During chart review, you note that a 78-year-old woman with hypertension and heart failure is chronically taking lisinopril 20 mg daily and spironolactone 25 mg daily. She was just started by urgent care on trimethoprim-sulfamethoxazole DS BID for an uncomplicated UTI. Baseline serum potassium 3 months ago was 4.4 mEq/L and eGFR was 52 mL/min/1.73m². Which of the following is the most important monitoring action during this antibiotic course?

Question

Accepted Answer

✅ D. Check a serum potassium and basic metabolic panel within 3–5 days of starting TMP-SMX
Trimethoprim structurally resembles the potassium-sparing diuretic amiloride and blocks epithelial sodium channels in the distal nephron, which decreases potassium excretion and increases the risk of hyperkalemia. This risk is markedly amplified in elderly patients and in patients already taking ACE inhibitors (lisinopril) or aldosterone antagonists (spironolactone), which both suppress the renin-angiotensin-aldosterone system (RAAS) and reduce potassium excretion. Reduced renal function (eGFR 52) further increases risk. A serum potassium and renal function check within several days is essential; life-threatening hyperkalemia has been reported in this combination.

❌ A. Monitor liver function tests weekly
TMP-SMX can cause hepatotoxicity but it is not the most pressing monitoring concern in this specific drug combination. The dominant risk in this patient is hyperkalemia from the TMP-SMX + ACE inhibitor + spironolactone stack.

❌ B. Monitor CBC for neutropenia
TMP-SMX can cause cytopenias (agranulocytosis, megaloblastic anemia, thrombocytopenia), but these are less acutely dangerous in a short UTI course than hyperkalemia in this medication context. Routine CBC monitoring is not the priority here.

❌ C. Monitor INR more frequently
TMP-SMX increases INR in patients on warfarin by inhibiting CYP2C9 and displacing warfarin from protein binding — but this patient is not on warfarin, so INR monitoring is not relevant.

:::danger
TMP-SMX + ACE inhibitor/ARB + spironolactone in an older adult = high risk of hyperkalemia and sudden death. Check potassium and renal function within 3–5 days, or choose a different antibiotic when possible.
:::

Trimethoprim-Sulfamethoxazole

✅ Indications

⚙️ Mechanism of Action

📏 Dosing

🚫 Contraindications

⚠️ Adverse Effects

🔬 Monitoring

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