What labs should be checked to diagnose and manage hyponatremia?

From the Guidelines

To diagnose and manage hyponatremia, initial laboratory tests should include serum sodium, osmolality, urine sodium and osmolality, serum potassium, glucose, creatinine, BUN, and thyroid function tests (TSH, free T4) 1. These tests are crucial for determining the type and cause of hyponatremia. Additional essential labs may include cortisol levels to rule out adrenal insufficiency, a complete blood count, and liver function tests to provide context about overall health status. Volume status assessment requires checking urine electrolytes (sodium, potassium) and calculating the fractional excretion of sodium. In cases of suspected SIADH, measuring serum uric acid and phosphate levels can be helpful. For patients on medications, drug levels may be necessary, particularly for diuretics, antipsychotics, or antidepressants. Serial monitoring of serum sodium is crucial during treatment to prevent overly rapid correction, which can lead to osmotic demyelination syndrome, as guided by the 2021 practice guidance by the American Association for the Study of Liver Diseases 1. Key considerations in managing hyponatremia include the severity and chronicity of the condition, with mild hyponatremia (Na 126-135 mEq/L) in cirrhosis without symptoms not requiring specific management apart from monitoring and water restriction, while more severe cases may require water restriction, cessation of diuretics, albumin infusion, or the use of vasopressin receptor antagonists with caution 1. The goal rate of increase of serum sodium should be 4-6 mEq/L per 24-hour period, not to exceed 8 mEq/L per 24-hour period to ameliorate the risk of ODS, especially in patients with severe hyponatremia (<120 mEq/L) at the time of liver transplantation 1.

Some key points to consider in the management of hyponatremia include:

• The importance of monitoring serum sodium levels closely during treatment to avoid overly rapid correction 1
• The use of vasopressin receptor antagonists for short-term treatment (≤30 days) in cirrhosis with caution 1
• The reservation of hypertonic saline for short-term treatment of patients with symptomatic or severe hyponatremia or those with imminent liver transplantation 1
• The consideration of multidisciplinary coordinated care to mitigate the risk of ODS in patients with severe hyponatremia undergoing liver transplantation 1.

Overall, the management of hyponatremia requires a tailored approach based on the underlying cause, severity, and chronicity of the condition, as well as careful monitoring to prevent complications such as osmotic demyelination syndrome, as recommended by the most recent and highest quality study 1.

From the Research

Labs to Check for Hyponatremia

To diagnose and manage hyponatremia, several labs should be checked, including:

• Serum sodium level to confirm hyponatremia, defined as a serum sodium concentration of <135 mmol/L 2, 3
• Serum osmolality to determine the effective serum tonicity and exclude pseudohyponatremia 4
• Urine osmolality to determine whether water excretion is normal or impaired 4
• Urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) 4, 5
• Urea and uric acid levels, which are typically low in SIADH 5
• Thyroid-stimulating hormone (TSH) and cortisol levels to rule out hypothyroidism and hypocorticism, respectively 4, 6
• Arterial blood gases to assess for any acid-base disturbances 4

Additional Tests

In certain cases, additional tests may be necessary, such as:

• High-dose adrenocorticotropic hormone (ACTH) test to evaluate adrenal function 6
• Low-dose ACTH test to assess for central hypoadrenalism 6
• Imaging studies, such as MRI or CT scans, to evaluate for pituitary or hypothalamic disease 6

Interpretation of Lab Results

The interpretation of lab results is crucial in diagnosing and managing hyponatremia. For example:

• A high urine osmolality (>600 mOsm/kg) may indicate SIADH or another cause of inappropriate antidiuretic hormone secretion 5
• A low urine sodium level (<20 mmol/L) may indicate hypovolemia, while a high urine sodium level (>40 mmol/L) may indicate SIADH 4, 5
• Low levels of urea and uric acid may be seen in SIADH, but are less specific in elderly patients 5