How to Measure Urine Osmolality in Hospitalized Elderly Patients with Suspected Hyponatremia
For elderly patients with suspected hyponatremia, obtain a spot urine sample and send it to the laboratory for direct osmolality measurement using freezing point depression, while simultaneously measuring urine sodium on the same sample. 1
Critical First Step: Confirm True Hyponatremia
Before measuring urine osmolality, you must first establish that the patient has true hypo-osmolar hyponatremia by measuring serum osmolality directly (not calculated) to exclude pseudohyponatremia. 1, 2 The threshold for true hypo-osmolality is <280 mOsm/kg. 1, 2
- If direct serum osmolality measurement is unavailable, use the validated equation: Osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with an action threshold of >295 mmol/L indicating dehydration rather than hyponatremia. 3
- Check serum glucose and urea to ensure they are within normal range, as elevated levels falsely elevate osmolality and must be normalized before interpretation. 1, 4
Obtaining the Urine Sample
Collect a random spot urine sample (not a 24-hour collection) when serum osmolality is confirmed <280 mOsm/kg. 1
- Send the sample for both urine osmolality AND urine sodium measurement simultaneously, as interpretation requires both values together. 1, 2
- The laboratory will measure urine osmolality using freezing point depression, which is the gold standard direct measurement method. 3
Interpreting Urine Osmolality Results
Urine osmolality <100 mOsm/kg indicates appropriate ADH suppression and suggests primary polydipsia. 1, 2
Urine osmolality >100 mOsm/kg indicates impaired water excretion and requires evaluation of urine sodium:
- Urine sodium <20 mmol/L suggests hypovolemic hyponatremia (volume depletion). 1, 2
- Urine sodium >40 mmol/L suggests SIADH or renal/endocrine causes. 1, 2
Critical Pitfalls to Avoid
Do NOT rely on single urine osmolality measurements in patients with fever or critical illness, as non-osmotic ADH stimulation causes hourly fluctuations ranging from 100-600 mOsm/kg. 1, 5 In febrile patients, body temperature correlates with urine osmolality fluctuations, making interpretation unreliable. 5
Do NOT use urine specific gravity, urine color, or clinical signs (skin turgor, mouth dryness) to assess hydration status in elderly patients, as these have inadequate diagnostic accuracy. 3, 4 The ESPEN guidelines give these methods a Grade A recommendation against their use. 4
Check for unmeasured osmoles (ethanol, mannitol, glycine) that can interfere with osmolality interpretation and obscure the diagnosis. 1, 6
Special Considerations for Elderly Patients
Elderly patients have baseline higher urine osmolality (mean 581 ± 174 mOsm/kg, with normal range 407-755 mOsm/kg), with females having significantly higher values than males. 7 This age-related difference reflects reduced renal concentrating ability and must be considered when interpreting results. 3
All older adults should be considered at risk for low-intake dehydration and screened when they contact healthcare or experience clinical deterioration. 8 However, note that urine osmolality is useful for evaluating hypovolemic hyponatremia but NOT for assessing low-intake dehydration, which requires serum osmolality measurement instead. 1