How this instrument works
The calculation estimates how far current sodium sits above the reference point of 140 mEq/L, then scales that gap by an age- and sex-adjusted estimate of how much of the body is water in the first place. A non-elderly man is assumed to be 60% water by weight; a non-elderly woman, or a man past 65, is assumed to be 50%; a woman past 65 is assumed to be 45%. Lean tissue holds far more water than fat does, which is why the male figure starts higher, and total body water share tends to decline with age in both sexes, which is why the fraction steps down again once either sex crosses that same threshold.
Those age- and sex-adjusted fractions trace back to a chapter by Oh and Carroll in the textbook Fluid, Electrolyte, and Acid-Base Disorders, edited by Arieff and DeFronzo and published by Churchill Livingstone in 1995 — the real origin of the numbers this instrument uses, even though the formula is very often attributed elsewhere.
That elsewhere is usually a 2000 New England Journal of Medicine review on hypernatremia by Adrogué and Madias — a paper frequently miscredited as the source of this exact equation when it actually does the opposite: it critiques the static formula's assumptions and argues for a more dynamic approach based on the composition of whatever fluid is actually being infused. Read this result as a reasonable starting estimate for planning fluid replacement, not as a one-time number that replaces watching how a patient's own sodium responds to treatment.
- Set Sex, since it changes which total-body-water fraction applies.
- Enter Age in years — the fraction steps down past 65.
- Enter Weight in kilograms.
- Enter current serum Sodium in mEq/L, then read the estimated deficit in liters, treating it as a starting point for replacement planning rather than a final figure.
Worked example — 40-year-old man, 70 kg, sodium 155 mEq/L
Under 65, so the male fraction stays at 0.6. Sodium ratio: 155 ÷ 140 ≈ 1.107, minus 1 leaves 0.107. Multiply through: 0.6 × 70 × 0.107 ≈ 4.5 L — the estimated shortfall of plain water behind this sodium reading.
Change the inputs to a 70-year-old, 60 kg woman at sodium 160: age steps her fraction down to 0.45. Ratio: 160 ÷ 140 ≈ 1.143, minus 1 leaves 0.143. Multiply through: 0.45 × 60 × 0.143 ≈ 3.86 L — a smaller body and a lower water fraction, even at a higher sodium, producing a somewhat smaller estimated deficit than the first case.
Questions
Is this formula really from Adrogué and Madias?
No, and it's a common mix-up. The age- and sex-adjusted body-water fractions used here trace back to a 1995 textbook chapter by Oh and Carroll. Adrogué and Madias's well-known 2000 review discusses hypernatremia at length, but its actual contribution was to critique this conventional formula's assumptions and propose a more dynamic, infusate-based alternative — not to originate the equation itself.
Why do the total-body-water fractions differ by sex and age?
Lean tissue holds far more water than fat does, and men carry more lean mass on average, which is why the non-elderly male fraction of 0.6 sits above the non-elderly female figure of 0.5. Total body water also tends to decline with age in both sexes, which is why the fraction steps down again past 65 — to 0.5 for men and 0.45 for women.
Is the result an exact volume to infuse?
No — treat it as a starting estimate for planning, not a prescription. Real fluid replacement needs to proceed gradually, with sodium rechecked repeatedly along the way, because the body's actual response depends on ongoing losses, intake, and how quickly correction is tolerated — none of which a single calculation can capture.
Why has this static formula been criticized?
Because it assumes a fixed body-water fraction and a single target sodium, when real replacement involves fluids with their own composition being infused into a body that's still losing or retaining water on its own. Adrogué and Madias argued for calculating the expected sodium change from the specific fluid being given, updated as therapy proceeds, rather than leaning on one static number computed up front.
Does this account for ongoing water losses?
No. It estimates the deficit already present at the moment of calculation, not water being lost going forward through fever, sweating, or an underlying condition like diabetes insipidus. Ongoing losses have to be added on top and reassessed as treatment continues.
How fast should sodium be corrected once a deficit is estimated?
Slowly, and this instrument doesn't set that pace — correcting chronic hypernatremia too quickly risks cerebral edema, so clinicians typically target a gradual reduction over roughly the first day and beyond, guided by repeated sodium checks rather than the single starting estimate.
References
- Oh MS, Carroll HJ. 'Regulation of intracellular and extracellular volume,' in Fluid, Electrolyte, and Acid-Base Disorders, 2nd ed. (Arieff AI, DeFronzo RA, eds.), Churchill Livingstone, 1995 — book chapter
- Adrogué HJ, Madias NE. 'Hypernatremia.' N Engl J Med. 2000 — critique of the conventional formula (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.