The calculation of sodium deficit is necessary when aiming to keep the patient in the state of optimal electrolyte balance.
The provided equation below uses the patient's weight, serum sodium (Na⁺) concentration, and the desired serum sodium concentration.
Assignment to the correct age/sex group is also needed for the best estimation of total body water, which is included in the sodium deficit formula.
What is sodium deficit?
Sodium deficit is the quantity of sodium missing from the patient's body in the state of hyponatremia, expressed in millimoles (mmol).
Hyponatremia is a low sodium level in the blood serum, most commonly described as a sodium concentration below 135 mEq/L (or mmol/L). The first symptoms are usually headaches, confusion, nausea, and imbalance.
Severe hyponatremia is a state where the patient's sodium level is below 125 mEq/L (or mmol/L). The symptoms of the sodium level falling are dry mucus membranes, cold and clammy skin, tremors, and seizures.
Remember that hyponatremia should be corrected slowly, as in case of overcorrection, there is a risk of osmotic demyelination. The optimal sodium correction rate is 0.5 mEq/L per hour. You can use our calculator for hyponatremia correction rate to estimate the flow of fluid you're planning to use.
During fluid supplementation, you should frequently test electrolytes in the patient's blood serum. Note that in the state of hyperglycemia, the sodium measurements can be distorted. Use the sodium correction calculator if your patient's blood sugar is high.
What's the sodium deficit formula?
We calculate the sodium deficit using this equation:
Sodium deficit = TBW × (desired_sodium - serum_ sodium)
TBW– Total body water, calculated by multiplying the patient's weight by the age/sex coordinate, which is 60% for children and adult males, 50% for adult females and elderly males, and 45% for elderly females;
desired_sodium– Target sodium concentration, usually it's 140 mEq/L; and
serum_sodium– Measured level of sodium concentration in the patient's blood serum.
The result is expressed in millimoles (mmol).
What's the normal level of sodium in blood?
The normal sodium level in the blood is between 135 and 145 mEq/L, with 140 mEq/L as the usual target value used to calculate the sodium deficit in the patient's blood.
Hyponatremia is the term used to refer to low levels of sodium in the body, below 135 mEq/L. Sodium is essential to keep water osmotic balance in the cells. More water can enter cells when sodium levels are too low, causing them to swell.
How do I find the sodium deficit of a woman of 55 kg and 125 mEq/L?
To find the sodium deficit of an adult woman:
Use the sodium deficit formula:
sodium_deficit = TBW × (desired_sodium - serum_ sodium)
TBW– Total body water. For a woman, this is 50% of her body weight.
desired_sodium– Normal sodium level; use 140 mEq/L.
serum_sodium– Sodium concentration in the patient's blood.
Estimate the total body water (
TBW = 55 kg × 0.5 = 27.5 kg
Substitute values in the
sodium_deficitformula to get the corresponding value:
sodium_deficit = 27.5 kg × (140 mEq/L - 125 mEq/L)
sodium_deficit = 412.5 mmol
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