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urine sodium

Urine sodium interpretation

Urine sodium test is often used to help determine the cause of an abnormal sodium blood level. Urine sodium levels may be tested in people who have abnormal blood sodium levels to help determine whether an imbalance is due to consuming too much sodium or losing too much sodium. Urine sodium testing is also used for people with abnormal kidney tests to help your doctor determine the cause of kidney disease and to help guide treatment. Urine sodium test checks whether your kidneys are removing sodium from your body. Urine sodium test may be used to diagnose or monitor many types of kidney diseases.

For adults, normal urine sodium values are generally 20 mEq/L in a random urine sample and 40 to 220 mEq/L per day (40 to 220 mmol/24 hours) 1.. Your result depends on how much fluid and salt you take in. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test result.

Sodium is an electrolyte present in all body fluids and is vital to normal body function, including nerve and muscle function. Sodium, along with other electrolytes such as potassium, chloride, and bicarbonate (or total CO2), helps cells function normally and helps regulate the amount of fluid in the body. While sodium is present in all body fluids, it is found in the highest concentration in the blood and in the fluid outside of the body’s cells. This extracellular sodium, as well as all body water, is regulated by the kidneys.

You get sodium in your diet, from table salt (sodium chloride or NaCl), and to some degree from most of the foods that you eat. Most people have an adequate intake of sodium. Your body uses what it requires and your kidneys eliminate the rest in your urine. Your body tries to keep your blood sodium within a very narrow concentration range. It does this by:

  • Producing hormones that can increase (natriuretic peptides) or decrease (aldosterone) the amount of sodium eliminated in urine
  • Producing a hormone that prevents water losses (antidiuretic hormone or ADH, sometimes called vasopressin)
  • Controlling thirst; even a 1% increase in blood sodium will make a person thirsty and cause that person to drink water, returning the sodium level to normal.

Abnormal blood sodium is usually due to some problem with one of these systems. When the level of sodium in the blood changes, the water content in the body also changes. These changes can be associated with too little fluid (dehydration) or with too much fluid (edema), often resulting in swelling in the legs.

The Food and Nutrition Board recommends a sodium intake of less than 2300 mg per day for adults. People normally obtain adequate amounts of sodium in their daily diet, but it is important not to exceed this recommended maximum amount. Common dietary sources of sodium are often processed food to which salt is added during preparation, such as cheeses, soups, pickles, and pretzels. Additionally, other processed, commercially prepared, or restaurant foods are generally high in sodium.

For people who are sodium-sensitive or have hypertension, reducing sodium intake can lead to markedly beneficial health effects. But even if you don’t have high blood pressure, limiting sodium as part of a healthy diet may decrease your risk of developing blood pressure problems and heart disease.

Urine sodium concentrations must be evaluated in association with blood sodium levels. Your body normally elimiates excess sodium, so the concentration in the urine may be elevated because it is elevated in the blood. Sodium may also be elevated in the urine when the body is losing too much sodium; in this case, the blood level would be normal to low. If blood sodium levels are low due to insufficient intake, then urine sodium concentrations will also be low.

  • Decreased urinary sodium levels may indicate dehydration, congestive heart failure, liver disease, or nephrotic syndrome.
  • Increased urinary sodium levels may indicate diuretic use or Addison disease.

Sodium levels are often evaluated in relation to other electrolytes and can be used to calculate a quantity termed anion gap. The anion gap is useful in identifying the presence of unknown substances such as toxins in the blood.

Urine sodium pathophysiology

  • Sodium is a major cation of the extracellular fluid.
  • Sodium is responsible for almost 50% of the osmotic strength of the plasma. Sodium helps to maintain the osmotic pressure of the extracellular fluid.
  • The daily diet contains 8 to 15 grams (130 to 260mmol) per day. Body requirement is only 1 to 2 mmol and excess is excreted in the urine by the kidney.
  • Aldosterone stimulates kidneys Decreases renal losses and increases reabsorption.
  • A natriuretic hormone (ADH) produced in response to increase sodium (Na+) by decreasingsodium (Na+) reabsorption and increase sodium (Na+) losses.
  • ADH control reabsorption of water at distal tubules affect sodium (Na+) level by dilution of urine.
  • Sodium transport is an active process.
urine sodium pathophysiology

Normal urine sodium

Normal urine sodium levels is 40-220 mEq/L per 24 hours (40 to 220 mmol/24 hours) 1.

Children 6 to 10 years

  • Male 41 to 115 mEq/L per 24 hours
  • Female 20 to 69 mEq/L per 24 hours

Children 10 to 14 years

  • Male 63 to 177 mEq/L per 24 hours
  • Female 48 to 168 mEq/L per 24 hours

High sodium in urine

A higher than normal urine sodium level may be due to:

  • Certain medicines, such as water pills (diuretics)
  • Low function of the adrenal glands
  • Inflammation of the kidney that results in salt loss (salt-losing nephropathy)
  • Too much salt in the diet
  • Dehydration
  • Adrenocortical deficiency
  • Diabetic Ketoacidosis
  • Chronic renal failure
  • Hypothyroidism
  • Toxemia of pregnancy
  • Syndrome of inappropriate ADH secretion (SIADH).

Low urine sodium

A lower than normal urine sodium level may be a sign of:

  • Adrenal glands releasing too much hormone (hyperaldosteronism)
  • Not enough fluid in the body (dehydration)
  • Diarrhea and fluid loss
  • Congestive heart failure
  • Kidney problems, such as long-term (chronic) kidney disease or kidney failure
  • Scarring of the liver (cirrhosis)
  • Cushing’s syndrome
  • Malabsorption
  • Inadequate sodium intake
  • Diaphoresis
  • Pulmonary emphysema
  • Inadequate intake of sodium.

Renal failure

The urine sodium concentration tends to be low in prerenal disease, being less than 20 mEq/L (in an attempt to conserve sodium), while the concentration is high in acute tubular necrosis (>40-50 mEq/L) 2. However, calculation of the fractional excretion of sodium (FeNa), using the following formula, is a more reliable means of differentiating prerenal disease from renal failure 3:

  • Fractional excretion of sodium (FeNa) = [(urine sodium/plasma sodium)/(urine creatinine/plasma creatinine)] x 100

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 sodium is less than 10 mEq/L in congestive heart failure, nephrotic syndrome, and cirrhosis; urine sodium is greater than 20 mEq/L in renal failure with impaired water excretion
  • Hypovolemic patients – Urine sodium is less than 10 mEq/L in excessive gastrointestinal 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 sodium 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
References
  1. Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. New York, NY: Elsevier; 2018.
  2. Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978 Jul. 89 (1):47-50.
  3. Kaplan AA, Kohn OF. Fractional excretion of urea as a guide to renal dysfunction. Am J Nephrol. 1992. 12 (1-2):49-54.
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