Purpose of Serum Osmolality in Hyponatremia
Measuring serum osmolality is the essential first step in evaluating hyponatremia because it distinguishes true hypotonic hyponatremia from pseudohyponatremia and hypertonic hyponatremia, fundamentally determining whether the patient has actual cellular overhydration requiring treatment. 1, 2
Primary Diagnostic Function
Serum osmolality measurement serves to classify hyponatremia into three critical categories that require completely different management approaches 3:
- Hypotonic hyponatremia (serum osmolality <280 mOsm/kg): True hyponatremia with cellular overhydration requiring treatment 2, 3
- Isotonic hyponatremia (serum osmolality 280-295 mOsm/kg): Pseudohyponatremia from laboratory artifact, hyperlipidemia, or hyperproteinemia—no treatment needed 4, 2
- Hypertonic hyponatremia (serum osmolality >295 mOsm/kg): Caused by hyperglycemia or mannitol administration—requires treating the underlying cause, not the sodium 4, 3
Clinical Significance
The main deleterious effect of hyponatremia is cell overhydration, which occurs only when effective osmolality is reduced. 3 Without measuring serum osmolality, you risk treating patients who don't need treatment (pseudohyponatremia) or missing the actual problem (hyperglycemia causing hypertonic hyponatremia). 2, 3
Diagnostic Algorithm After Osmolality Measurement
Once hypotonic hyponatremia is confirmed (osmolality <280 mOsm/kg), the diagnostic workup proceeds with 1, 2:
- Urine osmolality: Determines if ADH secretion is appropriately suppressed (<100 mOsm/kg) or inappropriately elevated (>100 mOsm/kg) 2, 5
- Urine sodium concentration: Differentiates between hypovolemic causes (<30 mmol/L) versus SIADH or other causes (>20-40 mmol/L) 1, 2
- Volume status assessment: Classifies as hypovolemic, euvolemic, or hypervolemic hyponatremia 1, 4
Common Pitfalls
Never skip serum osmolality measurement and proceed directly to treatment—this is the most critical error in hyponatremia management. 2, 3 Pseudohyponatremia requires no treatment, and treating it with hypertonic saline or fluid restriction can cause harm. 4, 2
Laboratory parameters are much more reliable than physical examination for determining the true nature of hyponatremia, with physical exam having only 41.1% sensitivity and 80% specificity for volume status assessment. 1, 3
For hyperglycemia-induced hyponatremia, correct the measured sodium by adding 1.6 mEq/L for each 100 mg/dL glucose above 100 mg/dL to determine the true sodium level. 1