Sodium is the main positively charged ion in the fluid outside cells, and the blood test measures its concentration in serum or plasma. It is ordered as part of an electrolyte panel, often when fluid balance, kidney function, or medicines that affect salt and water are being assessed. The range printed on your report decides how the value is read.
Sodium is often the first electrolyte on a report and one of the easiest to misread, because the number describes a concentration rather than a store. A change in the water around the sodium can move it as much as a change in sodium itself.
What the test measures
Sodium is a mineral that carries a positive charge when it dissolves in body fluid, which makes it an electrolyte. Most of the body's sodium sits outside cells, in the fluid that surrounds them, and it is the main positively charged ion there. It holds water in that space and helps set the volume of the fluid around cells.
The blood test measures the concentration of sodium in serum or plasma: the amount of sodium per volume of liquid. That distinction matters. A concentration reflects the balance between sodium and water, so a result can move because water changed while sodium stayed the same. The test does not measure the total amount of sodium in the body, and a blood result is not a measure of salt intake.
Why it may be ordered
- As part of an electrolyte panel, which usually reports sodium with potassium, chloride, and bicarbonate.
- In a basic metabolic panel, alongside urea, creatinine, and glucose.
- When a clinician is assessing fluid balance, for example after vomiting, diarrhoea, or a fever.
- When kidney function is being reviewed, because the kidneys decide how much sodium and water are kept or passed.
- When a medicine that changes how the body handles salt and water is being taken, because the result is read with the medicine list.
- When a condition that affects water balance, including some hormone conditions, is being assessed.
It is also measured in urine, but a urine sodium result answers a different question and is not comparable with a blood result.
The unit on your report
| What you may see | Unit | What it reflects |
|---|---|---|
| Sodium, Na, Na+ | mmol/L | The sodium concentration in serum or plasma, in millimoles per litre. |
| Sodium | mEq/L | The same concentration in milliequivalents per litre. For sodium the two labels give the same number, because each ion carries a single charge. |
| Serum sodium, plasma sodium | mmol/L | The sample type. Serum and plasma are prepared differently, but the unit and the meaning are the same. |
| Electrolytes, Na/K/Cl | mmol/L | A grouped heading on some reports. Each electrolyte beneath it is a separate result with its own range. |
Some reports print both units side by side. Read the range that matches the unit you are looking at, because a range is written for a unit. Do not compare a blood sodium value with a urine sodium value; they are different measurements.
Why this page does not print a range
A reference range is built for a particular laboratory, method, and population. Sodium ranges can differ between laboratories, between analytical methods, and between age groups or sexes, and pregnancy changes the way the body handles water and sodium. Some reports also print a range that the laboratory has adjusted for its own instrument.
The range printed on the report that produced your value is the one that applies. Doctano keeps that printed range with the result and does not replace it with a general range from the internet. When a report prints mmol/L and mEq/L side by side, the range that belongs to the unit you are reading is the one to use.
What a high value can mean
A result above the report's range means the sodium concentration in the sample is higher than the laboratory's reference interval. In practice that usually means there is relatively less water in the blood than sodium, not that the body has too much sodium. The term hypernatraemia describes a high sodium concentration; it is a description of the value, not a diagnosis.
Possible contributors include water loss through fever, vomiting, or diarrhoea; reduced water intake, which can happen when thirst or access to fluid is impaired; increased water loss in urine, including osmotic diuresis when blood glucose is high; some hormone conditions that affect water balance; and some medicines. The result does not confirm or rule out any of these, and a clinician reads it with the fluid assessment and the rest of the panel.
What a low value can mean
A result below the report's range means the sodium concentration is lower than the laboratory's interval. Low sodium is often described as hyponatraemia. It usually reflects relatively more water than sodium rather than a true shortage of sodium in the body, and it can appear when the body holds on to water.
Possible contributors include heart, liver, or kidney conditions that change fluid handling; a syndrome of inappropriate antidiuretic hormone secretion, in which the body holds water; diuretics and some other medicines; adrenal insufficiency; a low thyroid result; and a high blood glucose, which draws water out of cells. Some laboratory methods can also report a misleadingly low sodium when protein or lipid levels are very high, and the report may comment on this.
A low result is not specific to one cause. A clinician reads it with the person's fluid status, other electrolytes, glucose, kidney function, and medicines.
What can move the value
- Water balance. Fluid intake and fluid loss through illness, sweat, or urine all change the concentration.
- Blood glucose. A high glucose draws water out of cells and lowers the measured sodium.
- Protein and lipid levels. Very high levels can affect some laboratory methods and produce a misleading result.
- Medicines. Diuretics, some antidepressants, and some epilepsy medicines can change sodium handling, and a clinician can say whether a medicine is relevant.
- Kidney and hormone function. The kidneys and hormones such as aldosterone and vasopressin decide how much sodium and water are kept.
- Sample handling. A sample taken from an arm with an intravenous line running, or one that is delayed or diluted, can give a value that does not reflect the body.
What to compare it with
Read sodium with the rest of the electrolyte panel: potassium, chloride, and bicarbonate. Urea and creatinine add the kidney view, glucose explains a dilutional effect, and a serum osmolality, where it is reported, helps show whether the sodium result fits the measured concentration of the blood. In many reports the sodium line is the entry point to the whole fluid picture rather than a standalone result.
How to see the trend
Doctano reads the sodium value, its unit, and the reference range printed on the report. It keeps the range with the result, normalizes the marker name across laboratories, and places the value on the profile's timeline. Each point still opens the report it came from.
Because sodium appears as mmol/L on some reports and mEq/L on others, it is worth checking which unit each report used. Verify the value against the original report before treating a change as real, especially when the two results came from different laboratories.
What this page is not
This page explains the test in general. It does not interpret your result, diagnose a condition, or recommend treatment. It is not medical advice. Verify the value and the unit against the original report, and discuss the result with a qualified professional.