-Despite this study's poor methodology,
its simplicity makes it suited
to highlight a critical shortcoming of furosemide --
its variable bioavailability. The below scenarios highlight the clinical uncertainty this generates.
-If I prescribe a 40 mg tablet to an office patient, the "average" systemic level will be equivalent to 20 mg IV. However, because the bioavailability varies between 20-85%, 40 mg PO may be the bioequivalent of 8 mg IV (20% of oral dose) or 32 mg IV (80% of oral dose).
-If an inpatient with heart failure requires 40 mg IV furosemide, should she be transitioned to the standard 80 mg pill? Perhaps. But if her metabolism results in a bioavailability closer to 80%, the discharge dose should be 50 mg PO. Yet if her metabolism is closer to 20%, the appropriate oral dose is 200 mg.
-Should the above patient re-presents with dyspnea 2 weeks later, can this be ascribed to
medication non-adherence? Or was the furosemide under-dosed as a result of low bioavailability?
-Table: Data from the authority on diuretics, Craig Brater, confirm this paper's findings. This is why more recent loop diuretics such as bumetanide and torsemide are my preferred oral agents. The narrower the range in bioavailability, the more predictable the therapeutic response.
-Disclosures: I have no conflicts to declare.