From the Guidelines
SIADH is characterized by euvolemic hypoosmolar hyponatremia with low serum osmolality (< 275 mosm/kg) and inappropriately high urine osmolality (> 500 mosm/kg).
Key Diagnostic Features
- Hyponatremia (serum sodium < 134 mEq/L)
- Hypoosmolality (plasma osmolality < 275 mosm/kg)
- Inappropriately high urine osmolality (> 500 mosm/kg)
- Inappropriately high urinary sodium concentration (> 20 mEq/L)
- Absence of hypothyroidism, adrenal insufficiency, or volume depletion ### Laboratory Findings
- Urine osmolality of > 300 mosm/kg
- Urinary sodium level of > 40 mEq/L
- Serum osmolality of < 275 mosm/kg
- Serum uric acid concentration of < 4 mg/dL, as noted in 1 These findings are consistent with the diagnosis of SIADH, which is often seen in patients with small cell lung cancer (SCLC) 1.
From the Research
Osmolality in SIADH Diagnosis
- Osmolality plays a crucial role in the diagnosis of Syndrome of Inappropriate Antidiuretic Hormone (SIADH) 2, 3, 4, 5, 6
- Urine osmolality in excess of plasma osmolality is one of the cardinal criteria for the diagnosis of SIADH 2
- A urine osmolality >100 mOsm/kg, along with a urine sodium concentration above 40 mEq/L, can indicate SIADH 5
- High urine osmolality (>500 mOsm/kg) and persistence of high urine sodium concentrations may be useful in predicting poor response to fluid restriction in SIADH 4
- Measured serum osmolality and urinary osmolality can help in diagnosing SIADH, with values of 255 mOsm/kg and 130 mOsm/kg, respectively, reported in a case study 5
Key Findings
- The diagnosis of SIADH should be considered if the five cardinal criteria are fulfilled, including hypotonic hyponatraemia, natriuresis, urine osmolality in excess of plasma osmolality, absence of oedema and volume depletion, and normal renal and adrenal function 2
- Different types of osmotic dysregulation of antidiuretic hormone have been reported, with possibly fundamental deviations in treatment need and efficacy 3
- Fluid restriction is the mainstay of therapy for symptomatic patients with chronic SIADH, with a goal of restricting fluid intake to 800-1200 mL/24 hours 6