From the Guidelines
Urinary sodium is most accurately measured through a 24-hour urine collection, but a random "spot" urine sodium concentration that is greater than the potassium concentration can also be used as a reliable alternative, correlating with a 24-hour sodium excretion greater than 78 mmol/day with approximately 90% accuracy.
Measurement Methods
- A 24-hour urine collection is considered the gold standard for measuring urinary sodium excretion, as it provides a comprehensive assessment of sodium output over a full day 1.
- However, this method can be cumbersome and prone to errors due to incomplete collection.
- The completeness of a 24-hour urine collection can be verified by measuring urinary creatinine excretion, with men expected to excrete more than 15 mg/kg/day and women more than 10 mg/kg/day 1.
Alternative Method
- A random "spot" urine sample can be used as an alternative, with a sodium-to-potassium ratio greater than 1 indicating a 24-hour sodium excretion greater than 78 mmol/day with high accuracy 1.
Clinical Significance
- Urinary sodium measurement is crucial in managing patients with cirrhosis and ascites, as it helps guide diuretic therapy and assesses the need for sodium restriction 1.
- A urinary sodium excretion greater than 78 mmol/day is often used as a threshold to consider patients for dietary sodium restriction alone or in combination with diuretics 1.
From the Research
Measurement of Urinary Sodium
- Urinary sodium can be measured using a 24-hour urine sample, which is used to measure urinary osmolality and urinary sodium concentration 2.
- A spot urine sample can also be used to measure urinary sodium concentration, and it has been found to be sufficient for diagnostic purposes in some cases 2, 3.
- The urinary sodium/urinary creatinine ratio can be used to estimate urinary sodium excretion, and it has been found to be a reliable and practical method 3.
- Urinary sodium can be measured in spot urine samples, and it can be used to differentiate between prerenal and intrarenal acute kidney injury (AKI) 4.
Methods of Measurement
- 24-hour urine collection: This method involves collecting all urine produced by a patient over a 24-hour period, and it is used to measure urinary sodium excretion 2, 3.
- Spot urine sample: This method involves collecting a single urine sample from a patient, and it can be used to measure urinary sodium concentration 2, 3.
- Urinary sodium/urinary creatinine ratio: This method involves measuring the ratio of urinary sodium to urinary creatinine in a spot urine sample, and it can be used to estimate urinary sodium excretion 3.
Clinical Applications
- Diagnosis of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH): Urinary sodium concentration can be used to diagnose SIADH 2.
- Diagnosis of acute kidney injury (AKI): Urinary sodium concentration can be used to differentiate between prerenal and intrarenal AKI 4.
- Estimation of urinary sodium excretion: The urinary sodium/urinary creatinine ratio can be used to estimate urinary sodium excretion, which can be useful in the management of hypertension 3.