. By contrast, in the fluid-loss formula, the electrolyte composition of the fluid is subtracted from the patient’s [Na^{+}]_{s} and 1 L is subtracted from TBW. |

Clinical Utility | Formulas aid clinicians in making quantitative projections of the effect of prescribed fluid therapy and ongoing fluid losses on patient’s serum sodium. Adjustments in fluid therapy over time are facilitated by applying the two formulas as often as needed utilizing the intercurrent data of the patient. |

Utilization of fluid-loss formula in the management of hyponatremia is only required when ongoing fluid losses (renal and extrarenal) are substantial (>1 L/d); in that case, the effect of the fluid loss on the patient’s [Na^{+}]_{s} should be included in the computation of prescribed fluid therapy. | |

If the fluid-loss formula predicts correction of hyponatremia at an inappropriately rapid rate, a hypotonic infusate (e.g., 0.45% NaCl, 5% dextrose in water) must be used at a rate determined by the infusate formula. | |

Limitations | Reliability of projections depends on utilizing a reasonable approximation of TBW. A substantial overestimate of TBW would decrease the projected effect of infusates and fluid losses on [Na^{+}]_{s} risking overcorrection of hyponatremia. |

The estimated TBW (in liters) is calculated as a fraction of body weight. This fraction is 0.6 in children, 0.55 in men, and 0.5 in women.

^{25}^{,}^{55}^{,}^{56}TBW, total body water.