Fluid and electrolyte questions, made predictable
Written and reviewed by Dana Whitfield, RN, MSN · 7 min read · Updated September 2026
Short answer
Electrolyte items are among the most predictable on the exam, because each imbalance has one thing that kills people and the credited answer is nearly always the one that addresses it. Potassium and magnesium threaten the heart, sodium threatens the brain, and calcium threatens the airway and the heart. Learn the danger, not the list of signs.
Learn each one by its lethal complication
Hypokalaemia and hyperkalaemia both cause dysrhythmias, so a potassium outside range with any cardiac symptom is a report-and-act item. Hyponatraemia causes cerebral oedema, so the findings that matter are neurological — confusion, seizure — not the nausea. Hypocalcaemia causes tetany and laryngospasm, which is an airway problem. Hypermagnesaemia depresses respiration and reflexes.
Memorising twelve signs per imbalance is why this topic feels enormous. Memorising one lethal complication per imbalance covers most of what the exam actually asks, because the questions are written around the thing that hurts the client.
The pairs that move together
Potassium and magnesium travel together: a potassium that will not correct is usually waiting on magnesium, and that is a recurring item. Calcium and phosphate move in opposite directions. Sodium follows water, which is why the treatment for hyponatraemia is often restricting fluid rather than giving salt — the sodium is diluted, not missing.
That last one produces a classic distractor. An option offering sodium to a fluid-overloaded hyponatraemic client looks logical and is wrong for exactly the reason the item exists.
Where the drugs come in
Most electrolyte items on the exam arrive attached to a medication. Loop diuretics drop potassium. ACE inhibitors and potassium-sparing diuretics raise it. Digoxin becomes toxic in hypokalaemia, which is why a low potassium in a digoxin client is an urgent finding rather than a mild one.
Never push IV potassium. It is the single most reliably wrong option on the whole exam, and it appears regularly because it is the intuitive answer for a low number.
How to practise it
Work items rather than tables. For each one, name the imbalance, name its lethal complication, and only then look at the options — the correct answer is nearly always the one addressing that complication or the assessment that confirms it.
When you get one wrong, write down whether you misread the value, missed the drug that caused it, or knew the imbalance and chose a safe-sounding intervention. Those three failures need three different fixes.
Whatever you take from this, the next step is the same: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this guide covers, there are ten more on the practice questions hub, and the pricing page spells out what the free tier includes.
Common questions
What electrolyte values should I memorise for the NCLEX?
Potassium 3.5–5.0, sodium 135–145, calcium 9–10.5, and magnesium 1.3–2.1 mEq/L cover most items. Knowing what each imbalance threatens matters more than the exact boundary.
Why is IV potassium push always wrong?
Because it causes fatal dysrhythmias. Potassium is always diluted and infused with a pump at a controlled rate, and an option offering a push is there to be rejected.
What is the most tested electrolyte on the NCLEX?
Potassium, because so many common drugs move it and because both directions are dangerous to the heart. Digoxin plus hypokalaemia is one of the most repeated pairings on the exam.