From the Guidelines
Normal urine osmolality ranges from 300 to 900 mOsm/kg, while normal urine sodium typically ranges from 40 to 220 mEq/L (or mmol/L) in a 24-hour collection. These values can vary significantly based on hydration status, diet, and kidney function. Urine osmolality reflects the kidney's concentrating ability, with higher values indicating more concentrated urine as seen in dehydration, while lower values suggest dilute urine as in overhydration or certain kidney disorders. According to 1, inappropriately high urine osmolality (> 500 mosm/kg) and inappropriately high urinary sodium concentration (> 20 mEq/L) are characteristic of the Syndrome of Inappropriate Antidiuretic Hormone (SIADH). However, a more recent study 1 suggests that a urine osmolality of < 700 mOsmol/kg or a specific gravity of < 1.020 indicates euhydration, while > 900 mOsmol/kg indicates dehydration.
Key Points to Consider
- Urine sodium levels are influenced by sodium intake, aldosterone levels, and kidney function.
- Low urine sodium (<20 mEq/L) may indicate volume depletion, heart failure, or cirrhosis, while high values can reflect excessive sodium intake or certain medications like diuretics.
- These measurements are particularly useful in evaluating fluid and electrolyte disorders, kidney function, and distinguishing between different causes of hyponatremia or acute kidney injury.
- First-morning urine samples typically provide the most accurate assessment of concentrating ability.
Clinical Application
In clinical practice, it is essential to consider the patient's hydration status, diet, and kidney function when interpreting urine osmolality and sodium levels. For instance, a patient with a urine osmolality of 400 mOsm/kg and a urine sodium level of 30 mEq/L may be considered normal if they are well-hydrated and have normal kidney function. However, if the patient is dehydrated or has kidney disease, these values may indicate an underlying disorder. Therefore, it is crucial to consider the clinical context and use these values in conjunction with other diagnostic tests to guide patient management.
From the Research
Urine Osmolality
- Urine osmolality indicates the ability of the kidney to concentrate the urine and reflects the antidiuretic action of vasopressin 2.
- Low urine osmolality was independently associated with a greater risk of CKD progression (hazard ratio [HR], 1.71; 95% confidence interval [CI], 1.12-2.59) 2.
- The mean osmolality was 603.8 mOsm/kg and 629.1 mOsm/kg in those with and without decreased eGFR and/or albuminuria, respectively 3.
- Higher osmolality was significantly associated with lower eGFR among adults with eGFR ≥ 60 mL/min/1.73m2 3.
Urine Sodium
- Plasma osmolality, urine osmolality, and urine sodium can help to differentiate among the different etiologies of hyponatremia 4.
- Urine electrolytes and urine osmolality are useful in the clinical diagnosis of patients with fluid, electrolytes, and acid-base disorders, emphasizing their physiological basis, their utility, and the caveats and limitations in their use 5.
Normal Ranges
- There is no specific normal range for urine osmolality and urine sodium mentioned in the provided studies.
- However, a study found that osmolality ≥ 500 mOsm/kg was associated with lower eGFR (adjusted ß -1.13,95% CI -1.98, -0.28) 3.