Low Potassium on a Blood Test: What It Means
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 31 October 2026
Low potassium on a blood test: normal ranges in Spain, the UK and US, common causes such as water pills and diarrhoea, warning signs and next steps.
You don't have to figure this out alone
Dr. Pedro Yelamo Vera is an independent, bilingual GP licensed in Spain. Book a 15-minute video consultation in English or Spanish for €50, with a private electronic prescription issued under Spanish law when clinically appropriate.
Low potassium on a blood test means the level is below about 3.5 mmol/L. A small dip is common and often causes no symptoms. The usual causes are water pills (diuretics), vomiting or diarrhoea. A level under 3.0 mmol/L needs a doctor today, because very low potassium can upset the heart rhythm.
- Typical range: about 3.5 to 5.1 mmol/L. Each lab prints its own range.
- Commonest causes: water pills, vomiting, diarrhoea and laxatives.
- A mild dip often gives no symptoms. Larger drops cause weakness, cramps and palpitations.
- Under 3.0 mmol/L: see a doctor today. Fainting or chest pain: call 112.
- Do not start potassium tablets on your own. Ask a doctor first.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
What is a normal potassium level?
MedlinePlus gives a normal range of 3.5 to 5.1 mmol/L. Mayo Clinic uses 3.6 to 5.2 mmol/L. UK labs follow a shared range of 3.5 to 5.3 mmol/L. The differences are small. Always read the range printed next to your result.
Units can look different, but the number is the same. US reports often say mEq/L. Spanish and UK reports usually say mmol/L. For potassium, 1 mEq/L equals 1 mmol/L.
How do I find potassium on a Spanish blood report?
Look for potasio or the letter K. It often sits with sodium (sodio) under iones or bioquímica. An "L" or a star next to the value means it is below the range. In the UK it is part of the "U&Es" (urea and electrolytes). In the US it is part of a basic or full metabolic panel.
What causes low potassium?
Two causes top the list. Mayo Clinic names water pills, which make you lose potassium in urine. They are often used for high blood pressure, heart disease or swelling. The MSD Manual puts gut losses first. Common causes are:
- Water pills (diuretics), such as thiazide and loop diuretics.
- Gut losses: vomiting, diarrhoea, or using laxatives for a long time.
- Low magnesium, which makes the kidneys lose potassium.
- Heavy sweating and heavy drinking.
- Some medicines: insulin and asthma relievers such as salbutamol. Both shift potassium into cells, often only for a short time.
- Eating a lot of liquorice (regaliz), which acts like the hormone aldosterone.
- Hormone and kidney conditions, such as too much aldosterone, Cushing syndrome or rare inherited kidney problems.
- Eating disorders with vomiting or laxative use.
A poor diet alone is a rare cause. The MSD Manual calls it very uncommon.
Can the result be wrong?
Yes, sometimes. A guide from an NHS hospital lab explains two traps. If a sample sits in a warm van or room before testing, potassium can read falsely low. This is more common in hot weather, which matters in a Spanish summer. A very high white cell count can also lower the reading in the tube. If you feel well and the drop is small, a repeat test is often the first step.
What are the symptoms of low potassium?
Mayo Clinic notes that low potassium rarely causes symptoms if you feel well. When it does, MedlinePlus lists:
- Tiredness and muscle weakness.
- Muscle cramps or twitches.
- Constipation.
- Tingling or numbness.
- Palpitations or skipped beats.
A large drop can cause an irregular heartbeat, dizziness or fainting. The risk is higher if you have heart disease or take digoxin. In rare cases very low potassium causes paralysis.
Got a low potassium result and not sure what to do?
You can book a video visit with a GP licensed in Spain, in English or Spanish. He reads your whole report, checks your medicines and explains whether you need a repeat test or more care.
How low is too low?
Sources use slightly different cut-offs. This is a fair summary:
- 3.0 to 3.5 mmol/L (mild): often no symptoms. A doctor looks for the cause and repeats the test.
- Under 3.0 mmol/L: Mayo Clinic says this can be life-threatening and needs urgent care. See a doctor today. You will likely need a heart tracing (ECG).
- Under 2.5 mmol/L, or any level with symptoms: the NHS lab guide advises urgent hospital care. Go to urgencias.
Call 112 if you faint, have chest pain, have a fast or irregular heartbeat with dizziness, or are too weak to stand or breathe properly.
What happens next?
The doctor first finds the cause. MedlinePlus says they may also check magnesium, sugar, calcium, kidney and thyroid tests, and sometimes aldosterone. Low magnesium is often found with low potassium. The NHS lab guide says potassium is hard to correct until magnesium is fixed too.
Treatment depends on the cause and how low the level is:
- Mild cases are often helped by potassium-rich foods, such as bananas, oranges, potatoes, spinach, beans and fish.
- Some people need potassium tablets. A doctor decides this.
- If a water pill is the cause, the doctor may change it. Do not stop a heart or blood pressure medicine on your own.
- Very low levels are treated in hospital, often through a drip.
Mayo Clinic advises not to start potassium supplements without talking to a doctor. Too much potassium is also dangerous, especially with kidney disease.
What about children and pregnancy?
Vomiting and diarrhoea are a common route to low potassium at any age, including a tummy bug in a child. MedlinePlus advises contacting a doctor right away after a lot of vomiting or heavy diarrhoea. For a child, or in pregnancy, do not wait: see a doctor today. Children also have their own potassium ranges, so the adult numbers above do not apply to them.
For a tummy bug in adults, see our guide to food poisoning and gastroenteritis in Spain.
Talking it through with a doctor by video
Dr. Yelamo is a GP licensed in Spain. He sees adults, children and pregnant patients by video, every day from 07:30 to 20:30 (Spain time). In the visit he will:
- Read your full blood report, not just the potassium line.
- Ask about water pills, laxatives, steroids, inhalers and liquorice.
- Check for vomiting, diarrhoea and warning signs.
- Explain whether you need a repeat test, an ECG or urgent care.
He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
A prescription is never guaranteed: it is issued only where clinically appropriate after a genuine evaluation, as a private electronic prescription under Spanish law. This service is for urgent, non-emergency care. In an emergency, call 112.
See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.
This guide is general information. It does not replace a consultation with your doctor.
Frequently asked questions
Is low potassium serious?
A mild dip is common and often harmless, but the cause should be found. A level under 3.0 mmol/L needs a doctor today because it can affect the heart rhythm.
What is the most common cause of low potassium?
Water pills (diuretics) and gut losses from vomiting, diarrhoea or laxatives are the most common causes.
Can I raise my potassium with food?
Foods such as bananas, oranges, potatoes, spinach, beans and fish can help in mild cases. A doctor should still look for the cause.
Should I take potassium supplements?
Not without asking a doctor. Too much potassium is also dangerous, especially if you have kidney disease.
Can a blood sample give a falsely low potassium?
Yes. A sample left in a warm place before testing, or a very high white cell count, can make potassium read low. A repeat test helps.
What is potasio on a Spanish blood test?
Potasio is the Spanish word for potassium. It is shown as K, in mmol/L or mEq/L, which give the same number.
Sources
- MedlinePlus: Low blood potassium
- Mayo Clinic: Low potassium (hypokalemia)
- MSD Manual: Hypokalemia (low level of potassium in the blood)
- Royal United Hospitals Bath NHS: Hypokalaemia, a guide for GPs
- University Hospitals Birmingham NHS: Potassium reference range
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
