Electrolytes on the ward: potassium, calcium, magnesium, phosphate
Treat the ECG, not just the number. Know the doses for high and low potassium, calcium and magnesium.
A 62-year-old man was admitted 2 days ago with 4 days of vomiting and diarrhoea. He takes furosemide for heart failure and omeprazole. Today he has muscle cramps, tingling around his mouth and palpitations.
Bloods: potassium 2.6 mmol/L, magnesium 0.45 mmol/L, calcium 1.95 mmol/L (7.8 mg/dL) with albumin 3.6 g/dL, phosphate normal. ECG: sinus rhythm, flat T waves, prominent U waves, QTc 510 ms.
Which do you replace first, through what line, how fast, and what do you check afterwards?
- ECG and cardiac monitorFor any severe potassium or calcium problem. The ECG tells you how urgent it is.
- Treat dangerous levels nowRepeat a surprising result, but don't wait for the repeat if the ECG is abnormal.
- Check magnesium every timeLow magnesium keeps potassium and calcium low.
- Correct calcium for albuminOr measure ionised calcium on a blood gas.
- Replace by the safest routeOral if possible. IV potassium only in ready-mixed bags, never as a push.
- Find the causeVomiting, diarrhoea, diuretics, drugs, kidneys, parathyroid, cancer, refeeding.
- Set a recheck timeWrite it on the chart. Don't leave it for the morning round.
- Potassium below 2.5 or above 6.5 mmol/L
- Any ECG change from potassium or calcium
- Arrhythmia, palpitations with long QT
- Corrected calcium above 3.5 mmol/L (14 mg/dL) or confusion
- Tetany, stridor or fits
- Severe weakness or breathing difficulty
- Severe hyperkalaemia (see the high potassium guide)
- Low magnesium behind resistant low potassium or calcium
- Hypercalcaemia from cancer or hyperparathyroidism
- Low calcium after thyroid or parathyroid surgery
- Refeeding syndrome
- A haemolysed or drip-arm sample
| Potassium (normal) | 3.5–5.0 mmol/L (same as mEq/L) |
| Severe low potassium | Below 2.5 mmol/L, or any level with ECG changes or weakness |
| Peripheral IV potassium | Usually no more than 10 mmol/hour, ready-mixed bags, maximum 40 mmol per litre |
| Corrected calcium (normal) | 2.2–2.6 mmol/L (8.8–10.4 mg/dL) |
| Dangerous high calcium | Above 3.5 mmol/L (14 mg/dL), or confusion at any high level |
| Dangerous low calcium | Below 1.9 mmol/L (7.6 mg/dL), tetany, stridor, fits or long QT |
| Magnesium (normal) | 0.7–1.0 mmol/L (1.7–2.4 mg/dL) |
| Phosphate (normal) | 0.8–1.5 mmol/L (2.5–4.5 mg/dL) |
Check every dose against a current drug reference and your hospital protocol.
On this page
Why electrolytes matter
Potassium, calcium and magnesium control how heart and muscle cells fire. Too much or too little causes arrhythmias, weakness, fits and death. Most electrolyte problems on the ward are caused by something you can see in the notes: vomiting, diarrhoea, diuretics, kidney injury, new drugs or feeding after starvation.
Three rules keep you safe:
- The ECG sets the urgency for potassium and calcium.
- Magnesium comes with potassium and calcium. Check and replace it.
- Fix the cause, or the level will fall again.
Is the result real?
- Treat now
- Repeat alongside treatment
Repeat quickly on a blood gas or fresh sample
Haemolysis, drip arm, EDTA contamination, delayed processing
| Situation | Clue |
|---|---|
| Haemolysed sample | High potassium, normal ECG, lab comments on haemolysis |
| Sample from a drip arm | Values match the fluid running (for example high glucose, low everything else) |
| EDTA contamination (purple tube filled first) | Very high potassium and very low calcium together |
| Delayed processing | High potassium after hours in transport |
| Low albumin | Low total calcium but normal corrected or ionised calcium |
Potassium
Low potassium (hypokalaemia)
Causes
- Losses from the gut: vomiting, diarrhoea, NG drainage, laxatives.
- Losses in urine: loop and thiazide diuretics, low magnesium, steroids, hyperaldosteronism.
- Shift into cells: insulin (DKA treatment), salbutamol, refeeding, alkalosis.
- Too little in: poor intake, IV fluids without potassium.
Symptoms and signs: weakness, cramps, constipation or ileus, palpitations. Severe: paralysis, breathing weakness, arrhythmias.
ECG: flat T waves, U waves, ST depression, long QT (often a long QU), ectopics, and in severe cases VT, torsades or VF. Low potassium makes digoxin toxicity more likely.
- Oral potassium chloride 40–80 mmol a day in divided doses
- Recheck in 24–48 hours
- IV 40 mmol in 1 litre 0.9% sodium chloride over 4–8 hours
- Check magnesium
- Recheck after each litre
- Cardiac monitor
- IV via ready-mixed bags
- Faster rates only through a central line in HDU
- Senior
- Never give potassium chloride as an IV push or bolus. It stops the heart.
- Use ready-mixed bags (for example 40 mmol in 1 litre). Concentrated ampoules must be fully mixed in a bag before use.
- Peripheral line: usually maximum 10 mmol/hour and 40 mmol per litre. Faster needs a central line and a cardiac monitor.
- Recheck potassium after each bag or every 4–6 hours on IV replacement.
High potassium (hyperkalaemia)
Causes: AKI or CKD, ACE inhibitors, ARBs, spironolactone, NSAIDs, trimethoprim, rhabdomyolysis, DKA (before insulin), Addison's disease, tumour lysis, haemolysed sample.
ECG: tall peaked T waves, flat or absent P waves, long PR, broad QRS, sine wave, then VF or asystole.
The full emergency treatment (calcium gluconate, insulin-dextrose, salbutamol, removal) is in the high potassium guide.
Calcium
Correct it for albumin first
About half of blood calcium is bound to albumin. If albumin is low, total calcium looks low even though the active (ionised) calcium is normal.
| Units | Formula |
|---|---|
| mg/dL (Pakistani labs) | Corrected calcium = measured calcium + 0.8 × (4.0 − albumin in g/dL) |
| mmol/L | Corrected calcium = measured calcium + 0.02 × (40 − albumin in g/L) |
Use the corrected calcium calculator, or measure ionised calcium on a blood gas (normal about 1.1–1.3 mmol/L).
High calcium (hypercalcaemia)
Causes: over 90% are primary hyperparathyroidism (usually outpatients, mild) or cancer (usually inpatients, severe: bone metastases, myeloma, PTH-related peptide from lung, breast, kidney cancers). Others: vitamin D excess, thiazides, lithium, TB and sarcoidosis, immobilisation, thyrotoxicosis.
Symptoms: "bones, stones, groans and moans": bone pain, kidney stones, constipation and abdominal pain, nausea, thirst and passing lots of urine, dehydration, confusion and drowsiness. ECG: short QT; arrhythmias when severe.
- RehydrateIV 0.9% sodium chloride, 3–4 litres over 24 hours if the heart allows. This is the most important step.
- Stop what raises calciumThiazides, calcium and vitamin D supplements, lithium.
- Bisphosphonate once rehydratedZoledronic acid 4 mg IV over 15 minutes (adjust for kidney function), or pamidronate. Works over 2–4 days.
- Find the causePTH first: high or normal = hyperparathyroidism; low = cancer or other cause.
- MonitorCalcium daily, fluid balance, kidney function, watch for overload.
Furosemide is not first-line for hypercalcaemia. Use it only to treat fluid overload once the patient is rehydrated.
Low calcium (hypocalcaemia)
Causes:
- After thyroid or parathyroid surgery (often 24–48 hours after; check calcium the next morning).
- Vitamin D deficiency: very common in Pakistan, especially in women who cover up, the elderly and those with dark skin.
- Low magnesium (stops PTH release).
- CKD, pancreatitis, massive transfusion (citrate), rhabdomyolysis, some drugs (bisphosphonates, denosumab).
Symptoms and signs: tingling around the mouth, fingers and toes, cramps, carpopedal spasm, Trousseau's sign (hand spasm when a BP cuff is inflated above systolic for 3 minutes), Chvostek's sign (facial twitch when tapping the facial nerve), laryngospasm and stridor, fits, long QT, arrhythmias, heart failure.
- Oral calcium 1–1.5 g elemental a day with vitamin D
- Treat cause
- Calcium gluconate 10% 10–20 mL IV over 10 minutes
- Then infusion
- Cardiac monitor
- Check magnesium
- After IV calcium, start an infusion (for example 100 mL of 10% calcium gluconate in 1 litre of 0.9% saline or 5% dextrose at 50 mL/hour, adjusted to levels), then oral calcium and vitamin D (often alfacalcidol or calcitriol after parathyroid surgery).
- Calcium gluconate irritates veins; calcium chloride is even more irritant and needs a large or central vein.
- Never put calcium in the same line as sodium bicarbonate (it precipitates).
Magnesium
Low magnesium: alcohol, PPIs (long-term), diuretics, diarrhoea, poor intake, refeeding, cisplatin. It causes cramps, tremor, arrhythmias (including torsades) and stubborn low potassium and calcium.
| Situation | Treatment |
|---|---|
| Torsades de pointes or severe with arrhythmia | Magnesium sulfate 2 g (8 mmol) IV over 10–15 minutes |
| Below 0.5 mmol/L, or symptoms | Magnesium sulfate 20 mmol IV over 4–12 hours (per protocol) |
| Mild, no symptoms | Oral magnesium; review PPI and diuretic |
Reduce doses and monitor closely in kidney failure: magnesium is excreted by the kidneys.
High magnesium is uncommon outside kidney failure and magnesium infusions (for example in pre-eclampsia): loss of reflexes, low BP, slow breathing. Treat with calcium gluconate and stopping the source.
Phosphate and refeeding syndrome
After days of little food (malnutrition, alcohol dependence, eating disorders, long illness, prolonged fasting, hunger strike), feeding drives phosphate, potassium and magnesium into cells. Levels crash within 2–4 days of starting food, causing weakness, breathing failure, heart failure, arrhythmias, confusion and death.
- Identify the riskLittle or no intake for 5 days or more, BMI below 16, recent weight loss, alcohol dependence, low baseline potassium, phosphate or magnesium.
- Check before feedingPotassium, phosphate, magnesium, glucose.
- Thiamine before and during100 mg orally three times daily (or IV) before starting feeds, plus a vitamin B complex.
- Start slowlyAbout 10 kcal/kg/day (5 kcal/kg/day in extreme cases), increasing over 4–7 days, with dietitian help.
- Monitor dailyPotassium, phosphate, magnesium for the first week. Replace as needed.
Investigations
| Test | Why |
|---|---|
| ECG | Urgency for potassium and calcium; QT |
| Urea, creatinine, sodium, potassium | Kidney function and trend |
| Magnesium, phosphate, albumin | Linked abnormalities |
| Venous blood gas | Quick potassium, ionised calcium, pH |
| Glucose | Insulin shifts, refeeding |
| PTH | High or low calcium: separates causes |
| Vitamin D | Low calcium, high calcium from excess |
| CK | Rhabdomyolysis |
| Urine potassium | Stubborn low potassium: kidney vs gut loss (specialist) |
| Myeloma screen, chest X-ray | High calcium with low PTH |
Treatment doses
Common mistakes
- Giving potassium as a push or running it too fast through a small vein.
- Replacing potassium for days without checking magnesium.
- Treating a haemolysed potassium without repeating it (or ignoring a real one because "it's probably haemolysed").
- Using total calcium without correcting for albumin.
- Giving furosemide before rehydrating a patient with high calcium.
- Starting full feeds in a starved patient without thiamine and electrolyte monitoring.
- Writing "repeat U&E in the morning" with no time and no plan.
Follow-up
- Write the next check time on the chart and hand it over.
- Stop or reduce the culprit: diuretics, PPIs, ACE inhibitors, spironolactone, thiazides.
- Treat the cause: anti-emetics, fluids for diarrhoea, investigation of high calcium, vitamin D replacement.
- Send outpatients with persistent abnormalities to the right clinic (endocrine for calcium, renal for potassium).
Frequently asked questions
What is the maximum rate of IV potassium through a peripheral line?
Usually 10 mmol per hour, in a ready-mixed bag with no more than 40 mmol per litre. Faster rates need a central line and cardiac monitoring.
Why replace magnesium when potassium is low?
Low magnesium makes the kidneys waste potassium and stops the parathyroid glands releasing PTH, so potassium and calcium won't stay up until magnesium is corrected.
How do I correct calcium for albumin?
In mg/dL, add 0.8 to the measured calcium for every 1 g/dL that albumin is below 4.0. In mmol/L, add 0.02 for every 1 g/L below 40.
What is the first treatment for high calcium?
IV 0.9% saline to rehydrate (3–4 litres over 24 hours if the heart allows), then a bisphosphonate such as zoledronic acid, and treatment of the cause.
What is refeeding syndrome?
Dangerous falls in phosphate, potassium and magnesium when a starved or malnourished patient starts eating again. Prevent it with thiamine, slow feeding and daily electrolyte checks.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A patient's potassium stays low despite 3 days of replacement. What should be checked?
Answer: Magnesium. Low magnesium causes renal potassium loss; potassium won't correct until magnesium is replaced.
- Thyroid function: Not first.
- Calcium: Less relevant.
- Sodium: Not the main cause.
- Phosphate: Not the main cause.
2. What is the usual maximum rate of IV potassium through a peripheral line on a general ward?
Answer: 10 mmol per hour. Use ready-mixed bags; faster rates need a central line and cardiac monitoring.
- 40 mmol per hour: Needs a central line and monitoring.
- 100 mmol per hour: Dangerous.
- No limit: Unsafe.
- As a 20 mmol bolus: Never give potassium as a bolus.
3. A patient has corrected calcium 3.8 mmol/L (15.2 mg/dL), confusion and dehydration. What is the first treatment?
Answer: IV 0.9% sodium chloride. Severe hypercalcaemia: rehydrate with IV saline, then a bisphosphonate, and find the cause.
- Furosemide first: Only after rehydration, if at all.
- Oral bisphosphonate first: Rehydrate first.
- Thiazide diuretic: Raises calcium.
- IV calcium gluconate: Wrong direction.
4. A patient on long-term omeprazole has refractory hypokalaemia and hypocalcaemia with tremor and prolonged QT. Magnesium is 0.4 mmol/L. What should be given first?
Answer: IV magnesium sulfate. Hypomagnesaemia causes: PPIs, diuretics, alcohol, diarrhoea, malabsorption, aminoglycosides, cisplatin, refeeding. Magnesium is needed for PTH secretion/action (→ hypocalcaemia) and renal potassium retention (→ refractory hypokalaemia). Severe/symptomatic: IV magnesium sulfate (e.g. 8 mmol over 15 min for arrhythmia/torsades, then infusion); oral for mild. Stop the PPI if possible.
- More oral potassium only: Potassium will not correct without magnesium.
- Calcitriol alone: Calcitriol does not fix magnesium.
- Stop all fluids: Stopping fluids does not help.
5. A 65-year-old woman with breast cancer has confusion, polyuria and constipation. Calcium is 3.6 mmol/L. What is the first treatment?
Answer: IV 0.9% saline. Severe hypercalcaemia (> 3.5 mmol/L) or symptomatic: rehydrate with IV 0.9% saline (3–4 L/day as tolerated), then IV bisphosphonate (zoledronic acid or pamidronate; effect in 2–4 days). Calcitonin acts fast but briefly; steroids for lymphoma/granulomatous disease or vitamin D toxicity; denosumab if renal failure. Malignancy causes via PTHrP or bone metastases; primary hyperparathyroidism is the commonest outpatient cause.
- IV bisphosphonate before fluids: Fluids come before bisphosphonates.
- Furosemide before fluids: She is dehydrated, so fluids come first.
- Oral calcium restriction only: Dietary restriction is not enough at 3.6 mmol/L.
6. A thin young man has tetany. His serum calcium is 6 mg/dL, phosphate is low and urinary calcium is very low. PTH is high. What is the most likely cause?
Answer: Vitamin D deficiency. Vitamin D deficiency reduces calcium and phosphate absorption. The low calcium raises PTH (secondary hyperparathyroidism), which wastes phosphate in the urine, so phosphate is low. In hypoparathyroidism, phosphate is high and PTH low; in CKD, phosphate is high.
- Primary hypoparathyroidism: PTH would be low and phosphate high.
- Chronic kidney disease: Phosphate would be high.
- Pseudohypoparathyroidism: PTH is high but phosphate is high (end-organ resistance).
- Primary hyperparathyroidism: Calcium would be high.
7. In a rat given calcium chloride to make it hypercalcaemic, which vitamin D metabolite will the kidney make more of?
Answer: 24,25-dihydroxycholecalciferol. High calcium suppresses PTH. With less PTH, the kidney's 1-alpha-hydroxylase falls and 24-hydroxylase rises, so 25-hydroxyvitamin D is converted to the inactive 24,25-dihydroxy form instead of active calcitriol. This reduces calcium absorption from the gut.
- 1,25-dihydroxycholecalciferol (calcitriol): Production falls when calcium is high (PTH low).
- 25-hydroxycholecalciferol: Made in the liver; not regulated by calcium in this way.
- 7-dehydrocholesterol: A skin precursor converted by UV light.
- Ergocalciferol: Plant-derived vitamin D2.
References and dose sources
- UK Kidney Association — hyperkalaemia
- Society for Endocrinology
- Resus Council UK — ALS
- NICE CG32 — nutrition support (refeeding)
- Potassium chloride dosing: Drugs.com: Potassium chloride dosage (see also our potassium chloride dose page)
- Calcium gluconate dosing: Drugs.com: Calcium gluconate dosage (see also our calcium gluconate dose page)
- Magnesium sulfate dosing: Drugs.com: Magnesium sulfate dosage (see also our magnesium sulfate dose page)
- Sodium chloride (normal saline, 3% saline) dosing: Drugs.com: Sodium chloride dosage (see also our sodium chloride (normal saline, 3% saline) dose page)
- Thiamine (vitamin B1) dosing: Drugs.com: Thiamine dosage (see also our thiamine (vitamin b1) dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.