From the Guidelines
The first test to diagnose hyponatremia is a serum sodium measurement, typically obtained through a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP) blood test. This test measures the concentration of sodium in the blood, with hyponatremia defined as a serum sodium level below 135 mmol/L, as suggested by guidelines on hyponatremia in the general patient population 1. When a patient presents with symptoms that might suggest hyponatremia, or when routine bloodwork reveals a low sodium level, this initial measurement confirms the presence of hyponatremia.
The importance of this test is highlighted by the fact that hyponatremia is associated with increased mortality and morbidity, particularly neurological complications, and reduced survival after liver transplantation, as noted in patients with advanced cirrhosis 1. Key points to consider when interpreting the results of this test include:
- The definition of hyponatremia as a serum sodium concentration lower than 130 mmol/L in patients with advanced cirrhosis, but also considering reductions below 135 mmol/L as significant in the general patient population 1
- The differentiation between hypovolaemic and hypervolaemic hyponatremia, which can both occur in patients with cirrhosis and have different underlying causes and implications for treatment
- The need for additional tests after confirming hyponatremia to determine the underlying cause, including assessments of volume status, serum and urine osmolality, and urine sodium concentration.
From the Research
Diagnosis of Hyponatremia
To diagnose hyponatremia, the first step is to measure the serum sodium level. According to 2, hyponatremia is defined as a serum sodium concentration of less than 135 mEq/L. The following are the key points to consider when diagnosing hyponatremia:
- Measure serum sodium level to confirm hyponatremia
- Assess the severity of hyponatremia, which can be mild (130-134 mEq/L), moderate (125-129 mEq/L), or severe (less than 125 mEq/L)
- Evaluate the patient's symptoms, which can range from mild (nausea, vomiting, weakness) to severe (delirium, confusion, impaired consciousness, ataxia, seizures)
Diagnostic Approach
The diagnostic approach to hyponatremia involves differentiating between hypotonic and nonhypotonic hyponatremia, as well as assessing the patient's volume status. As stated in 3, the initial step is to differentiate hypotonic from nonhypotonic hyponatremia, and then further differentiate hypotonic hyponatremia on the basis of urine osmolality, urine sodium level, and volume status. The key points to consider are:
- Differentiate between hypotonic and nonhypotonic hyponatremia
- Assess urine osmolality, urine sodium level, and volume status to determine the underlying cause of hyponatremia
- Use recently identified parameters, such as fractional uric acid excretion and plasma copeptin concentration, to further improve the diagnostic approach
Tests to Diagnose Hyponatremia
The following tests can be used to diagnose hyponatremia:
- Serum sodium level measurement
- Urine osmolality test
- Urine sodium level test
- Assessment of volume status, including measurement of blood pressure, pulse, and jugular venous distension As mentioned in 4, these tests can help determine the underlying cause of hyponatremia and guide treatment.
Treatment of Hyponatremia
Treatment of hyponatremia depends on the severity and duration of the condition, as well as the underlying cause. According to 5, treatment options include:
- Fluid restriction
- Hypertonic saline infusion for severe or symptomatic hyponatremia
- Vasopressin receptor antagonists, urea, and loop diuretics to increase renal free water excretion
- Treatment of the underlying cause, such as hypothyroidism or heart failure, as mentioned in 6