American Board of Internal Medicine (ABIM) Certification Practice Exam

When should thiazide diuretics be switched to loop diuretics for managing hypertension?

1. When GFR is less than 45 mL/min/1.73 m2

2. When GFR is less than 60 mL/min/1.73 m2

3. When GFR is less than 30 mL/min/1.73 m2

4. When GFR is less than 15 mL/min/1.73 m2

The correct answer is: When GFR is less than 30 mL/min/1.73 m2

Thiazide diuretics are often used in the treatment of hypertension, but their effectiveness is significantly diminished when renal function declines. Specifically, thiazides are effective primarily in patients with normal or mildly decreased kidney function but can become ineffective at lower glomerular filtration rates (GFR). Switching from thiazide diuretics to loop diuretics is recommended when the GFR falls below 30 mL/min/1.73 m². At this level of renal impairment, thiazide diuretics may not adequately promote natriuresis (the excretion of sodium in the urine) due to reduced renal blood flow and glomerular filtration. Loop diuretics, on the other hand, are potent diuretics that can still exert their effects in patients with more significantly impaired renal function. This capability to work even when GFR is lower makes loop diuretics a better choice for managing fluid overload and hypertension in such patients. Thus, the recommendation to switch to loop diuretics aligns with clinical practice guidelines, considering the diminished pharmacologic effect of thiazides when GFR falls below 30 mL/min/1.73 m². This switch helps ensure effective management of blood pressure and fluid status despite