Sodium Deficit Calculator - Total Body Water and mmol Replacement

A sodium deficit calculator is a clinical screening tool that turns a patient's age and sex, body weight, current serum sodium, and target serum sodium into the published sodium deficit in mmol plus the total body water. It uses the published age and sex water fractions.

• • Pre-rounds hyponatremia chart review: enter the basic metabolic panel, pick the age and sex category, and read the deficit in mmol.
• • Severe hyponatremia workup: use the calculator with serum sodium at or below 125 mEq/L to size the deficit.
• • Walk-through of the Omni example: recheck the worked example with a 55 kg adult female, serum Na 125 mEq/L, and target 140 mEq/L to confirm a deficit of about 413 mmol.
• • Compare to the correction rate tool: read the deficit with the Sodium Correction Rate Calculator so the plan stays inside the chronic envelope.

The default target of 140 mEq per liter sits inside the published 135 to 145 mEq per liter reference range, and the calculator floors the deficit at zero when the current reading is at or above the target. The calculator is a screening aid, not a stand-alone replacement order. Volume status, urine sodium, and serum osmolality still drive the final plan.

According to MedlinePlus, the normal serum sodium range is 135 to 145 mEq/L, and a value outside that range should be read alongside the patient's volume status.

The sodium deficit is the total millimoles of replacement, and Sodium Correction Calculator applies the Adrogué-Madias formula to plan the per-liter change, the infusate volume, and the 24-hour rate that the deficit is matched against.

The sodium deficit calculator applies the published total body water formula in three steps and returns the result in mmol. The same calculation drives any pre-round review of a basic metabolic panel that reports a serum sodium below 135 mEq per liter.

• category: combined age and sex category. Loads the published total body water fraction: adult male 0.60, adult female 0.50, elderly male 0.50, elderly female 0.45, child 0.60.
• weight: body weight in kilograms. Multiplied by the TBW fraction to estimate total body water in liters.
• serumSodium: current serum sodium in mEq per liter. The hyponatremia cutoff is 135 mEq per liter.
• targetSodium: target serum sodium in mEq per liter. The default is 140 mEq per liter, inside the 135 to 145 mEq per liter reference range.
• TBW: total body water in liters, equal to body weight times the water fraction.
• sodiumDeficit: sodium deficit in mmol, equal to total body water times the sodium gap. One mEq of sodium equals one mmol.

The same formula is used in the Buffington and Abreo hyponatremia review, the Merck Manual chapter on hyponatremia, the AAFP 2015 review, and the Omni Calculator reference. The default 140 mEq per liter target keeps the gap inside the 135 to 145 mEq per liter reference range.

According to PubMed Hyponatremia A Review, total body water is about 60% of body weight in adult males and children, 50% in adult females and elderly males, and 45% in elderly females.

The deficit in mmol is one side of the correction plan and the rate of change in mEq/L per hour is the other, and Sodium Correction Rate Calculator turns the same basic metabolic panel plus a planned 24-hour change into the published chronic correction rate that the deficit is matched against.

Four concepts drive the result. Naming them keeps the calculator from being read as a stand-alone replacement order.

Total body water scales the result to the patient, the sodium gap controls the size of the deficit, the deficit is the actual replacement number, and the cutoff decides whether the patient sits inside the published correction envelope.

The same basic metabolic panel that drives the sodium deficit is also where pseudohyponatremia from hypertriglyceridemia shows up, and Sodium Change Calculator applies the 0.002 mEq/L per mg/dL rule to return the corrected sodium that should be entered into the deficit calculation.

The form works from a single age and sex category plus a small set of basic metabolic panel values. Each input should come from the most recent chart review.

1. 1 Pick the age and sex category: choose adult male, adult female, elderly male, elderly female, or child.
2. 2 Enter the body weight: type the body weight in kilograms. The calculator multiplies the weight by the TBW fraction to estimate total body water.
3. 3 Enter the current serum sodium: type the current serum sodium in mEq per liter from the latest basic metabolic panel.
4. 4 Enter the target serum sodium: type the target serum sodium in mEq per liter. The default of 140 mEq per liter sits inside the published reference range.
5. 5 Read the deficit and clinical guidance: the result panel shows the sodium deficit, total body water, TBW fraction, sodium gap, band label, and clinical guidance line.

A 55 kg adult female with serum sodium 125 mEq per liter. Pick adult female, weight 55, current Na 125, target Na 140. The calculator returns TBW 27.5 L, gap 15 mEq/L, sodium deficit 413 mmol, Hyponatremia band, and a guidance line.

Severe hyponatremia is often read alongside a low serum bicarbonate on the same basic metabolic panel, and Bicarbonate Deficit turns the same weight plus the measured bicarbonate into a parallel deficit number that helps frame the metabolic acidosis workup.

Using a sodium deficit calculator gives the bedside team a single published number to anchor the hyponatremia plan, with the same units and water fractions on every chart review.

• • Published total body water math: returns the sodium deficit in mmol and the total body water in liters using the published age and sex water fractions.
• • Five age and sex categories covered: adult male, adult female, elderly male, elderly female, and child with the published water fractions preloaded.
• • Hyponatremia band and clinical guidance: labels the current serum sodium as Hyponatremia or Normal using the 135 mEq per liter cutoff and pairs the deficit with a clinical guidance line.
• • Same mmol unit as the correction literature: the deficit in mmol lines up with the published correction literature.
• • Default 140 mEq/L target: the target defaults to 140 mEq per liter so the gap sits inside the 135 to 145 mEq per liter reference range.
• • Shared language with the care team: hospitalists, nephrologists, pharmacists, and the bedside nurse can all read the same mmol, liters, and mEq per liter numbers.

The same calculator is used for chart review, hyponatremia workup, and the screening step before any serum sodium correction plan. The most useful pattern is to run the calculator before orders, recheck the basic metabolic panel at 4 to 6 hours, and re-enter the new serum sodium to update the deficit.

Most of the basic metabolic panels that trigger a sodium deficit calculation also report a sodium, a chloride, and a bicarbonate, and Anion Gap Calculator reads those three numbers to flag a high, normal, or low anion gap on the same chart review.

Five patient and lab variables can move the result by more than a full band.

• • The sodium deficit is a screening aid, not a stand-alone replacement order. Volume status, urine sodium, serum osmolality, and the trend of the basic metabolic panel still drive the final plan.
• • Children, pregnant patients, and patients on chronic parenteral nutrition can have atypical total body water fractions, so the calculator should be read alongside the clinical context when the result is borderline.

The next step is always a basic metabolic panel recheck at 4 to 6 hours. A plan that would cross the published 10 mEq per liter per 24 hours chronic correction ceiling should pause and trigger a volume status and serum osmolality review. Patients on vasopressin, thiazide diuretics, or with syndrome of inappropriate antidiuretic hormone secretion can have a paradoxical rise in serum sodium after the cause is removed.

According to Merck Manual, hyponatremia is a serum sodium below 135 mEq/L, the sodium deficit scales with total body water, and chronic hyponatremia should be corrected by no more than about 10 mEq/L per 24 hours.

The same basic metabolic panel that drives a sodium deficit calculation often reports a low albumin, and Corrected Calcium Calculator applies the published 0.8 mg/dL per 1 g/dL Payne rule to return an albumin-adjusted calcium that helps frame the broader electrolyte review.