Sodium is an electrolyte present in all body fluids and is vital to normal body function, including nerve and muscle function. This test measures the level of sodium in the blood and/or urine.
Electrolytes are minerals that carry a charge and exist in your body fluids. Sodium and other electrolytes such as potassium, chloride, and bicarbonate (or total CO2) help 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.
We get sodium in our diet, from table salt (sodium chloride or NaCl), and to some degree from most of the foods that we eat. Most people have an adequate intake of sodium. The body uses what it requires and the kidneys eliminate the rest in the urine. The body tries to keep the blood sodium within a very narrow 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, 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.
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.
How is the test used?
A sodium blood test is used to detect an abnormal sodium level, including low sodium (hyponatremia) and high sodium (hypernatremia). It is often used as part of an electrolyte panel or basic metabolic panel for a routine health exam.
A blood sodium test may also be used to:
- Help detect, evaluate, and monitor electrolyte imbalances and/or acid-base (pH) imbalances (acidosis or alkalosis)
- Detect the cause and help monitor treatment in people with dehydration or excess fluid buildup (edema)
- Help evaluate and monitor a variety of chronic or acute illnesses involving the brain, lungs, liver, heart, kidney, thyroid, or adrenal glands
- Monitor people taking medications that can affect sodium levels, such as diuretics
Urine sodium levels may be tested if you have abnormal blood sodium levels to help determine whether an imbalance is due to, for example, consuming too much sodium or losing too much sodium. Urine sodium testing may also be used if you have abnormal kidney test results to help your healthcare practitioner determine the cause of kidney disease and to help guide treatment.
As the meals we eat on a given day affect our sodium levels, a 24-hour urine sodium tests can be helpful to your healthcare practitioner to reduce the impact of a single meal on the urine sodium result.
When is it ordered?
- Weakness, fatigue
- In severe cases, coma
However, there may be no symptoms when the sodium level falls slowly. That is why sodium levels are often checked even if you have no specific symptoms.
Sodium blood testing may be ordered when have signs and symptoms of high sodium, such as:
- Dry mucous membranes (e.g., mouth, eyes)
- Less frequent urination
- Muscle twitching
- In severe cases (extremely high concentrations), restlessness, acting irrationally, coma or convulsions
Electrolyte panels and basic metabolic panels are also commonly ordered on a regular basis when monitoring treatment of certain conditions, including high blood pressure, heart failure, and liver and kidney disease.
What does the test result mean?
Sodium levels are typically interpreted along with results from other tests done at the same time, such as the results of other electrolyte tests. Low and high sodium levels can be caused by various conditions and diseases.
Examples of conditions that can cause a low blood sodium (hyponatremia) include:
- Losing too much sodium, most commonly from conditions such as diarrhea, vomiting, excessive sweating, use of diuretics, kidney disease or low levels of cortisol, aldosterone and sex hormones (Addison disease)
- Drinking too much water as might occur during exercise
- Excess fluid buildup in the body (edema) caused by heart failure, cirrhosis, and kidney diseases that cause protein loss (nephrotic syndrome)
- Conditions that lead to too much anti-diuretic hormone (ADH), causing a person to keep too much water in the body; this can happen with several diseases, particularly those involving the brain and the lungs, many kinds of cancer, and with use of some drugs.
Examples of conditions that can cause a high blood sodium level (hypernatremia) include:
- Dehydration, losing too much water without drinking enough water, is a common cause.
- Cushing syndrome
- Diabetes insipidus, a condition caused by too little ADH
- Rarely, too much salt in the diet
Sodium urine concentrations must be compared with sodium blood levels. The body normally eliminates excess sodium, so the concentration in the urine may be elevated because it is elevated in the blood. It 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 concentrations will also be low.
- Decreased urine sodium levels may indicate dehydration, congestive heart failure, liver disease, or nephrotic syndrome.
- Increased urine 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. For details, read the article Electrolytes and Anion Gap.
My sodium level is only slightly out of range. What does this mean?
Your sodium result is interpreted by your healthcare practitioner within the context of other tests that you have had done as well as other factors, such as your medical history. A single high or low result may or may not have medical significance. Generally, this is the case when the test value is only slightly higher or lower than the reference range. This is why healthcare practitioners may repeat a test on you and why they may look at results from prior times when you had the same test performed.
On the other hand, a result outside the range may indicate a problem and warrant further investigation. Your healthcare practitioner will evaluate your test results and determine whether a result that falls outside of the reference range means something significant for you.
What is the recommended dietary salt intake?
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.
Your taste for salt is both acquired and reversible. As you use less salt, your preference for it will lessen.
Is anyone at particular risk for low or high sodium levels?
Yes. People who have long-lasting diarrhea and/or vomiting, profuse sweating, burns, Addison disease, kidney disease, or congestive heart failure may have low sodium levels. People with dehydration, diuretic use, Cushing syndrome, cystic fibrosis, neurological disorders, hypothyroidism, or renal failure may have high sodium levels.
Should athletes and others pay particular attention to re-hydrating after playing sports or exercising to keep their sodium levels up?
Yes. During prolonged and strenuous exercise, both water and sodium are lost through sweating. To maintain the correct balance of water and sodium in the body, people may need not only to drink water but to ensure adequate sodium intake, whether through salty foods or specially formulated "sports drinks."
Is there anything else I should know?