Severe hypokalemia is an electrolyte emergency in clinical care. A serum potassium (K+) at or below 2.5 mmol/L associates with cardiac dysrhythmias and death, but patients with acute hypokalemia are also at increased risk of hyperkalemia during rapid correction. Therefore, prompt repletion and careful surveillance during correction are recommended. Yet the serum K+ that is measured to steer this correction requires at least 5-15 minutes and is typically measured “as needed” after correction. Blood must be drawn, transported, centrifuged, and resulted, and in that interval the patient’s true serum K+ may have already changed.