CCRN Fluid and Electrolyte Review: Potassium, Sodium, Calcium, Magnesium and Phosphate

Where fluids and electrolytes sit on the CCRN (Adult) test plan

The Adult CCRN test plan, for exams taken on and after November 12, 2025, lists "fluid and electrolyte imbalances" among the 22 problems of the Multisystem category, which is 16% of the exam. It also lists "manage patient fluid and electrolyte balance" among the general nursing actions that apply in every category. So expect electrolytes both as the subject of an item and as a detail inside a cardiac, renal or endocrine scenario. AACN publishes no item count for a single problem.

This review covers seven imbalances, plus volume depletion. The clinical points come from MSD Manual Professional chapters, each with a full review dated June 2025 (June 2026 for volume depletion), read on November 6, 2026. It gives no drug doses. Doses, infusion rates and replacement scales belong to your unit's protocols and the prescriber's orders.

Seven electrolyte imbalances at a glance

Thresholds are MSD's. Laboratories set their own reference ranges, so a value printed in a stem may sit a little differently from the cutoffs here.

High calcium, high magnesium and high phosphate are not reviewed here. High phosphate appears with tumor lysis syndrome in the CCRN hematology and immunology review.

Hyperkalemia: read the ECG, then sort the treatments by what they do

MSD says hyperkalemia is usually asymptomatic until cardiac arrhythmias develop, which is why the ECG matters more than how the patient feels. The changes come in an order:

1. Early: a longer PR interval, a shorter QT interval, and tall, symmetric, peaked T waves. These are frequently visible once potassium is above 5.5 mEq/L.
2. Above 6.5 mEq/L: the QRS widens, the P wave disappears, and nodal and ventricular escape rhythms appear.
3. Finally: the QRS degenerates into a sine wave, followed by ventricular fibrillation or asystole.

MSD adds that potassium levels vary widely among patients with similar ECG changes, so treat the tracing in front of you.

Check the sample first when the picture does not fit. A hemolyzed blood sample is the most common cause of a falsely high result. MSD says to consider pseudohyperkalemia in patients without risk factors or ECG abnormalities, and to repeat the test with care to avoid hemolysis. A patient with kidney injury and peaked T waves is not that patient.

Sort the treatments by function. Exam items often turn on which treatment does what, and how fast.

Three safety points from the same chapter are easy to test. Calcium is given with caution to a patient taking digoxin. Calcium chloride can cause tissue necrosis if it extravasates, so MSD says to give it only through a correctly positioned central venous catheter. And evidence for IV sodium bicarbonate in hyperkalemia is limited. Glucose checks after insulin follow from the hypoglycemia risk above. Items like these depend on reading a value in context, and full CCRN prep includes practice in reading lab values inside ICU scenarios.

Hypokalemia: weakness, U waves and the magnesium check

MSD grades hypokalemia by level. Mild (3.0 to 3.4 mEq/L) rarely causes symptoms. Moderate (2.5 to 3.0) generally causes muscle weakness and may lead to paralysis and respiratory failure. Severe (below 2.5) causes ECG changes or severe symptoms. Cardiac effects are usually minimal until potassium is below 3 mEq/L, and then the ECG shows a sagging ST segment, a depressed T wave and a growing U wave. Premature beats, tachyarrhythmias and heart block can follow, and eventually ventricular fibrillation.

Points that change answers:

• Digoxin and heart disease raise the stakes. MSD says patients with significant heart disease and patients receiving digoxin are at risk of conduction abnormalities even with mild hypokalemia.
• IV potassium has limits. MSD caps the concentration for peripheral veins and the routine infusion rate, and allows faster replacement for a hypokalemic arrhythmia only with continuous cardiac monitoring and hourly potassium checks, usually through a central vein. The numbers are in your unit's protocol.
• Dextrose is the wrong diluent. MSD says glucose solutions are avoided for dilution, because the rise in insulin can make hypokalemia transiently worse.
• Check magnesium. When hypokalemia occurs with hypomagnesemia, MSD says both must be corrected to stop ongoing renal potassium wasting.
• DKA is a special case. The body is short of potassium even when the serum level is high. MSD says IV potassium is deferred until the serum potassium starts to fall. The CCRN endocrine review covers the potassium-before-insulin rule.

Hyponatremia: correct slowly unless the brain is in danger

MSD defines hyponatremia as a serum sodium below 136 mEq/L, caused by an excess of water relative to solute. It can occur with low, normal or high extracellular volume, and the treatment direction depends on which:

Symptoms are mainly neurologic and are worse when sodium falls quickly. The rule that carries the most items is the pace of correction. MSD says serum sodium should not rise by more than 8 mEq/L over the first 24 hours, because too-rapid correction can cause osmotic demyelination syndrome. That syndrome can bring flaccid paralysis, dysarthria and dysphagia over days or weeks after the episode, and the damage is often permanent. MSD says it is more common in patients with alcohol use disorder, undernutrition or other chronic debilitating illness.

The exception is the patient whose brain is at risk now. For rapid-onset hyponatremia (known onset within 24 hours) with severe neurologic symptoms such as seizures, MSD describes rapid correction by 4 to 6 mEq/L with hypertonic saline, in an intensive care unit, with sodium checked every 2 hours. After that first rise, correction slows so the total stays within 8 mEq/L in the first 24 hours.

Three more points from the chapter:

• Volume replacement can overshoot. In hypovolemic hyponatremia, MSD notes that once volume is restored the kidneys can excrete large amounts of water and the sodium can normalize too quickly. A Passy inference for the bedside: a sudden rise in urine output during correction is worth reporting along with the next sodium.
• Glucose pulls sodium down. Sodium falls by about 1.6 mEq/L for every 100 mg/dL rise in glucose above normal, so a low sodium in severe hyperglycemia is partly a water shift.
• Cerebral salt wasting is not SIADH. After traumatic brain injury or CNS surgery, with a urine sodium above 40 mEq/L, MSD says to consider cerebral salt wasting, and that fluid should not be restricted in these patients because restriction can cause cerebral vasospasm.

The urine-and-sodium logic that separates SIADH from diabetes insipidus is in the endocrine review's DI and SIADH table. For a limit you keep forgetting, AI Rescue in full CCRN prep can give you a memory trick under the explanation.

Hypernatremia: a water deficit, replaced on a schedule

Hypernatremia is a serum sodium above 145 mEq/L. MSD describes it as a deficit of total body water relative to sodium, usually from limited access to water or an impaired thirst mechanism. That is why it shows up in patients who cannot ask for a drink: intubated, sedated, confused or dependent on tube feeding. Thirst is the major symptom, and a patient who cannot communicate may show it as agitation. The major signs are neurologic: confusion, neuromuscular excitability, hyperreflexia, seizures or coma.

Treatment replaces intravascular volume and free water, and the timing is the testable part. MSD says hypernatremia that developed within the last 24 hours should be corrected over the next 24 hours. Hypernatremia that is chronic or of unknown duration should be corrected over 48 hours, with serum osmolality lowered no faster than 0.5 mOsm/L per hour, to avoid cerebral edema. The nursing work is accurate intake and output, serial sodium levels, neurologic checks, and watching potassium and other electrolytes, which MSD says should be monitored during replacement.

Low calcium, magnesium and phosphate travel together

These three are often abnormal in the same patient, and one can block the correction of another.

Hypocalcemia

Ionized calcium is the physiologically active form. Low albumin lowers the total calcium without lowering the ionized fraction: MSD says total calcium moves by about 0.8 mg/dL for every 1 g/dL change in albumin. When low ionized calcium is suspected, MSD says to measure it directly even if the total is normal. Alkalosis lowers ionized calcium, so tetany can appear without a markedly low total. The CCRN ABG guide covers how to identify an alkalosis.

Severe hypocalcemia, a total below 7 mg/dL, may cause hyperreflexia, tetany, laryngospasm or generalized seizures, and the ECG typically shows a prolonged QTc. Two bedside signs point to latent tetany:

• Chvostek sign: facial twitching when the facial nerve is tapped lightly just in front of the ear. MSD notes it is also present in up to 10% of healthy people.
• Trousseau sign: carpal spasm when a blood pressure cuff is inflated to 20 mm Hg above systolic pressure for 3 minutes.

MSD's treatment for tetany is IV calcium gluconate. It warns that calcium infusions are hazardous in patients receiving digoxin and should be given slowly, with continuous ECG monitoring, after hypokalemia has been checked for and corrected. MSD also lists transfusion of more than 10 units of citrated blood among the causes; the hematology review has the transfusion context.

Hypomagnesemia

MSD says hypomagnesemia is common in hospitalized patients and often occurs with hypokalemia and hypocalcemia. The serum level can be normal even when body stores are low. Two statements from the chapter explain most exam items:

• Suspect it when other values resist treatment. MSD says to suspect magnesium deficiency in unexplained hypocalcemia or refractory hypokalemia, even when the serum magnesium is normal.
• Fix it to fix the others. Concurrent hypokalemia and hypocalcemia are difficult to correct until magnesium has been repleted.

During replacement, MSD says to monitor the serum magnesium frequently, particularly in renal insufficiency or with repeated parenteral doses. It also notes that IV magnesium sulfate given alone can worsen hypocalcemia, because sulfate binds ionized calcium. Rhythm treatment itself is in the CCRN cardiovascular review.

Hypophosphatemia

MSD names a short list of settings for clinically significant acute hypophosphatemia: the recovery phase of DKA, acute alcohol use disorder, severe burns, parenteral nutrition, refeeding after prolonged undernutrition, and severe respiratory alkalosis. A level below 1 mg/dL is severe. The effects reach several systems: muscle weakness, respiratory failure, heart failure, seizures and coma, rhabdomyolysis, hemolytic anemia, and impaired white cell and platelet function.

For the nurse, the pattern to catch is a malnourished patient who weakens after feeding starts. MSD reserves IV phosphate for a level below 1 mg/dL, severe symptoms, or when oral replacement is not feasible. It says to monitor calcium and phosphate during therapy and to avoid rapid administration, which can cause hypocalcemia, hyperphosphatemia and calcium phosphate deposits in tissue. Potassium phosphate also delivers potassium, so MSD prefers sodium phosphate when renal function is impaired or the serum potassium is above 4 mEq/L.

Volume depletion: what the body shows before the labs do

Sodium concentration and volume are separate questions. MSD's volume depletion chapter (full review June 2026) says the serum sodium can be high, low or normal in a volume-depleted patient. The concentration describes water relative to sodium; the volume describes how much sodium and water the body holds.

MSD grades the signs by how much extracellular fluid is lost:

Two cautions come with that table. Skin turgor may be low in older patients whatever their volume status, and dry mucous membranes do not always track volume, especially in mouth breathers. Supporting labs include a BUN-to-creatinine ratio often above 20:1 and, with normally working kidneys, a urine sodium usually below 15 mEq/L.

For severe deficits MSD gives IV 0.9% saline or a buffered solution such as Ringer's lactate. It says some evidence seems to favor Ringer's lactate, which may reduce hyperchloremic metabolic acidosis, but that the question is not settled. Resuscitation targets in shock are a separate topic, covered in the CCRN sepsis and shock guide. Volume overload from transfusion is in the hematology review, and fluid problems in kidney injury are in the GI, renal and skin review.

A worked ICU case: one value that will not correct

This case is original and fictional. Mr. Castellano, 54, has alcohol use disorder and was admitted two days ago with pneumonia after weeks of poor intake. He takes a loop diuretic at home. This morning:

• Potassium is 2.9 mEq/L. It was 2.8 yesterday, and he received two rounds of replacement since then.
• Magnesium is 1.1 mg/dL.
• He reports tingling around his mouth, and his hand spasms when the blood pressure cuff stays inflated.
• Phosphate is 2.3 mg/dL, and tube feeding started three hours ago.

Why is the potassium not rising? Because the magnesium is low. MSD says both deficiencies must be corrected to stop renal potassium wasting, and both alcohol use disorder and loop diuretics are on its list of causes of low magnesium. The nurse reports the magnesium alongside the potassium and anticipates magnesium replacement, not simply a third round of potassium.

What do the tingling and the cuff spasm mean? They are signs of tetany, and the cuff finding is a Trousseau sign. The nurse asks for an ionized calcium and keeps him on continuous ECG monitoring. A potassium below 3 mEq/L is the range where MSD says ECG changes typically occur, and severe hypocalcemia can lengthen the QTc. The low calcium is also unlikely to hold until magnesium is repleted.

What should the nurse watch as feeding continues? The phosphate. Refeeding after prolonged undernutrition and alcohol use disorder are both on MSD's short list for acute hypophosphatemia, and his level is already below 2.5 mg/dL. Muscle weakness and respiratory failure, both among the clinical features MSD lists, are the effects to watch for and report with the next level.

The case is one patient, three linked values and one order of work: find the deficit that is blocking the others.

How to study fluids and electrolytes for the CCRN

Learn each imbalance as four facts: the threshold, the danger sign, the first move and the correction limit. Then practice them inside scenarios, because the test plan also places electrolytes inside other problems. Tumor lysis is in the hematology review, rhabdomyolysis and burns are in the CCRN multisystem emergencies guide, and the anion gap is in the ABG guide.

For practice at the category's weight, full CCRN prep lets you work on Multisystem alone, with 352 questions. To check a baseline first, the free CCRN practice questions include three Multisystem items among their 20, and the free CCRN study plan spreads the six categories over dated weeks. The Adult test plan guide lists all 22 Multisystem problems, and the CCRN guide library has every other review.

This review is a study aid. It was checked against the sources listed on November 6, 2026 and has not been reviewed by a specialist nurse. At the bedside, follow your unit's protocols and the treating team's orders.