Urine Sodium

Urine sodium (Na) analysis is usually ordered when it is necessary to distinguish between various forms of renal failure and to classify hyponatremia.[1]

Normal urine sodium values include the following[2] :

• Reference range: 40-220 mEq/day or 40-220 mmol/day (SI units)
• Spot urine: >20 mEq/L
• Fractional excretion of sodium (FE _Na): 1-2%

Hyponatremia

In evaluating the possible causes of hyponatremia, first establish the patient's volume status. The following algorithm is useful in diagnosing cause as it relates to volume:

• Edematous/hypervolemic patients - Urine Na is less than 10 mEq/L in congestive heart failure, nephrotic syndrome, and cirrhosis; urine Na is greater than 20 mEq/L in renal failure with impaired water excretion
• Hypovolemic patients - Urine Na is less than 10 mEq/L in excessive gastrointestinal (GI) losses, skin losses, or third spacing (eg, pancreatitis); urine Na is greater than 20 mEq/L in diuretic use, salt-wasting nephropathy, and hypoaldosteronism; it is typically greater than 40 mEq/L in cerebral salt wasting 
• Normovolemic patients - The main differential is syndrome of inappropriate antidiuretic hormone secretion (SIADH) versus psychogenic polydipsia; urine osmolarity is inappropriately high in SIADH, being greater than plasma osmolarity, and urine Na values are above 40 mEq/L; high urine osmolarity is also found in reset osmostat, hypothyroidism, and adrenal insufficiency and in some cases of renal failure; in contrast, psychogenic polydipsia is characterized by low urine osmolarity

Renal failure

The urine Na concentration tends to be low in prerenal disease, being less than 20 mEq/L (in an attempt to conserve Na), while the concentration is high in acute tubular necrosis (>40-50 mEq/L).[3]  However, calculation of the fractional excretion of Na (FeNa), using the following formula, is a more reliable means of differentiating prerenal disease from renal failure[4] :

FeNa = [(urine Na/plasma Na)/(urine creatinine/plasma creatinine)] x 100

Specifics for indirect integrated chip technology (ICT) urine Na measurement (ARCHITECT c system, Abbott Laboratories) are as follows[5] :

• Specimen type - Urine
• Container - Urine collection container; no preservative should be added to the container.  
• Collection method - Random or 24-hour specimen. 
• Specimen volume - A minimum 15 μL sample volume is needed for ICT assays 
• Other instructions - At 2-8°C, urine can be stored for Na measurement for 45 days. 
• Results expressed in mmol/L are equivalent to mEq/L; results expressed as mmol/day are equivalent to mEq/day.
• To convert results from mmol/L to mmol/day (24-hour urine Na excretion), use the following formula: 24-hour excretion = [(V x c) ÷ 1000] mmol/day, where V is 24-hour urine volume (mL) and c is analyte concentration (mmol/L)

Related tests include the following:

• Serum Na
• Serum electrolytes
• Urine osmolality

Measurement of the urine Na concentration is vital in determining the integrity of tubular reabsorptive function. Low urine Na concentration indicates not only intact reabsorptive function but also the presence of a stimulus to conserve Na, whereas a high urine Na concentration may signify salt-wasting etiologies.

Indications

Indications for testing of urine Na include the following:

• Diagnosis of SIADH
• Differentiation of prerenal azotemia from acute tubular necrosis
• Assessment of hypervolemic hyponatremia, in which the urine Na is less than 10 mEq/L in all cases except renal failure with impaired water excretion
• Assessment of hypovolemic hyponatremia, in which urine Na is less than 10 mEq/L in cases of excessive GI losses, skin losses, or third spacing (eg, pancreatitis); the urine Na is over 20 mEq/L in diuretic use, salt-wasting nephropathy, and hypoaldosteronism; it is usually over 40 mEq/L in cerebral salt wasting
• Assessment of normovolemic hyponatremia, with the main differential being SIADH versus psychogenic polydipsia; in SIADH, urine osmolarity is inappropriately high, reaching a greater value than plasma osmolarity, and urine Na is over 40 mEq/L; high urine osmolarity is also found in reset osmostat, hypothyroidism, and adrenal insufficiency and in some cases of renal failure; low urine osmolarity is found in psychogenic polydipsia

Considerations

In all clinical assessments mentioned above, one must understand the effect of various drugs, particularly diuretics and intravenous (IV) fluids, that impact urine Na concentrations. Conclusions cannot be derived when treatments that impact the excretion of Na and water are being used.