Critical Potassium Level for Spironolactone

A 75-year-old female patient with a history of congestive heart failure is prescribed spironolactone, a potassium-sparing diuretic, to help manage her fluid retention. Prior to starting the medication, the nurse must assess the patient's laboratory values, especially her potassium levels. Spironolactone is known for causing hyperkalemia, particularly in patients with compromised renal function. Due to this, the nurse notes a potassium level of _____ mEq/L. What is the most critical potassium level above which the nurse should contact the prescribing physician to discuss alternative treatment options?