Loop diuretic equivalency & route conversion

Convert between loop diuretics and between intravenous and oral routes.

What is this for?

Loop diuretics differ in potency per milligram and, critically, in oral bioavailability. Furosemide absorption is erratic and averages around 50%, which is why its oral dose is double the intravenous dose. The other loops are essentially completely absorbed and convert one to one between routes.

How to use it

Select the current loop diuretic and enter the dose.
Select the target agent.
If you are also changing route, select the direction. The ratio applied is that of the target agent.

Worked example

A patient on intravenous furosemide 40 mg is being switched to intravenous bumetanide. What is the equivalent dose?

Answer: 40 mg × (1 ÷ 40) = 1 mg of bumetanide. The classic equivalency to memorise is furosemide 40 mg = torsemide 20 mg = bumetanide 1 mg.

Clinical pearls & pitfalls

Furosemide is the only common loop with a 1:2 IV to PO ratio. Oral furosemide bioavailability ranges from roughly 10% to 100% between and within patients, which is the single biggest reason for apparent diuretic resistance.
The equivalency to hold in memory is furosemide 40 = torsemide 20 = bumetanide 1. Ethacrynic acid 50 mg completes the set and is reserved for true sulfonamide allergy.
Loop diuretics have a threshold dose–response. Below the threshold nothing happens; above it, increasing the dose increases the response until a ceiling. This is why doubling the dose is more effective than increasing the frequency in a patient not responding.
In decompensated heart failure, gut wall oedema impairs oral absorption. Switching to intravenous administration, or to a more reliably absorbed oral agent, often restores response without any increase in equivalent dose.
Ethacrynic acid is not a sulfonamide and is the option for patients with a documented severe sulfonamide reaction, though cross-reactivity with loop diuretics is far rarer than commonly assumed.

Assumptions & limitations

Equipotency values are approximate and were derived largely in patients with normal renal function.
In advanced renal impairment the dose–response curve shifts rightward and much larger doses are needed; the ratios between agents hold better than the absolute doses.
Does not account for differences in duration of action — torsemide acts longer than furosemide, which affects dosing frequency.
Route conversion assumes average bioavailability. Furosemide absorption in an individual patient may be far from average.

References

Ellison DH, Felker GM. Diuretic treatment in heart failure. N Engl J Med. 2017;377(20):1964-1975.
Heidenreich PA, et al. 2022 AHA/ACC/HFSA guideline for the management of heart failure. Circulation. 2022;145(18):e895-e1032.
Brater DC. Diuretic therapy. N Engl J Med. 1998;339(6):387-395.