What goes into the SIADH criteria
- Serum sodium (mEq/L)
- Serum glucose (mg/dL)
- Measured serum osmolality (optional) (mOsm/kg)
- Urine osmolality (mOsm/kg)
- Urine sodium (mEq/L)
- Urine potassium (for Furst ratio) (mEq/L)
- Clinically euvolemic (no edema, no signs of volume depletion)
- No diuretic in the past week
- Hypothyroidism and adrenal insufficiency excluded (TSH, morning cortisol)
- No significant kidney impairment
- BUN (optional) (mg/dL)
- Serum uric acid (optional) (mg/dL)
- Serum creatinine (for FE) (mg/dL)
- Urine creatinine (for FE) (mg/dL)
- Urine uric acid (for FE urate) (mg/dL)
- Urine urea nitrogen (for FE urea) (mg/dL)
How to interpret the result
- Essential: effective serum osmolality < 275 mOsm/kg; urine osmolality > 100 mOsm/kg; urine sodium > 30 mEq/L with normal salt and water intake; clinical euvolemia; no recent diuretics; normal thyroid, adrenal and kidney function
- Supporting: serum uric acid < 4 mg/dL; BUN < 10 mg/dL; FENa > 0.5%; FE urea > 55%; FE uric acid > 12%; failure to correct with 0.9% saline; correction with fluid restriction
- Urine osmolality ≤ 100: ADH is suppressed (primary polydipsia, low solute intake); urine sodium ≤ 30 with concentrated urine: low effective volume
- Furst ratio (urine Na + K) ÷ serum Na: > 1, fluid restriction will fail; 0.5–1, restrict to about 500 mL/day; < 0.5, restrict to about 1 L/day
Clinical pearls
- Send urine osmolality and sodium before giving saline or diuretics.
- SIADH is a diagnosis of exclusion: check TSH and a morning cortisol, and review drugs (SSRIs, carbamazepine, oxcarbazepine, cyclophosphamide, opioids, MDMA).
- Look for a cause: pneumonia and other lung disease, CNS disease, small cell lung cancer, pain, nausea and the postoperative state.
- FE uric acid stays high in SIADH even on diuretics, which makes it useful when urine sodium can't be interpreted.
- Normal saline can lower the sodium further in SIADH when urine osmolality is high.
- Treat moderate or severe symptoms with hypertonic saline first, and limit correction to 8 mEq/L in 24 h in high-risk patients.
Frequently asked questions
- What are the diagnostic criteria for SIADH?
- Hypotonic hyponatremia (effective serum osmolality < 275 mOsm/kg) with inappropriately concentrated urine (> 100 mOsm/kg), urine sodium > 30 mEq/L on normal intake, clinical euvolemia, no recent diuretics, and normal thyroid, adrenal and kidney function.
- What is the Furst ratio?
- (Urine sodium + urine potassium) ÷ serum sodium. Above 1, the urine is more concentrated in electrolytes than plasma and fluid restriction alone will not raise the sodium; 0.5 to 1 suggests a restriction of about 500 mL/day; below 0.5, about 1 L/day.
- How do you tell SIADH from hypovolemic hyponatremia?
- Both have concentrated urine. In hypovolemia urine sodium is usually below 30 mEq/L and BUN and uric acid tend to be high; in SIADH urine sodium is above 30 and uric acid and BUN are low. If unsure, a cautious saline challenge raises the sodium in hypovolemia but not in SIADH.
- Why is FE uric acid useful?
- It is above 12% in SIADH and usually stays high on diuretics, when urine sodium is unreliable.
Evidence
- Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170:G1-G47.
- Verbalis JG, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42.
- Furst H, et al. The urine/plasma electrolyte ratio: a predictive guide to water restriction. Am J Med Sci. 2000;319:240-244.
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For education only. Check the inputs and the population the score was validated in before acting on it. Not a substitute for clinical judgment.