Overview
This note summarizes the diagnostic approach to hyponatremia, focusing on identifying its underlying cause using laboratory algorithms and clinical context, along with key pitfalls in interpretation.
Initial Diagnosis and Classification
- Hyponatremia is diagnosed by finding serum sodium less than 135 mmol/L on blood tests.
- Determine etiology by assessing serum osmolality to differentiate true from false hyponatremia.
- Low serum osmolality indicates true hyponatremia.
- Normal serum osmolality suggests pseudo-hyponatremia, often due to high fats or proteins.
- High serum osmolality (factitious hyponatremia) is typically from hyperglycemia, mannitol, or glycine infusions.
True Hyponatremia: Next Steps
- Check urine osmolality or specific gravity to assess ADH activity.
- Low urine osmolality indicates ADH-independent processes such as low solute diets (tea and toast syndrome, beer drinker's potomania) or psychogenic polydipsia.
- Psychogenic polydipsia may require acute therapy with hypertonic saline if symptomatic.
- Renal failure patients have urine osmolality near plasma osmolality and respond to fluid restriction or dialysis.
ADH-Dependent Hyponatremia
- High urine osmolality suggests ADH-dependent hyponatremia.
- Assess patient's volume status: hypervolemic (heart failure, cirrhosis, nephrotic syndrome), hypovolemic (GI, renal, or other losses), or euvolemic (hypothyroidism, adrenal insufficiency, SIADH).
Limitations in Volume Assessment
- Clinical assessment of volume status is imprecise; experts are correct only about 50% of the time.
- Biochemical markers such as urine sodium and serum uric acid are used for further differentiation.
Laboratory Characterization
- Hypervolemic and hypovolemic hyponatremia: urine sodium <20 mmol/L, high or high-normal uric acid.
- Euvolemic hyponatremia: urine sodium >20 mmol/L, low uric acid.
Caveats and Pitfalls
- Recent diuretic use may falsely elevate urine sodium, especially in heart failure.
- Low dietary sodium or NPO status can artificially lower urine sodium in euvolemic patients.
- Uric acid may be artificially low due to gout medications like allopurinol.
Recommendations / Advice
- Use an algorithmic approach integrating lab results and clinical context to identify the underlying cause of hyponatremia.
- Be cautious interpreting urine sodium and uric acid in patients on diuretics, restricted diets, or uric acid–lowering therapy.