The NCLEX does not test whether you know that a potassium of 2.8 is low. It tests whether you know what that means for this patient and what the nurse does first. Across the fluid and electrolyte questions in our bank, the same seven patterns account for most of the items. Learn the patterns and the individual questions stop being surprising.
1. The priority is the organ, not the number
“Which finding requires immediate action?” is never answered with a lab value; it is answered with the consequence. In hyponatremia the priority is the brain — confusion heading toward seizures. In hyperkalemia it is the strip. In hypermagnesemia it is the reflexes and then the breathing. Practice reading a panel and naming the organ at risk before you name the value.
Try it · the number is why, the brain is what
Hyponatremia
See the whole patient respond.
Click or hover any value to see why it changed.
Built for pattern recognition
Solid chips are direct physiology. Dashed chips are common clinical pairings — not automatic cause and effect.
Load an imbalance
Pick a direction, then a value — each opens its own simulator page.
Move any value
Bedside monitor · pattern
Hyponatremia
Excess water relative to sodium lowers serum tonicity, so water moves into brain cells.
Extracellular volume at baseline. The dashed line marks euvolemia.
What the nurse would notice
Hyponatremia across body systems
Headache, confusion, lethargy; seizures under about 120
Cramps, weakness, poor coordination
Nausea and vomiting with an acute fall
Concentrated urine if ADH is the cause
What the nurse does
Seizure precautions. Fluid restriction when volume is normal or high; isotonic saline when the patient is dry. 3% saline only for seizures or severe symptoms, ICU, on a pump.
Neuro checks and sodium every 2–4 hours during correction. Cap the rise near 8 mEq/L in 24 hours — over-correction causes osmotic demyelination, which is permanent.
Match the fluid to the volume status. Too much water is restricted; too little volume is replaced. The number is why; the brain is what.
Teaching model, not a clinical calculator. Patterns show high-yield directional relationships under the stated assumption. Real symptoms, ranges and treatment depend on cause, acuity, comorbidities and local protocols.
Signs & symptoms
Present on this panel. Tap any sign to see which values produce it.
How to think through fluid and electrolytes
Do not memorize eight disconnected lists. Move one value, follow what it changes, and connect the lab pattern to the symptoms you see at the bedside.
Excitable or suppressed?
Low calcium or magnesium removes the brakes: tingling, cramps, hyperreflexia, tetany, seizures. High levels add too much brake: weakness, hyporeflexia, lethargy, respiratory depression.
What can stop the heart?
Potassium and magnesium deserve rhythm attention. In dangerous hyperkalemia, stabilize the myocardium first, then shift potassium into cells, then remove it from the body.
Is circulation failing?
A dramatic lab can distract from volume loss. In DKA and major GI loss, restoring circulating volume comes before the correction students notice first.
Why values move together
- ECF → Na⁺ ↓ 2.1 — Sodium is a concentration, so it moves with the water. Add free water to the extracellular space and the sodium is diluted; lose free water and it concentrates.
- Na⁺ → Cl⁻ ↓ 16.0 — Chloride is the anion that rides along with sodium in the extracellular fluid. Where sodium goes, chloride follows — which is why they usually move together on a panel.
- Na⁺ → ECF ↑ 11.2 — A sodium that climbs is telling you water left (or was never replaced), so the extracellular volume falls with it. Hypernatremia is a water problem before it is a salt problem. The model reads TONICITY here — the glucose-corrected sodium — because a sodium diluted by hyperglycemia is not a sign of extra water.
Normal values
The reference bands this tool uses. Switch to Set and mark them from memory.
Reference ranges vary slightly between laboratories and textbooks. These match the ranges used across NurseSavvy.
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2. Anything changing the ECG comes first
A high potassium with peaked T waves outranks everything else on the page, and the first intervention is IV calcium to stabilize the myocardium — then shift, then eliminate. A question that gives you an ECG change and asks what to do first is testing whether you reach for calcium before insulin, dialysis or a binder. The full order is in hyperkalemia ECG changes in order.
3. Magnesium before potassium and calcium
When two values are low and one of them is magnesium, magnesium is replaced first, because neither potassium nor calcium will hold until it is. The distractor is always the potassium — it looks more urgent, and it is the one that will not work.
4. Volume before the electrolyte
In DKA, in massive GI loss, in burns, the first bag is isotonic fluid, not the correction the labs are shouting for. Insulin before fluids in DKA drives potassium and water into cells in a patient who is already a quarter down on volume — the pressure falls and the potassium crashes. The question usually reads “the provider orders an insulin infusion; what does the nurse do first?” and the answer is start the fluids and confirm urine output.
5. The lab that is lying
A sodium of 128 with a glucose of 620 corrects to about 136 and needs nothing. A total calcium of 7.8 with an albumin of 2.0 corrects to about 9.4. A potassium of 5.4 in DKA is a depleted patient’s shifted potassium and will fall the moment insulin starts. Questions built on these are testing whether you correct the value before you act on it.
6. Slow is the rule for correction
Sodium moves no faster than about 0.5 mEq/L an hour and 8–10 mEq/L in a day, in either direction, or the brain pays for it — cerebral edema one way, osmotic demyelination the other. Potassium is never IV push and runs at about 10 mEq an hour peripherally, on a pump. Calcium and magnesium are given slowly on the monitor. A question offering a fast correction is offering the wrong answer.
7. The assessment that gates the drug
On a magnesium drip: the patellar reflex, the respiratory rate and the blood pressure every hour, and hold the infusion when the reflex disappears or the rate falls under 12. Before potassium: urine output. Before digoxin: the potassium. Before insulin in DKA: the potassium again. These are the questions where the right answer is a check, not a treatment.
How to practise these
Load each pattern on the simulator, treat it, and treat it wrong — the wrong bag runs too, and watching the potassium fall after an early insulin push is worth more than any rationale. Then answer the questions the way the exam asks them: what is the priority, what do you do first, what do you check before the dose. The method for the whole unit is in how to learn fluid and electrolytes, and the couplings behind patterns 3 and 5 are in the six relationships.
Practise the seven patterns
Free account, no card. Fluid and electrolyte questions in every NGN format, tagged by pattern, with a rationale on every option.