Situation: A 61-year-old woman with type 2 diabetes and hypertension takes losartan 100 mg and spironolactone 25 mg daily and recently began using a salt substitute. She comes to the emergency room with generalized weakness and tingling in her legs. Her serum potassium is 7.0 mEq/L (3.5–5.0 mEq/L) on a non-hemolyzed sample, serum creatinine 1.0 mg/dL (88 µmol/L), capillary blood glucose 142 mg/dL (7.9 mmol/L), and her 12-lead electrocardiogram (ECG) shows tall, peaked T waves. She is on a cardiac monitor. All four orders are carried out. On non-hemolyzed samples, her potassium is 5.6 mEq/L 1 hour after the insulin but 6.2 mEq/L 4 hours later, although she has received no potassium, her urine output is adequate, and her capillary glucose has stayed between 110 and 160 mg/dL (6.1–8.9 mmol/L). Her daughter asks why the level rose again. Which explanation by the nurse is correct?

Insulin with dextrose lowers serum potassium within minutes by shifting it into cells, but the effect lasts only a few hours and total body potassium is unchanged. Potassium binders act over hours, so the level can rebound before enough potassium has been removed; it is rechecked and treatment is repeated or escalated as ordered. Calcium gluconate never lowered the potassium level.

Why the potassium rose again after insulin

The correct explanation is that insulin only shifts potassium from the extracellular fluid into cells; it does not eliminate potassium from the body. When the insulin effect fades, that potassium moves back out of the cells into the blood, so the serum level rises again.

Insulin with dextrose lowers serum potassium within minutes by activating the sodium–potassium ATPase pump, which drives potassium into skeletal muscle and other cells. This intracellular shift is temporary. Total body potassium is unchanged, and once the insulin is metabolized, the potassium re-equilibrates back into the extracellular space. In this patient, the initial drop from 7.0 mEq/L to 5.6 mEq/L at 1 hour reflects that shift. The rebound to 6.2 mEq/L at 4 hours reflects the waning of the insulin effect before the potassium binder has removed enough potassium from the gut.

Watch out! A potassium binder such as sodium polystyrene sulfonate or patiromer acts over hours, not minutes. During that lag, the serum potassium can rebound after the insulin wears off. This is why serial potassium checks and repeat or escalated treatment are required.

Key point! Calcium gluconate never lowers serum potassium. It only stabilizes the cardiac membrane against the arrhythmogenic effects of hyperkalemia. Therefore, the rebound cannot be explained by calcium gluconate wearing off and releasing potassium.

The case report in the provided abstract reinforces the same mechanism: rebound hyperkalemia occurred about 4 hours after discontinuation of a glucose–insulin–potassium infusion, even leading to a serious conduction disturbance ^[1]. That timing matches this patient’s course and underscores that insulin-based shifting is temporary unless potassium removal is also achieved.

The other options are incorrect. Sodium given with a binder does not pull potassium out of cells; it facilitates potassium excretion in the distal nephron. A potassium binder does not release bound potassium back into the blood in a clinically significant way once it has formed an insoluble complex in the gut.