SIADH — Diagnostic Criteria (Bartter & Schwartz Criteria)
📌 SIADH — Diagnostic Criteria (Bartter & Schwartz Criteria)
A. Essential Criteria
- Hypotonic hyponatremia
- Serum sodium <135 mEq/L
- Serum osmolality <275 mOsm/kg
- Inappropriately concentrated urine
- Urine osmolality >100 mOsm/kg despite low serum osmolality
(ADH should normally be suppressed → dilute urine)
- Urine osmolality >100 mOsm/kg despite low serum osmolality
- Euvolemic clinical status
- No signs of dehydration or edema
- Normal skin turgor, BP, JVP
- Elevated urine sodium
- Urine Na⁺ >30–40 mEq/L
(Normal salt intake; kidney not conserving sodium)
- Urine Na⁺ >30–40 mEq/L
B. Supportive / Additional Criteria
- Normal renal, thyroid, and adrenal function
- Normal creatinine
- Normal cortisol (to exclude adrenal insufficiency)
- Normal TSH/free T4 (to exclude hypothyroidism)
- Absence of diuretic use
- Especially thiazides (can mimic SIADH)
- Low serum uric acid
- Often <4 mg/dL
- Due to increased uric acid excretion
- Low BUN
- BUN <10 mg/dL
C. Optional / Additional Clues
- Fractional excretion of uric acid (FEUA) >12%
- Plasma ADH mildly elevated despite hyponatremia
📌 Quick Memory Trick (SIADH = “H.O.U.S.E. Na”)
Hypotonic hyponatremia
Osmolality low (serum)
Urine osmolality high
Sodium in urine high
Euvolemic
Na – no endocrine/renal failure, no diuretics
📊 Summary Table
| Parameter | Finding in SIADH |
|---|---|
| Serum Na⁺ | ↓ (<135) |
| Serum Osmolality | ↓ (<275) |
| Urine Osmolality | ↑ (>100) |
| Urine Sodium | ↑ (>30–40) |
| Volume Status | Euvolemic |
| BUN | Low |
| Uric Acid | Low |
| Renal / Thyroid / Adrenal Function | Normal |
| Diuretics | Absent |
1. Which of the following is an essential biochemical criterion of SIADH?
Low serum osmolality with hyponatremia defines hypotonic hyponatremia—central to SIADH.
2. Urine sodium in SIADH is typically:
Urine sodium is elevated (>30–40 mEq/L) due to inappropriate renal sodium loss despite hyponatremia.
3. Which clinical volume status is seen in SIADH?
SIADH typically presents with clinically normal (euvolemic) status.
4. Serum uric acid in SIADH is usually:
Low uric acid (<4 mg/dL) supports SIADH due to increased renal uric acid losses.
5. Which of the following conditions must be excluded before diagnosing SIADH?
Hypothyroidism and adrenal insufficiency must be excluded before diagnosing SIADH.
6. In SIADH, urine osmolality is typically:
Urine is inappropriately concentrated (>100 mOsm/kg) despite hypotonic plasma.
7. Serum BUN in SIADH is typically:
SIADH often presents with low BUN (<10 mg/dL) due to dilution and reduced urea production.
8. Fractional excretion of uric acid (FEUA) in SIADH is:
FEUA >12% is supportive of SIADH.
9. Which of the following medications can cause SIADH?
Carbamazepine is a classic drug that increases ADH release/sensitivity.
10. The most common cause of SIADH in hospitalized patients is:
Pulmonary diseases (e.g., pneumonia, TB) are major triggers of SIADH.
11. Which malignancy is classically associated with SIADH?
SCLC produces ectopic ADH → classic paraneoplastic SIADH.
12. Which of the following findings helps differentiate SIADH from CSW?
SIADH = euvolemic; CSW = hypovolemic.
13. Which of the following is NOT a criterion for SIADH?
SIADH is euvolemic, not hypervolemic.
14. Which laboratory pattern is consistent with SIADH?
Hyponatremia with low serum Osm but high urine Osm = classic SIADH mismatch.
15. Which ADH-related mechanism causes SIADH?
SIADH = excessive ADH or enhanced renal sensitivity to ADH.
16. In SIADH, which of the following is expected?
Plasma becomes diluted → hypotonic hyponatremia.
17. Which of the following drugs treats SIADH by inducing nephrogenic diabetes insipidus?
Demeclocycline reduces renal response to ADH.
18. Which IV fluid is appropriate for symptomatic severe SIADH?
3% hypertonic saline is used for severe symptomatic hyponatremia.
19. Which of the following is a hallmark of SIADH?
Euvolemic hyponatremia is the core feature of SIADH.
20. Which test confirms the diagnosis of SIADH?
SIADH is diagnosed clinically using Bartter & Schwartz criteria—not via ADH measurement.
| Feature | SIADH | Cerebral Salt Wasting (CSW) |
|---|---|---|
| Primary Pathophysiology | Excess ADH → water retention | Renal salt loss due to ↓ sympathetic tone & natriuretic peptides |
| Volume Status | Euvolemic or slight hypervolemia | Hypovolemic (KEY difference) |
| Serum Sodium | Low | Low |
| Serum Osmolality | Low | Low |
| Urine Sodium | High (>30–40 mEq/L) | High (>40–60 mEq/L) |
| Urine Osmolality | High (>100 mOsm/kg) | High (>100 mOsm/kg) |
| Uric Acid | Low | Low (but ↑ with correction) |
| Fractional Excretion of Uric Acid (FEUA) | High (>12%) | High (>12%) |
| Response of Uric Acid to Correction of Na⁺ | Remains low | Returns to normal |
| BUN | Low | Normal–low |
| JVP / Physical signs | Normal JVP, no dehydration | ↓ JVP, dehydration signs |
| Urine Output | Normal–low | High (polyuria) |
| Pathology association | Lung disease, drugs, malignancy | CNS injury, SAH, meningitis, neurosurgery |
| ADH levels | High | Normal/low |
| Treatment | Fluid restriction, salt tabs, hypertonic saline, vaptans | Aggressive volume + sodium replacement (NS/3% saline + fludrocortisone) |
| Effect of NS infusion | No improvement or worsens hyponatremia | Improves Na⁺ and volume status |
| Central diagnostic difference | Water excess | Salt loss |
