Hyponatremia: diagnose and correct

The Diagnose tab follows the European guideline (ESE/ESICM/ERBP 2014, section 6) and states each step as the guideline frames it:

1. Exclude non-hypotonic hyponatremia. A measured osmolality under 275 mOsm/kg always means hypotonic hyponatremia (6.2.1.2). At 275 or above, look for effective osmoles (glucose, mannitol, glycine, hyperosmolar contrast), pseudohyponatremia (lipids, protein, immunoglobulin, paraprotein) and ineffective osmoles (urea, alcohols), then accept as hypotonic only if none is found (6.2.1.3).
2. Urine osmolality first (6.3.1.1). 100 mOsm/kg or less: relative excess water intake — primary polydipsia, low solute intake, beer potomania (6.3.1.2).
3. Then urine sodium from a spot sample taken with the blood (6.3.1.3). 30 mmol/L or less: low effective arterial blood volume (6.3.1.4); diuretics raise urine sodium, so a value this low still counts in a patient taking them. Above 30: assess ECF volume and diuretic use (6.3.1.5), and read with caution on diuretics.
4. SIAD is a diagnosis of exclusion. Essential criteria: effective osmolality under 275, urine osmolality over 100, clinical euvolemia, urine Na over 30 on normal salt and water intake, no adrenal, thyroid, pituitary or renal insufficiency, and no recent diuretics. Supplemental criteria: uric acid under 0.24 mmol/L (4 mg/dL), serum urea under 3.6 mmol/L, failure to correct with 0.9% saline, FENa over 0.5%, FEurea over 55%, FE uric acid over 12%, and correction with fluid restriction. The guideline suggests against measuring vasopressin (6.3.1.6).

US 2013 panel. It did not publish a diagnostic algorithm. It separates hypotonic from non-hypotonic hyponatremia and then classifies by volume status — hypovolemic, euvolemic, hypervolemic (Hoorn & Zietse 2017). The European group put urine tests ahead of the volume exam because the exam is unreliable. Both treat SIAD as a diagnosis of exclusion. The US panel's own threshold wording could not be checked against its primary text for this version, so the tool uses the European numbers throughout.

Causes of SIAD to look for (European Table 7): cancer, lung disease, nervous-system disease, drugs, and nonspecific stimuli such as nausea, pain, stress and anesthesia.

The European guideline uses 2.4 mmol/L per 100 mg/dL (5.5 mmol/L) of glucose above 100 mg/dL (Hillier 1999; rec 6.2.1.1 and Table 9). Hillier's experiments in healthy volunteers found 2.4 overall, significantly more than Katz's traditional 1.6. Up to 400 mg/dL, 1.6 performed well; above 400 mg/dL, 4.0 fitted better. The tool decides on the 2.4 value and shows 1.6 alongside. When the two land on opposite sides of 135, it says so — treat that patient as possibly having both a hyperglycemic and a hypotonic component, and recheck as glucose falls.

Total body water = weight × 0.6 (non-elderly men), 0.5 (non-elderly women), 0.5 (elderly men), 0.45 (elderly women) — Adrogué & Madias 2000, reproduced in the European guideline. Neither source defines "elderly"; this tool uses ≥ 65 years.

Values from Adrogué–Madias Table 2. Ringer's lactate's own K⁺ is not included by default; add it under "Added K⁺" if you want it counted.

The formula assumes a closed system: it ignores urine and other ongoing losses. When the stimulus for vasopressin disappears (volume restored, drug stopped, cortisol replaced, nausea or pain relieved) a water diuresis can raise Na⁺ far faster than predicted — in one hypertonic-saline series summarised by the European guideline, patients starting below 120 mmol/L rose more than predicted in 74% of cases, on average 2.4 times the predicted rise. Isotonic saline in SIADH can even lower Na⁺ if urine is more concentrated than the infusate.

Units mmol/L. Limits apply to chronic (> 48 h) or unknown-duration hyponatremia; the US panel framed them for Na⁺ ≤ 120. Documented acute hyponatremia (< 48 h — e.g. postoperative, marathon, ecstasy) carries much less demyelination risk. A limit is a value not to cross, not a target. Since 2023, observational studies linking faster correction with lower mortality have prompted commentaries calling for revision (e.g. "Hyponatremia treatment guidelines: a necessary course correction", Kidney Int 2026); a large group of guideline authors argued that current safeguards should stand until better evidence exists (Sterns et al., CJASN 2024). Neither guideline had been revised as of October 2026.

Severe (European definitions): vomiting, cardiorespiratory distress, abnormal and deep somnolence, seizures, coma (GCS ≤ 8). Moderately severe: nausea without vomiting, confusion, headache. Symptoms may have other causes — the milder the hyponatremia, the less likely it explains them.

• European 2014, severe: 150 mL 3% NaCl IV over 20 min; check Na⁺ at 20 min while giving a second 150 mL; repeat up to twice more or until Na⁺ has risen 5 mmol/L. Consider 2 mL/kg instead of 150 mL in markedly abnormal body size. If symptoms improve, stop hypertonic saline. If not, continue 3% aiming for +1 mmol/L per hour, stopping at +10 mmol/L total or Na⁺ 130; check Na⁺ every 4 h.
• European 2014, moderately severe: a single 150 mL 3% over 20 min; aim for +5 mmol/L per 24 h; check Na⁺ at 1, 6 and 12 h.
• US panel 2013: 100 mL 3% NaCl over 10 min, repeated up to 3 times as needed, aiming for a 4–6 mmol/L rise within a few hours; continuous 3% at 0.5–2 mL/kg/h is the listed alternative. Panel members advise reserving a third bolus for ongoing seizures or herniation signs (Rondon-Berrios & Sterns 2022).

• European 2014: intervene if Na⁺ rises > 10 mmol/L in the first 24 h or > 8 mmol/L in any 24 h thereafter. Stop active treatment. With expert input, consider 10 mL/kg of electrolyte-free water (e.g. D5W) over 1 h with strict monitoring of urine output and fluid balance, and desmopressin 2 µg IV, not more often than every 8 h.
• US panel 2013: re-lowering is reasonable in high-risk patients once the rise exceeds 8 mmol/L in < 24 h; in normal-risk patients it is optional after 10–12 mmol/L, but further rise should be prevented for the next 24 h (Sterns 2024). The panel's re-lowering regimen is D5W 3 mL/kg/h with hourly Na⁺ until back under the 24-h limit (as summarised by Warren 2023), with desmopressin to stop a water diuresis.
• A sudden rise in urine output to > 100 mL/h signals imminent overcorrection — check Na⁺ every 2 h until stable (European guideline).

The European guideline puts urine tests before the bedside volume exam, because clinical volume assessment has low sensitivity (0.5–0.8) and specificity (0.3–0.5).

1. Exclude non-hypotonic hyponatremia — hyperglycemia (corrected Na⁺ shown in the planner), mannitol, glycine irrigation, pseudohyponatremia from severe hyperlipidemia or hyperproteinemia.
2. Urine osmolality ≤ 100 mOsm/kg → relative excess water intake: primary polydipsia, low solute intake ("tea and toast", beer potomania).
3. Urine osmolality > 100 → check spot urine Na⁺ with a simultaneous blood sample.
4. Urine Na⁺ ≤ 30 mmol/L → low effective arterial volume: hypovolemic (GI losses, third spacing, remote diuretics) or hypervolemic (heart failure, cirrhosis, nephrotic syndrome).
5. Urine Na⁺ > 30 mmol/L → assess ECF volume and diuretic use: hypovolemic with renal salt loss (diuretics, primary adrenal insufficiency, salt-wasting nephropathy, cerebral salt wasting); euvolemic (SIAD, secondary adrenal insufficiency, hypothyroidism — rarely); hypervolemic with kidney failure or diuretics. Interpret with caution on diuretics.

Treatment follows the category: hypovolemia — 0.9% saline or a balanced crystalloid at 0.5–1.0 mL/kg/h (European), watching for a brisk aquaresis once volume is restored; SIAD — fluid restriction first; hypervolemia — fluid restriction and treat the underlying disease.