![]() U Osm ≤ 100 mOsm/kg H 2O: Dilute urine ( ADH is suppressed ) indicates excess water intake.Interpretation of urine osmolality (U Osm): to determine antidiuretic hormone ( ADH) activity.Consider also: urea (U Urea), uric acid (U UA).Serum studies: CBC, BMP consider uric acid.Diagnostic evaluation of hyponatremia based on serum osmolality Hypotonic hyponatremia Serum osmolality measurement is the first step in the evaluation of verified hyponatremia. Initiate treatment immediately for acute or severely symptomatic hyponatremia. Consider additional focused diagnostic evaluation to identify the underlying cause.Normal serum osmolality ( 280–295 mOsmol/kg H 2O ): isotonic hyponatre mia or pseudohyponatremia.Low serum osmolality ( 295 mOsmol/kg H 2O ): hypertonic hyponatremia.Measured in serum or calculated (see “ Hyponatremia formulas” below).Check the serum osmolality (S Osm): first step in the evaluation of confirmed hyponatremia.Exclude hyperglycemia : Check serum glucose.Pathophysiologyĭiagnostic approach to hyponatremia Isotonic hyponatremia should always be excluded as a cause of hyponatremia to avoid unnecessarily aggressive treatment. Due to very high amounts of protein or lipids in the plasma (e.g., hyperlipidemia, multiple myeloma), which then alter the plasma water concentration.Asymptomatic laboratory artifact falsely indicating hyponatremia when sodium has not been reduced or diluted.Definition : : low measured serum Na + concentration and normal serum osmolality.Definition: serum sodium level 295 mOsm/kg H 2O.Thiazide diuretic use and SIADH are the most common causes of hyponatremia in the emergency department. Severe hypoproteinemia (e.g., nephrotic syndrome).Excessive infusion of hypotonic (e.g., 0.45% NaCl) or sodium-free isotonic IV fluids.Water intoxication (dilutional hyponatremia).Decreased salt intake (e.g., “tea and toast” diet).Third space fluid loss (e.g., peritonitis, ascites).Dermal fluid loss (e.g., burns, sweating). #Concerning body fluid compartments free#Acute or chronic renal failure with low urine output (i.e., failure to excrete free water).Glucocorticoid deficiency ( adrenal insufficiency).Exercise-associated hyponatremia ( EAH).Recovery phase of acute tubular necrosis.Mineralocorticoid deficiency ( Addison disease ).Acute or chronic renal failure with high urine output ( polyuria).Normal or minimal changes in extracellular fluid volume.See the section “Core IM podcast 5 pearls on hyponatremia (episode 1: diagnosis)” for their show notes on this topic. A rapid increase of the serum sodium concentration can have damaging osmotic effects, as seen in osmotic demyelination syndrome. Treatment involves careful correction of the sodium deficit and/or fluid imbalance. Some conditions with very high protein (e.g., multiple myeloma) or glucose levels (e.g., DKA) in the blood may result in a laboratory artifact of falsely low serum sodium concentrations ( pseudohyponatremia). Investigating the cause of hyponatremia requires consideration of the patient's volume status and renal ability to retain sodium. The onset can be acute or chronic and symptoms are predominantly neurological and often nonspecific (e.g., nausea, headache, confusion). Causes of hyponatremia include dehydration, excessive free water intake (e.g., primary polydipsia), and increased release of ADH causing reabsorption of free water in the kidneys (e.g., SIADH, CHF). Sodium is the most important osmotically active particle in the extracellular space and is closely linked to the body's fluid balance. Hyponatremia is a state of low sodium levels ( < 135 mEq/L). ![]()
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