- 01Electrolyte results are concentrations per liter of blood water
Each line is the amount of a charged particle per liter of blood water, so it moves when water moves, not only when the body's total store changes. That is why a low sodium can appear with no sodium lost at all: extra water has diluted it, so adding salt is rarely the fix. It is also why potassium, about 98% of which sits inside cells, can read high when cells leak into the tube, and why a high potassium is repeated promptly and carefully.
Your electrolyte lines report concentrations: how much sodium, potassium, chloride and bicarbonate sit in each liter of serum, the watery part of your blood left after it clots. An electrolyte is a particle that carries an electric charge once dissolved (an ion), and the charge is what lets it move water, fire nerves and keep your heart in rhythm. A concentration changes when the water around it changes, so the sodium line mostly tells you about water balance and very little about how much salt you eat. Most small shifts come from hydration, medicines or the blood sample itself. A few specific numbers need care the same day, and they are listed in the severity table below.
The electrolyte lines on a metabolic panel
A basic metabolic panel (BMP) has eight tests: sodium, potassium, chloride, bicarbonate, glucose, calcium, blood urea nitrogen and creatinine. A comprehensive metabolic panel (CMP) adds six liver and protein tests on top of those eight. The electrolyte lines are the same on both. Ranges below are typical adult values; your lab's printed range is the one to compare against, because analyzers and populations differ (see how reference ranges are built).
| Line on your report | Typical adult range | What it mainly reflects | Most common reason it is off |
|---|---|---|---|
| Sodium (Na) | 135 to 145 mmol/L 2 7 | Water balance: how much water the body holds relative to its sodium | Low: thiazide diuretics (a common type of water pill for blood pressure), some antidepressants, drinking large amounts of water. High: too little water, often in people who cannot sense thirst or reach a drink |
| Potassium (K) | 3.5 to 5.0 mmol/L 4 | A small outside reading of the large potassium store inside your cells | High: the sample itself (a burst red cell, fist clenching), reduced kidney function, some blood-pressure medicines. Low: diuretics, vomiting, diarrhea |
| Chloride (Cl) | About 98 to 107 mmol/L (typical lab range; check your report) | Moves with sodium; also the acid-base partner of bicarbonate | The same water shifts that move sodium; low after repeated vomiting |
| Bicarbonate (CO2 or total CO2) | About 22 to 29 mmol/L (typical lab range; check your report) | Acid-base balance (how acidic the blood is): bicarbonate is the blood's main buffer, soaking up acid to keep blood pH steady | Low: diarrhea, kidney disease, acid build-up. High: vomiting, diuretics |
| Anion gap (calculated) | About 3 to 11 mmol/L on modern analyzers; older methods ran higher 6 | Sodium minus chloride and bicarbonate; a high gap flags acids the panel does not measure directly | High: acid build-up, for example from uncontrolled diabetes or kidney failure. Low: most often low albumin, the main blood protein, which also changes how calcium is read (corrected calcium) 6 |
No electrolyte has a separate "optimal" target that differs from the reference range. For these lines, being inside your lab's range is the goal, and the questions that matter are how far a value sits outside it, how fast it got there and whether you have symptoms.
That 98% figure explains why potassium behaves so differently from sodium. Anything that lets potassium leak out of cells raises the serum number without any change in the body's total store: red cells bursting in the tube (hemolysis), a hard fist clench during the draw, a sample left too long or chilled before processing 5. Insulin and shifts in acid-base balance move potassium in and out of cells too, which is why a very high glucose or a low bicarbonate can travel with an odd potassium.
A high potassium on a routine blood test is often caused by the sample itself (a hard fist clench, a burst red cell, a delayed tube), so the usual first step is a prompt, careful repeat draw.
In a 1990 study in the New England Journal of Medicine, repeated fist clenching during a blood draw raised measured serum potassium by more than 1 mmol/L, enough to push a normal value well into the flagged range 5. A falsely high result still gets repeated promptly, because a true high potassium can disturb heart rhythm.
Units: mmol/L, mEq/L and the neighbors that use mg/dL
US reports print sodium, potassium, chloride and bicarbonate in mmol/L or mEq/L (milliequivalents per liter, a count of electric charge), and because each carries a single charge the two units give the same number: 140 mmol/L of sodium is 140 mEq/L. The neighbors on the panel follow different rules, shown below; ionized calcium, the free fraction, is a separate order, and the lab units guide covers the conversions.
| Marker | Charge | Usual US unit |
|---|---|---|
| Sodium, potassium, chloride, bicarbonate | 1 | mmol/L (same number as mEq/L) |
| Calcium, magnesium | 2 | mg/dL |
| Phosphorus | Varies | mg/dL |
Sodium reports water balance far more than salt intake
Sodium is a ratio, the sodium in your blood divided by the water it is dissolved in. Your kidneys and a hormone called ADH (antidiuretic hormone, the signal that tells the kidneys to hold on to water) control the water side of that ratio closely. When ADH is high, from nausea, pain, some medicines or certain illnesses, the kidneys keep water the body does not need, the sodium is diluted and the line reads low, while the total amount of sodium in the body may be unchanged. In Adrogué and Madias's review, low sodium is framed as a relative excess of water compared with sodium, rather than a shortage of sodium itself 3. Doctors call a low sodium hyponatremia and a high one hypernatremia.
How extra water lowers a sodium number
Low sodium usually means the body is holding too much water for the sodium it has.
The same logic runs in reverse for high sodium. A value above 145 mmol/L almost always means the body is short of water relative to its sodium, most often in frail older adults or infants who cannot feel thirst or reach a drink 7. Two other effects can lower the sodium line: very high blood sugar pulls water into the blood and genuinely dilutes sodium, and very high triglycerides can make the lab reading falsely low. The sodium page explains both corrections, and the blood sugar guide covers glucose itself.
Why your diet usually is not the explanation
US dietary reference values set a sodium intake of 2,300 mg a day as the level above which cutting back lowers chronic disease risk, and an adequate potassium intake of 3,400 mg a day for men and 2,600 mg a day for women 1. Those numbers are about long-term heart and blood-pressure health. They do not usually show up in your blood test: in people with healthy kidneys, serum sodium and potassium stay in range across a wide span of intakes, because the kidneys excrete what is not needed 1. A high-salt week will not usually move your sodium line, and a low-fruit week will not usually move your potassium line.
Common, usually harmless reasons for a small shift
- Hydration on the day: being dehydrated concentrates the blood and can nudge sodium and chloride to the top of the range; a lot of water before the draw can nudge them down.
- A sample problem: red cells bursting in the tube (the report may say "hemolyzed"), fist clenching, a tight tourniquet, or a sample that sat for hours or was chilled can all raise potassium, as covered in the potassium section above 5.
- Recent vomiting: losing stomach acid lowers chloride and raises bicarbonate 6, and lowers potassium 4.
- Recent diarrhea: losing gut fluid lowers bicarbonate 6 and potassium 4.
- A medicine started or changed in the past few weeks, most often a diuretic, an ACE inhibitor or an ARB (two families of blood-pressure medicines), spironolactone (a water pill that holds on to potassium), an NSAID pain reliever such as ibuprofen, or an SSRI antidepressant 2 4. Do not stop or change a prescribed medicine because of a lab result; talk to the clinician who prescribed it first.
Reading the lines together
The lines carry more meaning as a group. Chloride usually rises and falls with sodium, so a low sodium with a low chloride points back to water. Bicarbonate and chloride pair up on acid-base. When bicarbonate falls, either chloride rises to take its place (common with diarrhea), or the anion gap widens, which signals an acid the panel does not measure, such as lactic acid (lactate) or the ketones of uncontrolled diabetes 6. Potassium is read against creatinine and eGFR, because the kidneys clear most of the daily potassium load 4. The kidney function guide explains those two markers, and how to read any lab result covers pattern reading in general.
Which numbers need same-day care
Example: a sodium result of 131 mmol/Lmmol/L
| Result | What it usually means | Typical next step |
|---|---|---|
| Sodium 130 to 134 mmol/L | Mild low sodium (hyponatremia) 2 | Review medicines and fluid intake with your clinician; usually repeated. Your clinician may add urine sodium and urine concentration (osmolality) tests, and sometimes thyroid and adrenal checks, to find what is holding the water (see urine tests explained) |
| Sodium 125 to 129 mmol/L | Moderate low sodium 2 | Contact your clinician within a day or two, the same day if you have symptoms; the same urine tests are often added |
| Sodium below 125 mmol/L | Profound low sodium 2 | Same-day medical care; an emergency with confusion, vomiting or drowsiness |
| Sodium above 145 mmol/L | High sodium (hypernatremia), usually too little water 7 | Clinician review; same-day care if confused or very drowsy |
| Potassium 5.1 to 5.9 mmol/L | Mildly high; often the sample 4 | A careful repeat draw within a few days, plus a kidney function check (creatinine, eGFR) |
| Potassium 6.0 mmol/L or higher | High potassium that can affect heart rhythm 4 | Same-day medical care, not a routine repeat |
| Potassium 3.0 to 3.4 mmol/L | Mildly low; often a diuretic or fluid loss 4 | Clinician review; magnesium is often checked too, because low magnesium keeps potassium low |
| Potassium below 3.0 mmol/L | Low potassium that can affect heart rhythm 4 | Same-day medical care |
| Low bicarbonate or high anion gap with diabetes, rapid or deep breathing, vomiting, abdominal pain or drowsiness | Possible serious acid build-up, such as diabetic ketoacidosis 6 | Emergency care now |
Potassium that is too high or too low can disturb heart rhythm, and in large observational studies both are linked to higher death rates, a U-shaped pattern that matters most for people with chronic kidney disease, heart failure or diabetes 4.
A sodium that drifted low over weeks is corrected slowly under supervision, because raising it too fast can injure the brain 2; the sodium page explains why.
How the test is done and when it is repeated
- Order: electrolytes are part of every BMP and CMP. There is rarely a reason to order them separately, though a single potassium or sodium can be run on its own for a repeat.
- Fasting: the electrolyte lines do not require fasting. Some clinicians ask you to fast because the same panel includes glucose.
- The draw: tell the person drawing your blood (the phlebotomist) if a previous potassium came back high. A relaxed hand, a short tourniquet time and prompt processing reduce false highs 5.
- Cadence: healthy adults have no set screening schedule for electrolytes. People taking diuretics, ACE inhibitors, ARBs or spironolactone, or living with kidney disease, usually have periodic checks set by their clinician 4.
- After a surprise result: a mildly off value is usually repeated within days to weeks, sooner for potassium; a red-flag value is handled the same day.
For the next level of detail, each line has its own page: sodium, potassium, chloride, bicarbonate and the anion gap. If a low potassium keeps coming back, ask your clinician whether your magnesium result should be checked. The magnesium compound page explains the supplement in general terms; do not start magnesium or potassium supplements to correct a lab value without medical advice, especially if you have kidney disease. The electrolytes topic and the kidney topic collect every related page.
Citations
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Sodium and Potassium. Washington, DC: The National Academies Press; 2019.
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-G47.
- Adrogué HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581-1589.
- Clase CM, Carrero JJ, Ellison DH, et al. Potassium homeostasis and management of dyskalemia in kidney diseases: conclusions from a KDIGO Controversies Conference. Kidney Int. 2020;97(1):42-61.
- Don BR, Sebastian A, Cheitlin M, Christiansen M, Schambelan M. Pseudohyperkalemia caused by fist clenching during phlebotomy. N Engl J Med. 1990;322(18):1290-1292.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162-174.
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.