What is a 24-Hour Urine Sodium Test?
The 24-Hour Urine Sodium Test measures the total amount of sodium (Na⁺) excreted in the urine over a complete 24-hour period.
Sodium is an essential electrolyte that helps regulate fluid balance, blood pressure, nerve function, and muscle activity. The kidneys control sodium levels by adjusting how much sodium is retained or eliminated in urine.
Because this test measures sodium excretion over an entire day, it can provide a better estimate of daily sodium excretion and dietary sodium intake than a single spot urine sample.
What is it used for?
The test may be used to:
Estimate daily sodium intake
Evaluate sodium and fluid balance
Investigate certain kidney disorders
Assess abnormal blood sodium levels in selected cases
Evaluate suspected abnormalities in renal sodium handling
Support assessment of hypertension
Monitor adherence to a prescribed low-sodium diet
Investigate certain endocrine or hormonal disorders affecting sodium balance
Why might I need this test?
Your healthcare provider may request a 24-hour urine sodium test if you have:
High blood pressure and need an assessment of sodium intake
Unexplained abnormalities in sodium or fluid balance
Suspected kidney disease or renal sodium-wasting
Certain adrenal or hormonal disorders
Edema or abnormal fluid retention
A medical need to monitor sodium intake
What happens during the test?
You will collect all urine produced during a 24-hour period in a special container.
Typically:
At the designated starting time, empty your bladder and do not collect this first urine.
Record the starting time.
Collect all urine passed afterward, including overnight urine.
At exactly 24 hours, collect the final urine specimen.
Return the complete collection to the laboratory.
Do not discard any urine during the collection period. Missing even one or more collections can make the result inaccurate.
Will I need to do anything to prepare?
Usually, no fasting is required.
Continue your usual diet unless your healthcare provider has specifically prescribed a sodium-restricted diet.
Tell your healthcare provider about medications and supplements you are taking, particularly diuretics and medications that affect sodium balance.
Do not change or stop medication unless instructed by your healthcare provider.
Are there any risks?
There are no significant medical risks associated with urine collection.
The main concern is an incomplete 24-hour collection, which can result in an inaccurately low sodium excretion measurement.
What do the results mean?
Results are commonly reported as mmol/24 hours or mEq/24 hours.
Result Possible interpretation
High urine sodium May reflect high dietary sodium intake, increased sodium excretion, or certain kidney/endocrine conditions
Low urine sodium May reflect low sodium intake, increased sodium retention, dehydration, or reduced effective circulating volume
Normal urine sodium Generally indicates sodium excretion within the laboratory's expected range
The laboratory's own reference interval should be used because ranges can vary depending on the method and clinical population.
24-Hour Urine Sodium and Dietary Sodium Intake
Most dietary sodium is ultimately excreted through the kidneys. Therefore, 24-hour urinary sodium excretion can be used as an objective estimate of sodium intake.
However, it is not a perfect measurement because:
Some sodium is lost through sweat and stool.
Collection may be incomplete.
Sodium intake can vary considerably from day to day.
Certain medications can alter sodium excretion.
A single 24-hour collection therefore represents sodium intake over approximately that collection period rather than necessarily a person's usual long-term intake.
What Can Affect Urine Sodium?
Urinary sodium excretion can be influenced by:
Dietary salt and sodium intake
Fluid intake
Diuretics
Kidney function
Blood pressure
Hormones such as aldosterone
Dehydration or volume depletion
Heart, liver, or kidney conditions
Sodium-restricted diets
24-Hour Urine Sodium vs. Spot Urine Sodium
24-hour urine sodium measures the total amount of sodium excreted during an entire day and is generally more useful when an accurate estimate of daily sodium excretion is required.
Spot urine sodium uses a single urine sample and is easier to obtain. It can be useful for assessing renal sodium handling in specific clinical situations, but hydration and urine concentration can substantially affect the result.
Important Note
A 24-hour urine sodium result should be interpreted together with the patient's diet, fluid status, blood pressure, kidney function, medications, and clinical condition.
If the test is being used to estimate dietary sodium intake, completeness of the 24-hour collection is essential. Urine creatinine may also be assessed to help evaluate whether the collection is likely to be complete.
Reference ranges vary according to the laboratory, method, age, and clinical context. Always interpret the result using the reference interval provided by the testing laboratory.
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