Although most thiazide diuretics lose their effectiveness in renal failure, metolazone remains active even when the glomerular filtration rate (GFR) is below 30-40 mL/min (moderate renal failure). This gives it a considerable advantage over other thiazide diuretics, since renal and hea failure often coexist and contribute to fluid retention(Refer: K. D. Tripathi&;s Essentials of Medical Pharmacology, 7th edition, pg no: 583)