A 24-year-old woman presents with vomiting and deep sighing respirations. Glucose is 540 mg/dL, arterial pH 7.08, bicarbonate 6 mEq/L, urine ketones strongly positive, and serum potassium 3.0 mEq/L. Intravenous fluids are being started. What is the correct approach to insulin and potassium?
- A Start regular insulin infusion immediately; give potassium only after 2 hours
- B Withhold insulin until serum potassium exceeds 3.3 mEq/L, replacing potassium first ✓
- C Start insulin with a simultaneous potassium infusion since potassium is above 2.5 mEq/L
- D Give sodium bicarbonate first to correct the acidosis, then start insulin
Correct answer: B. Withhold insulin until serum potassium exceeds 3.3 mEq/L, replacing potassium first
Explanation
Total body potassium is severely depleted even though the measured value reflects extracellular shift from acidosis. Insulin drives potassium into cells and can precipitate fatal hypokalemia, arrhythmia, and respiratory paralysis. Current guidance is to defer insulin until potassium rises above 3.3 mEq/L while giving 20 to 30 mEq/h of potassium. Option C tempts students who remember a 2.5 threshold, but that applies to never holding potassium, not insulin. Bicarbonate is reserved for pH below 6.9.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.