A 78-year-old client with HFrEF on lisinopril and spironolactone is prescribed trimethoprim-sulfamethoxazole for a UTI. Baseline potassium is 4.6 mEq/L and creatinine is 1.4 mg/dL. Which is the most appropriate nursing action?

TMP-SMX is widely effective for UTI, but trimethoprim has a structure similar to amiloride and inhibits the epithelial sodium channel in the distal nephron, decreasing potassium excretion. The combination with ACE inhibitors or ARBs (which suppress aldosterone), aldosterone antagonists (spironolactone, eplerenone), potassium supplements, NSAIDs, beta-blockers, or pre-existing renal impairment markedly increases the risk of severe hyperkalemia, with documented cases of arrhythmia, hospitalization, and death especially in older adults. Combined nursing/care-coordination action: (1) baseline and follow-up labs — potassium, creatinine, BUN — typically within 3 to 5 days of starting; (2) communicate with the prescriber and pharmacist about the risk and propose an alternative antibiotic when culture allows — nitrofurantoin if creatinine clearance over 30 mL/min, fosfomycin single dose, cephalexin, or other narrow-spectrum agents; (3) teaching about high-potassium foods (bananas, oranges, tomatoes, potatoes, salt substitutes containing KCl) to limit intake during therapy; (4) report symptoms of hyperkalemia — weakness, paresthesias, palpitations; obtain ECG if K rises showing peaked T waves, prolonged PR, widened QRS; (5) follow standard sulfa cautions for SJS, photosensitivity, hydration, and folate; (6) document allergies and review home meds for any other potassium-elevating drugs. Administering without checks, increasing spironolactone, or adding K supplement are unsafe.

TMP-SMX + ACEi/spironolactone + elderly with reduced renal function = high hyperkalemia risk. Communicate, monitor labs, propose an alternative antibiotic.