From the Guidelines
SIADH is diagnosed through a combination of laboratory tests and clinical evaluation, requiring findings of hyponatremia, hypoosmolality of the serum, and inappropriate urine concentration. The diagnosis of SIADH involves several key criteria, including decreased serum osmolality (less than 275 mOsm/kg), inappropriately concentrated urine (urine osmolality greater than 100 mOsm/kg), elevated urine sodium (typically greater than 20-30 mEq/L), and normal kidney, adrenal, and thyroid function 1. Clinicians must also confirm euvolemia (normal fluid volume status) and exclude other causes of hyponatremia such as diuretic use, hypothyroidism, and adrenal insufficiency.
Key Diagnostic Criteria
- Decreased serum osmolality (less than 275 mOsm/kg)
- Inappropriately concentrated urine (urine osmolality greater than 100 mOsm/kg)
- Elevated urine sodium (typically greater than 20-30 mEq/L)
- Normal kidney, adrenal, and thyroid function Additional tests may include measuring blood urea nitrogen, creatinine, uric acid, and sometimes direct measurement of antidiuretic hormone (ADH) levels 1. Once SIADH is confirmed, further investigation to identify the underlying cause is necessary, which may involve chest imaging to look for lung cancer, brain imaging for neurological causes, or medication review since many drugs can trigger SIADH.
Importance of Accurate Diagnosis
Accurate diagnosis of SIADH is crucial as it affects the treatment approach, which differs significantly from other causes of hyponatremia, and can help prevent severe complications such as hyponatremic encephalopathy 1.
From the Research
Diagnosis of SIADH
The diagnosis of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is based on several clinical and laboratory criteria, including:
- Clinical euvolaemic state with low serum sodium and osmolality 2
- Raised urine sodium and osmolality 2, 3
- Exclusion of pseudohyponatraemia and diuretic use 2
- Hyponatremia with serum sodium levels below 130 mmol/L 3, 4
- Urine osmolality >100 mOsm/kg and urine sodium concentration above 40 mEq/L 3
Laboratory Tests
Laboratory tests are necessary for the diagnosis of SIADH, including:
- Serum sodium and osmolality measurements 2, 3
- Urine sodium and osmolality measurements 2, 3
- Serum glucose, potassium, uric acid, renal, and liver function tests 3
- Adrenal function tests (cortisol, adrenocorticotropic hormone, renin, and aldosterone) 3
- Thyroid function tests 3
Clinical Assessment
A clinical assessment is also important in the diagnosis of SIADH, including: