"Best of" | Hyponatremia Diagnostics 5 Pearls

Understanding hyponatremia: common pitfalls and diagnostic approach

Hyponatremia, a condition characterized by low sodium levels, is a complex issue that often presents diagnostic challenges. The first key point to remember is that hyponatremia is primarily a water problem. The diagnostic approach involves understanding the activity of antidiuretic hormone (ADH), which controls water reabsorption in the kidneys. Urine osmolality is the main tool to assess ADH activity. A low urine osmolality indicates that ADH is suppressed, usually due to a low effective blood volume or excess water intake. On the other hand, a high urine osmolality suggests ADH activation, either due to low effective blood volume or inappropriate secretion. To differentiate between appropriate and inappropriate ADH secretion, urine sodium levels are examined. A low urine sodium concentration indicates that ADH is appropriately activated in response to low effective blood volume. Conversely, a high urine sodium concentration suggests inappropriate ADH secretion. In cases where urine sodium is indeterminate, fractional excretion of sodium (FENa) can be calculated to provide additional information. Serum uric acid levels can also be used to differentiate between SIADH and other causes of hyponatremia, with low levels indicating SIADH. However, caution must be taken with interpreting serum uric acid levels, as certain conditions may affect its accuracy. Overall, a thorough understanding of ADH activity, urine osmolality, urine sodium, and serum uric acid levels enables a more accurate diagnostic approach to hyponatremia.