Biomarker: Sodium, Urine

Overview

Urine sodium measures the concentration or total amount of sodium excreted by the kidneys. Sodium helps regulate extracellular-fluid volume and supports normal nerve and muscle function. High or low urine sodium may reflect dietary intake, fluid balance, kidney handling of sodium, medications, or conditions such as dehydration, heart failure, adrenal disorders, or kidney disease. The test is clinically useful for evaluating abnormal blood sodium levels, assessing renal sodium handling, investigating kidney disease, and estimating dietary sodium intake when a complete 24-hour collection is obtained.

Clinical Use Cases

Help evaluate the cause of hyponatremia or other abnormal blood sodium levels.
Assess whether the kidneys are appropriately conserving or excreting sodium.
Support evaluation of volume depletion, heart failure, cirrhosis, adrenal disorders, and renal salt-wasting conditions.
Assist in evaluating acute kidney injury, including calculation of fractional excretion of sodium (FENa) using paired urine and blood sodium and creatinine measurements.
Support assessment and monitoring of kidney disease.
Estimate dietary sodium intake in clinical research or population studies using complete 24-hour urine collections.
Monitor urinary sodium excretion during dietary or therapeutic interventions.

Specimen Types

Random, or spot, urine specimen.
Timed urine specimen, including shorter timed collections when clinically indicated.
Complete 24-hour urine collection, particularly for estimating sodium intake or total daily sodium excretion.
Paired urine and blood specimens when calculating FENa.

Measurement Methods

Ion-selective electrode potentiometry, commonly used in automated clinical chemistry analyzers.
Flame emission photometry, an established method for sodium measurement in urine.
Ion chromatography in specialized or research laboratories.
Atomic absorption or plasma emission spectrometry in specialized analytical settings.
Calculation of FENa from urine and plasma sodium and creatinine concentrations.

Test Preparation and Influencing Factors

Follow the collection instructions precisely, especially for a 24-hour collection, because incomplete collection can produce misleading results.
A random urine test generally does not require fasting, unless other tests ordered at the same time require it.
Maintain usual dietary sodium and fluid intake unless the healthcare professional provides different instructions.
Diuretics can increase urinary sodium and substantially affect interpretation.
Corticosteroids, nonsteroidal anti-inflammatory drugs, and prostaglandin-related medicines may affect results.
Do not stop prescribed medicines without medical advice.
Recent changes in dietary sodium or fluid intake can alter urinary sodium excretion.
Dehydration, diarrhea, heart failure, cirrhosis, adrenal disorders, and kidney disease may change sodium excretion independently of usual dietary intake.
Collection timing affects spot urine results because sodium excretion varies during the day and with recent meals.
Spot urine sodium reflects a limited time period and is less reliable than repeated complete 24-hour collections for estimating usual individual sodium intake.
Urine volume and creatinine excretion can affect concentration-based interpretation.
For FENa, urine and blood samples should be collected at approximately the same time, and recent diuretic exposure may reduce interpretive accuracy.