What Is Sodium in a Blood Test? Normal Range and What Results Mean

Part of our Comprehensive Metabolic Panel guide.

What is sodium in a blood test?

Sodium in a blood test is the concentration of the sodium electrolyte in the liquid part of your blood, reported in milliequivalents per liter (mEq/L). The normal adult reference range is about 135 to 145 mEq/L (MedlinePlus, Sodium Blood Test). Sodium is the body’s main extracellular electrolyte, and it works with potassium and chloride to control fluid balance, nerve signaling, and muscle function.

The test is almost always part of a larger panel rather than a standalone order. You will see it inside a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP), the routine blood draws ordered at most checkups. A lab technician collects blood from a vein in your arm, and an analyzer measures the sodium ion concentration in your serum or plasma. Results usually return within a day.

Why is sodium measured?

Sodium is measured because it is the single best blood indicator of your body’s water and electrolyte balance, and abnormal values flag problems with the kidneys, hormones, heart, or hydration. Because hyponatremia alone appears in up to 30% of hospitalized patients, clinicians treat sodium as an early warning signal (StatPearls, Hyponatremia).

Your kidneys, along with hormones such as antidiuretic hormone (ADH) and aldosterone, normally keep serum sodium inside that tight 135 to 145 mEq/L window. When the number drifts, it tells your clinician something upstream has changed. Common reasons a provider checks sodium include:

• Routine screening: a baseline value at an annual physical.
• Symptom workup: confusion, weakness, severe thirst, swelling, or seizures.
• Monitoring: tracking kidney disease, heart failure, liver disease, or diuretic use.
• Fluid status: checking dehydration or fluid overload in hospital patients.

What does a high sodium mean?

A high blood sodium, called hypernatremia, is a serum level above 145 mEq/L and almost always means your body has lost more water than sodium, not that you ate too much salt (Cleveland Clinic). It is fundamentally a water problem. Severe symptoms typically appear only when sodium climbs above 160 mmol/L.

The most frequent driver is plain dehydration: not drinking enough, fever, heavy sweating, vomiting, or diarrhea. Older adults and people who cannot access water freely are at higher risk because thirst signals weaken with age. Other causes include diabetes insipidus, certain kidney conditions, and some medications.

Typical signs of hypernatremia:

• Early: intense thirst, dry mouth, and reduced urination.
• Moderate: restlessness, irritability, and muscle twitching.
• Severe: confusion, lethargy, seizures, and in extreme cases coma.

Treatment focuses on carefully restoring water, because correcting a high sodium too quickly can be dangerous to the brain.

What does a low sodium mean?

A low blood sodium, called hyponatremia, is a serum level below 135 mEq/L and usually means you have too much water diluting your sodium, rather than a true sodium deficiency (kidney.org). It is the most common electrolyte abnormality in both hospital and outpatient settings (StatPearls).

The cause is often an underlying condition that makes the body retain water. Heart failure, kidney disease, and liver cirrhosis are frequent culprits. So is the syndrome of inappropriate antidiuretic hormone (SIADH), certain medications such as some diuretics and antidepressants, and, occasionally, drinking extreme amounts of water during endurance events. Symptoms track with how low and how fast the sodium drops.

Common signs of hyponatremia:

• Mild: nausea, headache, and fatigue.
• Moderate: confusion, irritability, and muscle cramps or weakness.
• Severe: seizures, decreased consciousness, and coma.

As with high sodium, clinicians correct a low value slowly and deliberately to protect the brain.

How is sodium interpreted with other markers?

Sodium is interpreted alongside the rest of your metabolic panel, never in isolation, because the same number can mean opposite things depending on your fluid status and the other electrolytes. A sodium of 130 mEq/L in a swollen heart failure patient points to a very different problem than the same value in a marathon runner.

Clinicians read sodium together with these companions on a BMP or CMP:

• Potassium: the other major electrolyte, reviewed in tandem to assess overall balance and kidney handling.
• Chloride and bicarbonate: used with sodium to calculate the anion gap and check acid-base status.
• BUN and creatinine: kidney markers that show whether the kidneys are causing or responding to the sodium shift.
• Glucose: very high blood sugar can pull water into the blood and lower sodium artificially.

This is why your clinician will not act on a single off value without the full picture, and often a repeat draw.

The insider nuance: a normal number can still hide a problem

Here is the nuance most patients miss: a sodium reading inside the 135 to 145 mEq/L range does not always mean your sodium handling is healthy, and how fast the value changed often matters more than the value itself. A sodium that dropped from 140 to 133 in two days can cause more symptoms than a stable 130 someone has carried for months, because the brain adapts to slow shifts but not sudden ones.

Two other quiet traps are worth knowing. Pseudohyponatremia happens when very high blood fats or proteins make sodium look falsely low on certain analyzers, even though the true level is normal. And severe high blood sugar can dilute sodium, so a low reading sometimes corrects on its own once glucose is controlled. These are reasons your clinician may repeat the test or order additional labs before concluding anything. The single number is a starting point, not a verdict.

Frequently asked questions

What is a normal sodium level in a blood test?

For most adults, the normal serum sodium range is about 135 to 145 mEq/L (MedlinePlus). Exact reference ranges vary slightly by laboratory, so always read your result against the range printed on your own lab report.

Is high sodium in a blood test caused by eating too much salt?

Usually no. High blood sodium (hypernatremia) is mostly a water problem, not a salt problem. It most often reflects dehydration or water loss from fever, sweating, vomiting, or diarrhea rather than dietary salt intake (Cleveland Clinic).

What does a low sodium level mean?

A sodium below 135 mEq/L (hyponatremia) usually means too much water is diluting your blood sodium. Common causes include heart failure, kidney or liver disease, certain medications, and SIADH (kidney.org, StatPearls).

Should I fast before a sodium blood test?

Not for sodium alone. However, sodium is usually drawn as part of a metabolic panel that may include glucose, and your clinician may ask you to fast for 8 to 12 hours for those other markers. Follow the instructions you are given.

What symptoms suggest a sodium imbalance?

Watch for nausea, headache, confusion, weakness, muscle cramps, intense thirst, or, in severe cases, seizures. These symptoms overlap for both high and low sodium, so a blood test is needed to tell them apart. Seek care for severe symptoms.

Sources

• MedlinePlus, Sodium Blood Test
• MedlinePlus, Sodium blood test (Medical Encyclopedia)
• StatPearls (NCBI Bookshelf), Hyponatremia
• Cleveland Clinic, Hyponatremia
• National Kidney Foundation, Hyponatremia (low sodium level in the blood)

This article is for general educational purposes and is not medical advice. It cannot diagnose or treat you and does not replace your clinician. Always discuss your lab results and any health decisions with a qualified healthcare professional.