From the Guidelines
For hyponatremia, the essential lab work should include serum sodium, serum osmolality, urine sodium, and urine osmolality, as these tests help determine the cause and guide treatment. Additional important tests include serum potassium, blood urea nitrogen (BUN), creatinine, glucose, thyroid-stimulating hormone (TSH), cortisol levels, and liver function tests 1. A complete blood count and assessment of volume status through physical examination are also necessary. Serum sodium confirms hyponatremia (typically <135 mEq/L), while serum osmolality helps distinguish between true hyponatremia (low osmolality) and pseudohyponatremia (normal osmolality). Urine studies help categorize the condition as either dilutional (SIADH, heart failure) or depletional (diuretic use, adrenal insufficiency). Glucose is important because hyperglycemia can cause factitious hyponatremia. Hormone tests like TSH and cortisol are needed because thyroid dysfunction and adrenal insufficiency can cause hyponatremia.
Some key points to consider when evaluating hyponatremia include:
- The importance of assessing volume status, as this can help guide treatment and determine the underlying cause of hyponatremia
- The need to evaluate urine sodium and osmolality, as these can help distinguish between different causes of hyponatremia
- The potential for certain medications, such as diuretics, to contribute to hyponatremia
- The importance of evaluating liver function, as liver disease can contribute to hyponatremia
It's also important to note that the treatment of hyponatremia depends on the underlying cause, and that a comprehensive diagnostic evaluation, including the lab work mentioned above, is essential for determining the best course of treatment. In some cases, treatment may involve fluid restriction, while in others it may involve the use of medications such as diuretics or aquaretic agents 1.
Key lab tests for hyponatremia include:
- Serum sodium
- Serum osmolality
- Urine sodium
- Urine osmolality
- Serum potassium
- Blood urea nitrogen (BUN)
- Creatinine
- Glucose
- Thyroid-stimulating hormone (TSH)
- Cortisol levels
- Liver function tests
- Complete blood count
These tests can help determine the underlying cause of hyponatremia and guide treatment. It's essential to prioritize these tests when evaluating a patient with hyponatremia, as they can help improve outcomes and reduce morbidity and mortality.
From the Research
Laboratory Evaluation of Hyponatremia
To evaluate hyponatremia, the following laboratory tests are recommended:
- Serum sodium level to confirm the diagnosis of hyponatremia 2, 3, 4, 5
- Serum osmolality to determine the effective serum tonicity 3
- Urine osmolality to determine whether water excretion is normal or impaired 3
- Urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion 3
- Thyroid-stimulating hormone and cortisol levels in difficult cases of hyponatremia 3
- Arterial blood gases in difficult cases of hyponatremia 3
- Basic metabolic panel to evaluate electrolytes, blood urea nitrogen, and creatinine 6
Additional Tests
Additional tests may be ordered based on the patient's clinical presentation and medical history, such as:
- Point-of-care ultrasonography to estimate volume status 5
- Other case-specific laboratory parameters to evaluate the underlying cause of hyponatremia 5
Interpretation of Laboratory Results
The interpretation of laboratory results should be done in conjunction with the patient's clinical presentation and medical history. For example:
- A urine sodium level less than 20 mmol/L is indicative of hypovolemia, whereas a level greater than 40 mmol/L is suggestive of the syndrome of inappropriate antidiuretic hormone secretion 3
- A serum osmolality less than 280 mOsm/kg indicates a hypo-osmolar state, which requires further evaluation to determine the underlying cause 3