Sodium Blood Test: What It Measures and What Results Mean
📊 Quick facts about sodium blood test
💡 Key things to know about sodium blood tests
- Normal sodium range is 135-145 mmol/L: Values outside this range may indicate fluid imbalance, kidney problems, or hormonal disorders
- No fasting required: The test can be performed at any time of day without special preparation
- Commonly tested with other electrolytes: Often included in basic metabolic panels with potassium, chloride, and creatinine
- Low sodium (hyponatremia) is the most common electrolyte disorder: Can be caused by medications, heart failure, liver disease, or excessive water intake
- High sodium (hypernatremia) usually indicates dehydration: Often caused by not drinking enough water, excessive sweating, or diarrhea
- Kidneys regulate sodium balance: Hormones from the adrenal glands and pituitary gland control how kidneys handle sodium
What Is a Sodium Blood Test?
A sodium blood test measures the concentration of sodium in your blood, typically reported in millimoles per liter (mmol/L). Sodium is essential for maintaining fluid balance, nerve transmission, and muscle function. The normal range is 135-145 mmol/L.
Sodium is one of the most important minerals (also called electrolytes) in your body. It plays a vital role in maintaining the proper balance between salt and water in your blood and tissues. Your heart, brain, kidneys, and adrenal glands all work together to keep sodium levels within a narrow, healthy range. When this balance is disrupted, it can affect virtually every organ system in your body.
The sodium blood test is one of the most commonly ordered laboratory tests in medical practice. It is frequently included as part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP), which provides a broader picture of your body's chemical balance and kidney function. Understanding what the test measures and what results mean can help you participate more actively in your healthcare.
Most of the sodium in your body is found in body fluids, particularly in your blood. A smaller amount is stored in your bones, and very little is found inside your cells. The concentration of sodium in your blood is tightly regulated because even small deviations from normal can have significant effects on how your cells function.
How sodium affects your body
Sodium serves several critical functions in your body. First, it helps regulate the total amount of water in your body by controlling fluid movement between your blood vessels and tissues. This is why conditions that affect sodium levels often also affect blood pressure. Second, sodium is essential for the transmission of nerve impulses, which allow your brain to communicate with the rest of your body. Third, sodium plays a key role in muscle contraction, including the rhythmic contractions of your heart muscle.
The balance between sodium and another electrolyte called potassium is particularly important for heart function. These two minerals work together to generate the electrical signals that keep your heart beating in a regular rhythm. Disturbances in either sodium or potassium levels can potentially cause dangerous heart rhythm abnormalities.
Where does sodium come from?
You get sodium primarily through the food you eat. Table salt (sodium chloride) is the most common source, but sodium is also naturally present in many foods and is added to most processed foods as a preservative and flavor enhancer. Most adults consume far more sodium than they need, which can contribute to high blood pressure and other health problems over time.
Your body has sophisticated mechanisms for maintaining sodium balance regardless of how much you consume. When you eat too much salt, your kidneys excrete the excess in your urine. When you don't get enough sodium or lose too much through sweating or illness, your kidneys conserve sodium to maintain proper levels. However, these regulatory mechanisms can fail in certain diseases or under extreme conditions.
When Do You Need a Sodium Blood Test?
Doctors order sodium blood tests when they suspect fluid or electrolyte imbalance, to monitor certain medications, or to evaluate symptoms like confusion, weakness, or dehydration. It's a common test in hospitals and is often part of routine health screenings.
Your doctor may order a sodium blood test for several reasons. In hospital settings, it is one of the most frequently performed tests because sodium disturbances are common in hospitalized patients and can indicate serious underlying conditions. Understanding when and why this test is ordered can help you better understand your healthcare.
One of the most common reasons for ordering a sodium test is when your doctor suspects that the balance between salt and fluid in your body has changed. This can happen in many different situations, from simple dehydration to complex medical conditions affecting the heart, kidneys, or endocrine system.
Symptoms that may prompt testing
Your doctor may order a sodium test if you have symptoms that could indicate an electrolyte imbalance. These symptoms can be vague and nonspecific, which is one reason why laboratory testing is so valuable for diagnosis. Common symptoms that might prompt testing include confusion or difficulty concentrating, extreme fatigue or weakness, muscle cramps or twitching, nausea and vomiting, headaches, and in severe cases, seizures or loss of consciousness.
It's important to understand that these symptoms can have many causes other than sodium imbalance. The blood test helps your doctor determine whether sodium levels are contributing to your symptoms and guides appropriate treatment.
Medical conditions requiring monitoring
Certain medical conditions require regular sodium monitoring because they affect the body's ability to regulate fluid and electrolyte balance. Heart failure is a common example, as the condition affects the kidneys' ability to properly regulate sodium and water. Patients with heart failure often retain fluid, which can dilute sodium levels in the blood.
Kidney disease is another condition that often requires regular sodium monitoring. The kidneys are primarily responsible for maintaining sodium balance, so when they are damaged or diseased, sodium levels may become abnormal. Liver disease, particularly cirrhosis, can also affect sodium regulation and requires monitoring.
Certain hormonal disorders directly affect sodium balance. For example, Addison's disease (adrenal insufficiency) can cause low sodium levels because the adrenal glands don't produce enough of the hormones that help retain sodium. Conversely, conditions that cause excessive aldosterone production can lead to sodium retention and elevated levels.
Medication monitoring
Many medications can affect sodium levels, making regular monitoring important for patients taking these drugs. Diuretics (water pills) are among the most common medications that affect sodium. These drugs cause the kidneys to excrete more sodium and water, which can sometimes lead to low sodium levels, especially if fluid intake doesn't keep pace with losses.
Certain antidepressants, particularly SSRIs (selective serotonin reuptake inhibitors), can cause a condition called SIADH (syndrome of inappropriate antidiuretic hormone secretion), which leads to low sodium levels. Pain medications, anti-seizure drugs, and some chemotherapy agents can also affect sodium levels. If you take any of these medications, your doctor may recommend periodic sodium testing.
Dehydration assessment
One common reason for checking sodium levels is to assess dehydration. When you lose fluids through vomiting, diarrhea, excessive sweating, or simply not drinking enough, your sodium concentration in the blood often rises. This is because you're losing water faster than you're losing sodium, causing the remaining sodium to become more concentrated.
Athletes who engage in prolonged physical activity, such as marathon runners, may need sodium testing because intense sweating can lead to significant sodium losses. Interestingly, some endurance athletes develop low sodium rather than high sodium if they drink too much plain water without replacing lost salt.
How Is the Sodium Blood Test Done?
The sodium blood test is a simple blood draw from a vein in your arm. No fasting is required, and you may be asked to sit briefly before the test. The sample is analyzed in a laboratory, with results typically available within 24 hours.
The sodium blood test is a straightforward procedure that most people find quick and relatively painless. Understanding what to expect can help reduce any anxiety you might have about the test.
Preparation
One of the advantages of the sodium blood test is that it requires no special preparation. Unlike some blood tests that require fasting, you can eat and drink normally before a sodium test. The test can be performed at any time of day, making it convenient to schedule.
However, you may be asked to sit or lie down for a few minutes before the blood is drawn. This is because the distribution of fluid in your blood vessels changes depending on whether you're standing, sitting, or lying down. Allowing a few minutes for your circulation to stabilize helps ensure more accurate results.
If you're taking any medications, continue taking them as prescribed unless your doctor specifically instructs otherwise. Be sure to tell your healthcare provider about all medications and supplements you're taking, as some can affect sodium levels.
The blood draw
The blood sample is typically taken from a vein in your arm, a procedure called venipuncture. A healthcare professional will clean the area with an antiseptic wipe, apply a tourniquet to make the vein more visible, and insert a small needle to collect the blood sample. The entire process usually takes less than five minutes.
Most people feel only a brief pinch when the needle is inserted. After the sample is collected, the needle is removed, and pressure is applied to the site to stop any bleeding. A small bandage is then applied, which you can usually remove after an hour or so.
Sample processing
After collection, your blood sample is sent to a laboratory for analysis. The sodium concentration is measured in either the plasma or serum portion of the blood. Plasma is the liquid part of blood that remains after blood cells are removed using an anticoagulant. Serum is similar but is obtained by allowing the blood to clot first.
Both plasma and serum measurements give equivalent results for sodium, so the choice between them is usually based on what other tests are being performed on the same sample. Laboratory reports may indicate the test as "P-Sodium" (plasma sodium) or "S-Sodium" (serum sodium), but both represent the same clinical information.
How Are Sodium Test Results Interpreted?
Normal sodium levels range from 135-145 mmol/L. Results are interpreted alongside your symptoms, other test results, and medical history. A single abnormal result doesn't necessarily indicate disease; your doctor will consider the complete clinical picture.
Understanding how to interpret your sodium test results can help you have more informed conversations with your healthcare provider. However, it's important to remember that laboratory values should always be interpreted by a qualified healthcare professional who can consider your complete medical situation.
The reference range
The normal reference range for sodium in adults is typically 135 to 145 millimoles per liter (mmol/L). This range is established by measuring sodium levels in a large group of healthy people and determining the range that includes approximately 95% of normal values. Values outside this range are considered abnormal and warrant further investigation.
It's important to note that sodium levels can vary slightly between individuals and even in the same person at different times. A result that is just slightly outside the reference range may not be clinically significant, especially if you feel well and have no symptoms. Your doctor will consider the degree of abnormality along with other factors when interpreting your results.
| Sodium Level | Classification | Possible Causes | Clinical Significance |
|---|---|---|---|
| 135-145 mmol/L | Normal | Healthy balance | No intervention needed |
| 130-134 mmol/L | Mild hyponatremia | Medications, mild illness | Monitor, may need investigation |
| <130 mmol/L | Moderate-severe hyponatremia | Heart/kidney/liver disease, SIADH | Requires medical evaluation |
| >145 mmol/L | Hypernatremia | Dehydration, excess salt intake | Fluid assessment needed |
Factors affecting interpretation
Your doctor interprets sodium results by considering them alongside your symptoms, medical history, and other test results. A single blood test provides a snapshot of your sodium level at one point in time, but the clinical picture requires more comprehensive assessment.
Other tests that are often performed alongside sodium include potassium (another important electrolyte), creatinine (a measure of kidney function), and blood urea nitrogen (BUN). Together, these tests help your doctor understand whether an abnormal sodium level is due to a problem with fluid balance, kidney function, or another underlying condition.
What Causes High Sodium Levels (Hypernatremia)?
High sodium levels (above 145 mmol/L) usually indicate dehydration from not drinking enough water, excessive sweating, fever, diarrhea, or vomiting. Less common causes include excessive salt intake, kidney problems, or hormonal disorders affecting aldosterone.
Hypernatremia, or high sodium levels in the blood, is less common than low sodium but can be equally serious. Understanding the causes can help you recognize risk factors and seek appropriate care.
Dehydration: The most common cause
The most frequent cause of high sodium levels is dehydration, which occurs when you lose more water than you take in. This can happen for many reasons. Not drinking enough water, especially in hot weather or during physical activity, is a common cause. Fever, vomiting, and diarrhea can all cause significant fluid losses that lead to elevated sodium if fluids aren't replaced.
Elderly individuals are particularly vulnerable to dehydration-related hypernatremia because the sensation of thirst often decreases with age, and some may have difficulty accessing fluids independently. Certain medications, particularly diuretics, can also contribute to dehydration.
Excessive salt intake
While less common than dehydration, consuming too much sodium can elevate blood sodium levels. This might occur from heavily salting food, consuming large amounts of salty processed foods, or in rare cases, from accidentally ingesting too much salt. The body normally compensates for high sodium intake by increasing thirst and excreting excess sodium through the urine, but these mechanisms can be overwhelmed in extreme cases.
Kidney and hormonal disorders
Certain kidney diseases can impair the body's ability to eliminate excess sodium, leading to elevated levels. Hormonal disorders can also play a role. For example, conditions that cause the adrenal glands to produce too much aldosterone (a hormone that promotes sodium retention) can lead to hypernatremia along with high blood pressure.
A rare condition called diabetes insipidus (unrelated to the more common diabetes mellitus) causes the body to produce large amounts of dilute urine, leading to severe dehydration and high sodium levels if fluid intake doesn't keep pace with losses.
Treatment of high sodium
Treatment for high sodium depends on the underlying cause. In most cases, the primary treatment is careful fluid replacement to gradually restore normal sodium levels. The correction must be done slowly because rapid changes in sodium levels can be dangerous. Your doctor will determine the appropriate rate of correction based on how quickly the high sodium developed and your overall clinical condition.
What Causes Low Sodium Levels (Hyponatremia)?
Low sodium levels (below 135 mmol/L) can result from drinking too much water without adequate salt, heart failure, kidney disease, liver disease, thyroid problems, certain medications (especially diuretics), and hormonal conditions like Addison's disease or SIADH.
Hyponatremia, or low sodium levels in the blood, is the most common electrolyte disturbance encountered in hospitalized patients. It can range from mild and asymptomatic to severe and life-threatening, depending on how low the sodium falls and how quickly the change occurs.
Dilution of body fluids
Many cases of low sodium result from dilution of body fluids rather than actual sodium loss. This can occur when someone drinks large amounts of water without taking in adequate sodium. Marathon runners and other endurance athletes are at risk if they consume excessive amounts of plain water during prolonged exercise without replacing the sodium lost through sweat.
Medical conditions that cause fluid retention can also lead to dilutional hyponatremia. Heart failure, liver cirrhosis, and kidney disease can all cause the body to retain water, which dilutes the sodium in the blood. In these cases, the total amount of sodium in the body may actually be normal or even elevated, but the concentration in the blood is low because of excess water.
Sodium loss
True sodium depletion can occur from prolonged vomiting or diarrhea, excessive sweating without adequate salt replacement, or certain kidney disorders that impair sodium retention. Patients who have been sick with a stomach virus and have consumed only water without eating may develop low sodium from a combination of sodium loss and dilution.
Medication effects
Many medications can cause low sodium levels. Diuretics, particularly thiazide diuretics, are among the most common culprits. These medications increase sodium excretion by the kidneys and can lead to hyponatremia, especially in elderly patients or those who also restrict their sodium intake.
Certain antidepressants, particularly SSRIs, can cause a condition called SIADH (syndrome of inappropriate antidiuretic hormone secretion). In this condition, the body retains water inappropriately, diluting sodium levels. Other medications associated with low sodium include certain anti-seizure drugs, pain medications, and some cancer chemotherapy agents.
Hormonal disorders
Several hormonal conditions can cause low sodium. Hypothyroidism (underactive thyroid) can impair the body's ability to excrete water, leading to dilutional hyponatremia. Addison's disease (adrenal insufficiency) causes low sodium because the adrenal glands don't produce enough of the hormones that help retain sodium.
Pregnancy
Sodium levels normally decrease slightly during pregnancy due to increased fluid volume. This is generally a normal physiological adaptation and doesn't require treatment. Sodium levels typically return to normal after delivery.
Treatment of low sodium
Treatment for low sodium depends on the cause and severity. Mild cases may require only fluid restriction or adjustment of medications. More severe cases may require careful sodium supplementation, but correction must be done gradually to avoid serious neurological complications. Your healthcare provider will determine the most appropriate treatment based on your specific situation.
Both rapid correction of low sodium and rapid changes in sodium levels can cause serious complications, including a condition called osmotic demyelination syndrome. This is why sodium abnormalities should always be managed by qualified healthcare professionals who can monitor the rate of correction.
Common Abbreviations and Terms
Sodium test results may be labeled as P-Sodium (plasma sodium), S-Sodium (serum sodium), or simply Na (the chemical symbol for sodium). All refer to the same measurement. The unit mmol/L (millimoles per liter) is the standard reporting unit worldwide.
When reading your laboratory results, you may encounter various abbreviations and terms related to sodium testing. Understanding these can help you better interpret your results and communicate with your healthcare team.
Test name abbreviations
The sodium test may be labeled differently depending on the laboratory. Common abbreviations include Na (the chemical symbol for sodium, from the Latin "natrium"), P-Na or P-Sodium (plasma sodium), and S-Na or S-Sodium (serum sodium). The "P" and "S" prefixes simply indicate whether the measurement was made in plasma or serum, but as discussed earlier, both give equivalent results.
Measurement units
Sodium levels are typically reported in millimoles per liter (mmol/L). This unit is used internationally and reflects the concentration of sodium ions in a given volume of blood. Some older references may use milliequivalents per liter (mEq/L), which gives the same numerical values as mmol/L for sodium.
Related terms
You may also encounter related medical terms in your test results or medical records. Hyponatremia refers to low sodium levels, while hypernatremia indicates high sodium levels. The prefix "hypo-" means low or under, while "hyper-" means high or over.
Frequently Asked Questions
The normal sodium level in blood is between 135 and 145 millimoles per liter (mmol/L). This reference range applies to both serum and plasma sodium measurements. Values outside this range may indicate dehydration, kidney problems, hormonal disorders, or medication side effects. Minor variations just outside the normal range are not always clinically significant and should be interpreted by your healthcare provider in the context of your symptoms and overall health.
High sodium levels (hypernatremia) can be caused by several factors. The most common cause is dehydration from not drinking enough water, excessive sweating, fever, diarrhea, or vomiting. Other causes include excessive salt intake, certain kidney diseases, and hormonal disorders that cause overproduction of aldosterone. Diabetes insipidus, a rare condition unrelated to regular diabetes, can also cause high sodium by increasing water loss through urine.
Low sodium levels (hyponatremia) can result from drinking too much water without adequate salt intake, heart failure, kidney disease, liver disease, hypothyroidism, and certain medications, especially diuretics. Addison's disease (adrenal insufficiency) and SIADH (syndrome of inappropriate antidiuretic hormone) are hormonal conditions that can cause low sodium. It's the most common electrolyte disorder in hospitalized patients.
No, fasting is not required before a sodium blood test. You can eat and drink normally before the test, which can be performed at any time of day. However, you may be asked to sit or lie down for a few minutes before the blood draw because the distribution of fluid in your blood changes depending on your position. This helps ensure more accurate results.
Sodium blood test results are typically available within 24 hours when samples are sent to an external laboratory. In hospital settings, results can often be obtained much faster, sometimes within 1-2 hours, using point-of-care testing devices or in-house laboratories. Your healthcare provider will discuss the results with you and explain what they mean for your health.
Serum is the liquid portion of blood after it has clotted and blood cells are removed. Plasma is the liquid portion before clotting, obtained by adding an anticoagulant to the blood sample. Both measurements give essentially identical sodium values and can be used interchangeably. Your test results may be labeled P-Sodium (plasma) or S-Sodium (serum), but both represent the same clinical information.
References and Sources
This article is based on evidence from peer-reviewed medical literature and clinical guidelines:
- Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. European Journal of Endocrinology. 2014;170(3):G1-G47. doi:10.1530/EJE-13-1020
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. New England Journal of Medicine. 2015;372(1):55-65. doi:10.1056/NEJMra1404489
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements. 2012;2(1):1-138.
- Verbalis JG, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. American Journal of Medicine. 2013;126(10 Suppl 1):S1-42. doi:10.1016/j.amjmed.2013.07.006
- Liamis G, et al. Electrolyte disorders in community subjects: prevalence and risk factors. American Journal of Medicine. 2013;126(3):256-263. doi:10.1016/j.amjmed.2012.06.037
- World Health Organization. WHO Model List of Essential In Vitro Diagnostics, 3rd edition. Geneva: WHO; 2021.
This information is provided for educational purposes only and is not intended to replace consultation with a qualified healthcare professional. If you have concerns about your sodium levels or any symptoms, please contact your healthcare provider.
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