About the Test
Purpose of the test
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 the treatment if you have 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 if you are 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 or losing too much of the mineral. Also, if you have abnormal kidney test results, urine sodium testing may help your health practitioner determine the cause of kidney disease and help guide treatment.
As the meals we eat on a given day affect our sodium levels, a 24-hour urine sodium test can be helpful to your health practitioner to reduce the impact of a single meal on the urine sodium result.
What does the test measure?
Sodium is an electrolyte in all body fluids that is vital to normal body function, including nerve and muscle function. This test measures sodium levels 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 regulate the body’s fluid. While sodium is present in all body fluids, it is found in the highest concentration in the blood and 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 foods 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, known as antidiuretic hormone (ADH) and, sometimes called vasopressin
- Controlling thirst; even a 1% increase in blood sodium will make you thirsty and cause you 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 shifts can be associated with too little fluid (dehydration) or too much fluid (edema), often resulting in leg swelling.
When should I get this test?
Sodium testing may be ordered as part of an electrolyte panel or metabolic panel during a routine health exam or when you are being evaluated for an acute or chronic illness.
A blood sodium test may be ordered when we have signs and symptoms of low sodium, such as:
- Weakness, fatigue
- Confusion
- Lethargy
- 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 having signs and symptoms of high sodium, such as:
- Thirst
- Dry mucous membranes (e.g., mouth, eyes)
- Less frequent urination
- Muscle twitching
- Agitation
- In severe cases (extremely high concentrations), restlessness, acting irrationally, coma, or convulsions
Sodium and other electrolytes may be measured when monitoring treatment with intravenous (IV) fluids or when there is a possibility of developing dehydration.
Electrolyte panels and basic metabolic panels are also commonly ordered regularly when monitoring the treatment of certain conditions, including high blood pressure, heart failure, and liver and kidney disease.
A random or 24-hour urine sodium test may be ordered when a blood sodium test result is abnormal to help determine the cause of the imbalance or to monitor treatment.
Finding a Sodium Test
How can I get a sodium test?
Sodium testing is usually performed at a doctor’s office or another medical setting like a hospital or lab. The tests are normally requested by a doctor but may be available without orders from your doctor at a walk-in lab.
Can I take the test at home?
The test is available as an at-home kit, though the collected sample will typically be urine, not blood.
How much does the test cost?
The cost of a sodium test will vary depending on factors such as where the test is done and whether you have health insurance. When ordered by a doctor, insurance typically covers the test, although you may have to pay a copay or deductible. Your doctor’s office, lab, and health plan can provide information about any out-of-pocket costs that may be your responsibility.
Taking a Sodium Test
The sodium test usually requires a blood sample, which is usually taken from your arm in a doctor’s office, health clinic, hospital, or lab. In some cases, a 24-hour urine sample is collected instead.
Before the test
Usually, no special preparation is required for a sodium test.
During the test
A blood sample is taken from a vein in your arm. The person taking the sample may tie a band around your upper arm and will clean the area where the needle gets inserted into your skin. A small amount of blood is drawn into a tube. You may feel a slight sting when the needle enters your skin.
The process of taking a blood sample usually takes less than three minutes.
After the test
At a doctor’s office or lab, you will be asked to apply gentle pressure to the site with a bandage or a piece of gauze after the needle is withdrawn. This will help stop bleeding and may prevent bruising. Next, the site will be bandaged. You may resume your normal activities following the test.
A blood draw is a very low-risk procedure. You may have slight bruising at the site where the blood sample was taken.
Sodium Test Results
Receiving test results
The doctor who ordered your sodium test may share the results with you, or you may be able to access them through an online patient portal. Sodium results are usually available within a few business days.
Interpreting test results
Sodium levels are typically interpreted along with results from other tests done simultaneously, such as the results of other electrolyte tests. Various conditions and diseases can cause low and high sodium levels.
Examples of conditions that can cause 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 ADH, causing you 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 Reference Range
The reference ranges1 provided here represent a theoretical guideline that should not be used to interpret your test results. Some variation is likely between these numbers and the reference range reported by the lab that ran your test. Please consult your health care provider.
| Age | Conventional Units2 | SI Units3 |
|---|---|---|
| 0-18 years | Not available due to wide variability. See child’s lab report for reference range. | |
| Adult | 136-145 mEq/L | 136-145 mmol/L |
| >90 years | 132-146 mEq/L | 132-146 mmol/L |
1 from Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 5th edition, St. Louis: Elsevier Saunders; 2011.
2 Conventional Units are typically used for reporting results in U.S. labs
3 SI Units are used to report lab results outside of the U.S.
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 so in the blood. It may also be elevated in the urine when the body loses too much sodium; in this case, the blood level would be normal to low. If blood sodium levels are low due to insufficient intake, 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.
You may want to ask your doctor the following questions:
- What is the recommended dietary salt intake?
- What can I do to decrease my low or high salt levels risk?
- Should I pay particular attention to rehydrating after playing sports or exercising to keep my sodium levels up?
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