NCLEX-RN cheat sheet · Fluids, electrolytes and acid-base
Fluids and electrolytes
Overload and deficit, sodium, potassium, calcium, magnesium and phosphate
AI-checkedChecked by AI against current guidelines, with spot-checks by a registered nurse.
The scale and the intake record catch most fluid problems. Sodium mostly affects the brain, potassium mostly affects the heart, and both can turn dangerous fast.
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At a glance
Too much or too little fluid
| Check | Overload | Deficit |
|---|---|---|
| Causes | Heart failure, kidney failure, cirrhosis, pregnancy | Vomiting, diarrhea, sweating, fever, bleeding, diuretics, not drinking enough |
| BP, pulse | High BP; bounding pulses | Low BP; fast, weak, thready pulse |
| Lungs, neck veins | Crackles (often first in the lower back of the lungs); distended neck veins | Clear; flat neck veins |
| Skin, mouth | Tight, swollen, shiny | Tenting; dry, sticky mouth |
| Urine | May fall | Low, dark, concentrated |
| Weight | Up over 1 kg a day | Down over 1 kg a day |
Trap: Heavy edema can hide a deficit: the vessels can still be dry, with low pressure and a fast, thready pulse.
Sodium: about 136 to 145 mEq/L, by lab
| High sodium | Low sodium | |
|---|---|---|
| Causes | Water loss (too little drinking, vomiting, diarrhea) or too much salt | Too much water or hypotonic IV fluid, diuretics; also heart failure, cirrhosis, kidney disease, SIADH, underactive thyroid, SSRIs |
| Cells | Shrink | Swell |
| Signs (brain) | Confusion, irritability, lethargy, seizures; severe thirst, sticky mouth | Headache, confusion, vomiting, cramps, seizures, coma |
| Treatment | Less salt, controlled free water (like dextrose 5% in water). Low-volume shock: isotonic fluid first | Isotonic saline if volume-depleted; restrict water if water excess. Severe symptoms: emergency 3 percent saline bolus protocol |
Watch: Check sodium serially, stay within the ordered correction limits, and watch mental status: seizures can follow.
Potassium: about 3.5 to 5.1 mEq/L
| High potassium | Low potassium | |
|---|---|---|
| Causes | Kidney failure, acidosis, potassium-sparing diuretics, ACE inhibitors, supplements | Vomiting, diarrhea, potassium-wasting diuretics, insulin, poor diet; low magnesium |
| Signs | Irritability, cramps, diarrhea; often silent until it shows in the heart rhythm | Weakness, lethargy, thready pulse, arrhythmias; makes digoxin more toxic |
| ECG | Peaked T from about 5.5 mEq/L. Above about 6.5: flat P, long PR, wide QRS; can slide into sine wave and arrest | ST dip, flat or inverted T, U waves, long QT (torsades risk) |
| Treatment | Severe: IV calcium gluconate first to protect the heart; insulin with dextrose; binder, loop diuretic or dialysis. Mild: cut dietary potassium, review drugs like spironolactone | Food, oral or IV potassium, run slowly. Check the kidneys first, recheck after; check magnesium |
Never IV push: IV push potassium can stop the heart. Give it diluted and slow: the label limits it to about 10 mEq an hour in most cases; faster needs continuous heart monitoring.
Calcium, magnesium and phosphate (use your lab's range)
| Normal | Too high | Too low |
|---|---|---|
| Total calcium about 8.6 to 10.2 mg/dL. Ionized calcium is a separate test with its own range | Immobility, cancer, overactive parathyroid. Slows things: nausea, vomiting, constipation, thirst, more urine, weak muscles | Low parathyroid hormone (as after the glands are removed), low vitamin D, kidney disease, high phosphorus. Twitchy: tingling lips, hands and feet, cramps, tetany, long QT |
| Magnesium about 1.5 to 2.4 mg/dL | Kidney failure, too much replacement, magnesium laxatives or antacids. Slows: slow, weak pulse, lethargy, weak reflexes, cardiac arrest | Poor diet, loop diuretics, alcohol use disorder, long-term PPIs. Excitable: tremor, leg cramps, dysrhythmias, tetany |
| Phosphorus about 2.5 to 4.5 mg/dL in adults | Kidney disease, crush injuries, too many phosphate enemas. Usually silent; the low calcium it causes can show as tingling and cramps | Alcohol misuse, burns, diuretics, starvation, recovering DKA; refeeding can drop it fast. Severe: weakness, confusion, seizures |
Not always opposite: Calcium and phosphate often move in opposite directions in parathyroid hormone patterns, but not always: low vitamin D can lower both.
Measuring fluid status
- Daily weights: same scale, same time of day, similar clothing. 1 kg in 24 h is about 1 liter: report it.
- Urine is about 1,500 mL a day. Adults: report under about 0.5 mL/kg/h, or a falling trend. 30 mL/h matches that only at 60 kg.
- Urine specific gravity, typically 1.005 to 1.030 by your lab, rises when urine is concentrated. Hematocrit and BUN rise when the blood is concentrated.
Follow policy: Report low BP by ordered parameters or policy (a systolic under 100 mmHg is one example), weighing baseline and symptoms.
Care
- Overload: head of bed up, diuretics as ordered, fluid and sodium limits (ice chips count in the limit). Watch the lungs.
- Deficit: treat the cause, IV fluids as ordered, preferred fluids every 1 to 2 hours, oral rehydration for losses. Check orthostatics.
- Moderate coffee or tea still counts; limit excess caffeine, alcohol, and prune juice with loose stools.
Priority and safety calls
- Treatment can swing a client from one problem to the other: reassess before every intervention, and listen to the lungs during IV fluids.
- New crackles and breathlessness during IV fluids: slow the infusion per protocol, raise the head of the bed, notify the provider.
- Report signs of shock at once: fast pulse and breathing, falling pressure, cold, clammy skin, confusion.
- A widening QRS with high potassium is an emergency.
- ★ Hemolyzed high potassium: report it, assess the client, expect an urgent repeat. A normal ECG doesn't rule out danger; don't delay indicated treatment.
- Insulin for high potassium goes with dextrose; check glucose often, sometimes hourly.
- Newly confused on a diuretic or an SSRI: check the sodium.
- After parathyroid surgery: ask about tingling, and report twitching or spasm at once.
- ★ Magnesium sulfate: lost knee reflexes or slow breathing mean toxicity. Stop the infusion, get help, and give IV calcium gluconate per protocol.
How the NCLEX tests it
- Overnight gain over 1 kg in heart failure: assess the lungs and breathing, and report it.
- Trap: assuming edema means full vessels, or weighing on a different scale.
- First drug for dangerous high potassium with ECG changes: IV calcium gluconate, then insulin with dextrose.
- High potassium: peaked T waves, wide QRS. Low potassium: flat T waves, U waves.
- Tingling plus a positive Chvostek's sign after parathyroid surgery: low calcium.
- Trousseau's sign: hand spasm with a BP cuff inflated above systolic for 3 minutes.
- Traps: magnesium antacids in kidney failure, and missing a low magnesium when potassium or calcium won't come up.
Practice it next
This sheet comes from Fluid balance: deficit and overload and Sodium and potassium and Calcium, magnesium and phosphate in our NCLEX-RN course. Try free practice questions, or see the lessons it was written from.
Quick self-quiz
- A weight change of 1 kg in 24 hours is about 1 ___ of fluid.
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liter - For adults, report urine under about ___ mL/kg/h.
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0.5 - Normal sodium is about 136 to ___ mEq/L.
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145 - Potassium is never given by IV ___.
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push - Peaked T waves can appear from a potassium of about ___ mEq/L.
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5.5 - A facial twitch when you tap over the facial nerve is ___'s sign.
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Chvostek - Lost knee reflexes on magnesium sulfate: ___ the infusion.
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stop
Sources
- Open RN, Nursing Fundamentals 2e, 15.4 Electrolytes
- Open RN, Nursing Fundamentals 2e, 15.2 Basic Fluid and Electrolyte Concepts
- Open RN, Nursing Fundamentals 2e, 15.6 Applying the Nursing Process
- KDIGO consensus on potassium disorders (PubMed Central)
- Society for Endocrinology emergency guidance: severe symptomatic hyponatremia (PubMed Central)
- Endotext, mineral homeostasis (NCBI Bookshelf)
- DailyMed, Potassium Chloride for Injection Concentrate label
- AHRQ PSNet WebM&M, A Fatal Twist in Pseudohyperkalemia (2024)
More fluids, electrolytes and acid-base cheat sheets
Educational use only. This sheet is a study aid for the NCLEX-RN, not a substitute for clinical judgment, your facility’s policy or the provider’s orders. Where practice varies, follow facility policy and state law. NCLEX® and NCLEX-RN® are registered trademarks of NCSBN, which does not endorse these materials.