A joint European clinical practice guideline on the diagnosis and treatment of hyponatremia defines hyponatremia as follows:
Mild: serum sodium concentration 130-135 mmol/L
Moderate: serum sodium concentration 125-129 mmol/L
Profound: serum sodium concentration < 125 mmol/L
Acute: documented as lasting < 48 hours
Chronic: documented as lasting ≥ 48 hours, or duration cannot be classified
The guidelines state that a measured serum osmolality < 275 mOsm/kg always indicates hypotonic hyponatremia because effective osmolality can never be higher than total or measured osmolality. However, if the calculated osmolality is < 275 mOsm/kg, hyponatremia can be hypotonic, isotonic, or hypertonic, depending on which osmotically active agents are present and whether they are incorporated in the formula.
To differentiate the causes of hypotonic hyponatremia, the guideline recommends interpreting the osmolality of a spot urine sample as the first step, as follows:
If urine osmolality is ≤ 100mOsm/kg, accept relative excess water intake as a cause of the hypotonic hyponatremia.
If urine osmolality is > 100mOsm/kg, interpret the urine sodium concentration on a spot urine sample obtained simultaneously with a blood sample.
Related guideline suggestions are as follows:
If urine sodium concentration is ≤ 30 mmol/L, accept low effective arterial volume as a cause.
If urine sodium concentration is > 30 mmol/L, assess extracellular fluid status and use of diuretics to further differentiate likely causes of hyponatremia.
Measuring vasopressin for confirming the diagnosis of SIADH is not suggested.
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Cite this: Romesh Khardori. Fast Five Quiz: Hyponatremia Essentials - Medscape - Oct 14, 2019.