From the Guidelines
The normal range for urine osmolality is typically <700 mOsmol/kg for euhydration, while the normal range for urine sodium is not explicitly defined in the most recent and highest quality study, but generally, a random “spot” urine sodium concentration that is greater than the potassium concentration correlates with a 24-hour sodium excretion greater than 78 mmol/day with approximately 90% accuracy 1. Urine osmolality reflects the kidney's concentrating ability, with higher values indicating more concentrated urine and lower values suggesting dilute urine. Dehydration typically increases urine osmolality as the body conserves water, while overhydration decreases it. These parameters are useful in evaluating fluid and electrolyte balance, kidney function, and various clinical conditions such as syndrome of inappropriate antidiuretic hormone secretion (SIADH), diabetes insipidus, and acute kidney injury. Values outside these ranges may warrant further investigation depending on the clinical context, and a comprehensive evaluation including serum sodium, serum osmolality, and urine osmolality is essential for diagnosis and management of related conditions 1. Some key points to consider include:
- Urine osmolality <700 mOsmol/kg suggests euhydration, while >900 mOsmol/kg indicates dehydration 1
- A random “spot” urine sodium concentration that is greater than the potassium concentration correlates with a 24-hour sodium excretion greater than 78 mmol/day with approximately 90% accuracy 1
- The detection of inappropriately diluted urine (that is, urinary osmolality <200 mOsm/kg H2O), in combination with high–normal or elevated serum sodium is pathognomonic for the diagnosis of diabetes insipidus (nephrogenic or central) and warrants early genetic testing if NDI is suspected 1
From the Research
Normal Ranges for Urine Osmolality and Urine Sodium
- Urine osmolality: The normal range for urine osmolality is not explicitly stated in the provided studies, but it is used to assess antidiuretic hormone action and the osmolality of the renal medulla, and to determine the etiology of polyuria and/or hypernatremia 2.
- Urine sodium: The normal range for urine sodium is also not explicitly stated, but it is used to detect a mild to moderate degree of reduction of the 'effective' intravascular volume, and to examine the renal response to hypokalemia or hyperkalemia 2.
Clinical Use of Urine Osmolality and Urine Sodium
- Urine osmolality and urine sodium are used in the differential diagnosis of hyponatremic states, and to assess the etiology of polyuria and/or hypernatremia 3, 4.
- The measurement of urine osmolality and urine sodium can help to differentiate among the different etiologies of hyponatremia, and to guide management strategies 4.
- The use of urine electrolytes and urine osmolality in the clinical diagnosis of fluid, electrolytes, and acid-base disorders must integrate information obtained from the history, the physical examination, and other laboratory data 5.
Limitations and Caveats
- There are no 'normal values' for urine osmolality and urine sodium, only 'expected values' relative to clinical situations 2.
- The interpretation of urine osmolality and urine sodium can be influenced by various factors, such as abnormal renal and adrenal function, and the use of diuretics 2.
- The measurement of urine osmolality and urine sodium should be used in conjunction with other laboratory data and clinical information to guide diagnosis and management 3, 5.