NCLEX-RN® Electrolytes Made Simple: Sodium, Potassium, Calcium, Magnesium

The four electrolytes NCLEX® tests most — what causes imbalances, the clinical picture you'll see in stems, and the safe nursing action for each.

Overview

Electrolyte disorders are one of the highest-yield topics on the NCLEX-RN®. They appear in cardiac questions, renal questions, endocrine questions, and many NGN case studies. The good news: most students only need four electrolytes — sodium, potassium, calcium, magnesium — and a small set of patterns.

This guide gives you the normal ranges, the symptoms of high and low for each, and the nursing actions the exam expects.

The four electrolytes at a glance

Four electrolytes Four boxes: sodium 135-145, potassium 3.5-5.0, calcium 9-10.5, magnesium 1.5-2.5. Normal ranges (adult) Sodium 135–145 mEq/L "Na" Potassium 3.5–5.0 mEq/L "K" Calcium 9.0–10.5 mg/dL "Ca" Magnesium 1.5–2.5 mEq/L "Mg"
Figure 1. The four electrolytes that account for most exam questions.

Sodium (Na) — water and brain

Think brain swelling vs brain shrinking.

  • Hyponatraemia (< 135) — confusion, headache, nausea, seizures (severe). Causes: SIADH, diuretics, water intoxication, heart failure. Action: seizure precautions, fluid restriction, slow correction (rapid correction → osmotic demyelination).
  • Hypernatraemia (> 145) — thirst, dry mucous membranes, restlessness, seizures. Causes: dehydration, diabetes insipidus, tube feeds without enough free water. Action: slow rehydration with hypotonic fluids; rapid correction → cerebral edema.

The shared trap: correct slowly for either. The brain is fragile.

Potassium (K) — heart and muscle

Think cardiac arrhythmias and weakness.

  • Hypokalaemia (< 3.5) — muscle weakness, cramps, flat T waves, U waves, dig toxicity risk, ileus. Causes: diuretics, vomiting, NG suction, low intake. Action: PO replacement first if mild; IV (always diluted, never push, max 10 mEq/hr peripheral, monitor cardiac).
  • Hyperkalaemia (> 5.0) — muscle weakness, peaked T waves, wide QRS, cardiac arrest. Causes: AKI, K-sparing diuretics, ACEi, crush injury, blood transfusion. Action: hold K, cardiac monitor, calcium gluconate (cardio-protective), insulin + dextrose (shifts K in), Kayexalate or dialysis (removes K).
Potassium and the ECG Two ECG sketches: low potassium shows flat T waves and U waves, high potassium shows peaked T waves and wide QRS. Potassium changes the ECG Low K (< 3.5) Flat T · U wave appears High K (> 5.0) Tall peaked T · wide QRS
Figure 2. The two K patterns are visually distinct on a strip — recognising them is high-yield.

Calcium (Ca) — neuromuscular excitability

Think bones, stones, groans, moans (high) and tetany (low).

  • Hypocalcaemia (< 9.0) — Trousseau's sign (carpal spasm with BP cuff), Chvostek's sign (facial twitch), tingling, tetany, laryngospasm, prolonged QT. Causes: hypoparathyroidism, post-thyroidectomy, vitamin D deficiency, blood transfusion (citrate). Action: IV calcium gluconate slow push, seizure precautions, monitor airway.
  • Hypercalcaemia (> 10.5) — fatigue, constipation, polyuria, kidney stones, confusion. Causes: hyperparathyroidism, malignancy. Action: hydration with normal saline, furosemide (calcium-wasting), bisphosphonates, weight-bearing if safe.

Magnesium (Mg) — quiet but critical

Think of magnesium as calcium's quieter cousin — symptoms are similar.

  • Hypomagnesaemia (< 1.5) — neuromuscular irritability, tremors, torsades de pointes, hyperactive reflexes, alcohol withdrawal cohort. Action: IV magnesium sulfate (slow), seizure precautions, cardiac monitor.
  • Hypermagnesaemia (> 2.5) — sedation, hyporeflexia, hypotension, respiratory depression, cardiac arrest. Causes: pre-eclampsia treatment overdose, antacid abuse in renal failure. Action: stop magnesium, calcium gluconate antidote, monitor RR and reflexes.

The OB connection: a pre-eclamptic patient on Mg drip needs deep tendon reflex, RR, and urine-output checks every hour. Loss of reflexes = early sign of toxicity.

A "high vs low" memory pattern

For most electrolytes, the same body system is affected — only the direction changes.

High vs low Grid showing for sodium brain swelling vs shrinking, potassium muscle weakness with ECG flat or peaked T, calcium tetany vs lethargy, magnesium hyperreflexia vs hyporeflexia. Same system, opposite direction NaBrain swellvsBrain shrink KFlat TvsPeaked T CaTetanyvsLethargy MgHyperreflexvsHyporeflex Memory tipCalcium & Mgare opposite-acting:low Ca = excitablelow Mg = excitablehigh Mg = sedated
Figure 3. The same body system shows opposite signs at opposite ends of the range.

FAQ

Which electrolyte abnormality is the most life-threatening?

Hyperkalaemia (K > 6.0) is the classic NCLEX® emergency because it can cause sudden cardiac arrest. Severe hyponatraemia (Na < 120) is close behind because of seizure risk. Both deserve immediate cardiac monitoring and provider notification.

Can I push potassium IV?

Never. Concentrated KCl must be diluted in IV fluid and infused no faster than 10 mEq/hour through a peripheral line. Faster rates require central access and continuous cardiac monitoring. IV-push potassium is fatal.

What is the antidote for magnesium toxicity?

Calcium gluconate IV. Recognise toxicity by absent deep tendon reflexes, falling respiratory rate, and falling blood pressure on a magnesium infusion (often a pre-eclamptic patient). Stop the magnesium first, then give calcium.

Why does low calcium cause Trousseau's and Chvostek's signs?

Both result from increased neuromuscular excitability when ionised calcium is low. Trousseau's: inflate a BP cuff above systolic for 3 minutes — carpal spasm appears. Chvostek's: tap the facial nerve in front of the ear — facial twitch appears. They are classic NCLEX® assessment items.

How fast should sodium be corrected?

Slowly — usually no more than 8–12 mEq/L in 24 hours. Rapid correction of hyponatraemia causes osmotic demyelination; rapid correction of hypernatraemia causes cerebral edema. The exam favours answers that include words like slow correction or gradual replacement.

See also:

Practice on RN Clarity: Question Bank · Flashcards · Drug Cards