From the Guidelines
Hyponatremia is diagnosed based on serum sodium levels, with a diagnosis typically made when serum sodium is less than 134 mEq/L.
Diagnostic Criteria
The diagnosis of hyponatremia, specifically in the context of the Syndrome of Inappropriate Antidiuretic Hormone (SIADH), involves several key criteria, including:
- 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 1.
Clinical Assessment
Clinical assessment of intravascular volume status and biochemical measurements in blood and urine are crucial for further investigation of hyponatremia. Assessing the effective arterial blood volume with the fractional excretion of urate can improve the accuracy of a diagnostic algorithm for SIADH to approach 95% 1.
Laboratory Findings
Laboratory findings consistent with SIADH include:
- 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 1.
Differential Diagnosis
It is essential to distinguish SIADH from other causes of hyponatremia, including paraneoplastic hyponatremia secondary to elevated atrial natriuretic peptide and non-ADH-mediated causes such as sodium wasting due to drug nephrotoxicity or iatrogenic IV infusion of hypotonic fluid 1.
From the Research
Diagnosis of Hyponatremia
The diagnosis of hyponatremia involves a systematic approach to laboratory diagnosis, including:
- Measurement of serum sodium concentration, with hyponatremia defined as a serum sodium concentration <135mmol/l 2, 3, 4
- Assessment of extracellular volume status on physical examination 5
- Measurement of effective serum tonicity (serum osmolality less serum urea level) 5
- Determination of urine osmolality to determine whether water excretion is normal or impaired 6, 5
- Measurement of urine sodium level to determine the presence of hypovolemia or the syndrome of inappropriate antidiuretic hormone secretion 5
Diagnostic Approach
The diagnostic approach to hyponatremia involves differentiating between hypotonic and nonhypotonic hyponatremia, and further differentiating hypotonic hyponatremia based on urine osmolality, urine sodium level, and volume status 6. Recently identified parameters, such as fractional uric acid excretion and plasma copeptin concentration, may also be used to improve the diagnostic approach 6.
Laboratory Evaluation
The laboratory evaluation of hyponatremia includes:
- Measurement of serum osmolality and serum urea level to determine effective serum tonicity 5
- Measurement of urine osmolality and urine sodium level to determine the presence of hypovolemia or the syndrome of inappropriate antidiuretic hormone secretion 5
- Determination of hormone levels, such as thyroid-stimulating hormone and cortisol, in difficult cases of hyponatremia 5