What are the diagnostic criteria for hyponatremia (low sodium levels in the blood)?

From the Guidelines

Diagnosing hyponatremia requires a systematic approach beginning with confirmation of low serum sodium levels (below 135 mEq/L) through laboratory testing. First, assess the patient's volume status by examining for signs of dehydration or fluid overload, as hyponatremia can be classified as hypovolemic, euvolemic, or hypervolemic 1. Measure serum osmolality to differentiate between true hyponatremia (low osmolality) and pseudohyponatremia (normal osmolality). Urine sodium and osmolality tests are essential to determine if the kidneys are appropriately conserving or excreting sodium. Additional laboratory tests should include assessment of kidney function, thyroid function, adrenal function (morning cortisol), and glucose levels. A thorough medication review is crucial as many drugs can cause hyponatremia, including diuretics, antidepressants, and anticonvulsants.

Key Diagnostic Steps

• Assess volume status to classify hyponatremia as hypovolemic, euvolemic, or hypervolemic
• Measure serum osmolality to differentiate between true and pseudohyponatremia
• Conduct urine sodium and osmolality tests to evaluate kidney function
• Review medications for potential causes of hyponatremia
• Explore potential underlying causes such as SIADH, heart failure, cirrhosis, or adrenal insufficiency 1

Importance of Accurate Diagnosis

Accurate diagnosis of hyponatremia is critical to guide appropriate treatment strategies and improve patient outcomes. Hyponatremia is associated with increased mortality and morbidity, particularly neurological complications, and reduced survival after liver transplantation 1. Therefore, a comprehensive diagnostic approach is essential to properly classify hyponatremia and initiate effective treatment.

From the Research

Diagnosis of Hyponatremia

To diagnose hyponatremia, the following steps can be taken:

• Measure the serum sodium concentration, with hyponatremia defined as a level of less than 135 mmol/L 2, 3, 4, 5
• Assess the patient's fluid volume status, categorizing them as hypovolemic, euvolemic, or hypervolemic 2, 3, 5
• Measure serum and urine osmolality, as well as urine sodium concentration, to help establish the primary underlying causes of hyponatremia 2, 5
• Take a history of concurrent illness and medication use, as certain conditions and medications can contribute to the development of hyponatremia 2, 5

Laboratory Evaluation

The laboratory evaluation of hyponatremia may involve:

• Measurement of effective serum tonicity (serum osmolality less serum urea level) 5
• Urine osmolality to determine whether water excretion is normal or impaired 5
• Urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion 5
• Hormone levels (thyroid-stimulating hormone and cortisol) and arterial blood gases in difficult cases of hyponatremia 5

Clinical Presentation

Hyponatremia can present with a range of symptoms, from mild and nonspecific to severe and life-threatening:

• Mild symptoms may include weakness, nausea, and headache 3
• Severe symptoms may include seizures, coma, and cardiorespiratory distress 3, 6
• The severity of symptoms depends on the rapidity of development, duration, and severity of hyponatremia 3