MDI’s and Emptyness II

On the 15th I blogged about research showing that Metered Dose Inhalers (MDI’s) are commonly misunderstood, and that there is no easy way to figure out if they’re empty.

This took me completely by surprise, as I thought the ‘float test’ was an accurate way to tell if medication remained, but the article said it wasn’t, the only way to be safe was to count down the number of doses expected to be in the container, and discard it when empty.

SO, I got a little time to go check on it today, and from the MDConsult drug reference, under “Information for Patients”:

…Discard the canister after 200 sprays have been used or 3 months after removal from the moisture-protective foil pouch, whichever comes first. Never immerse the canister into water to determine how full the canister is (“float test”).

and boy do I feel silly, as I thought the float test worked.

(The float test came about over one of the big frustrations of MDI’s: they have more propellant than medication, and thus will keep right on spraying medically useless propellant into the patients’ lungs instead of meds and propellant).

There’s a cautionary tale here: I was taught the float test as a pup, and it’s not unreasonable to think it would work. It makes intuitive sense and when I’ve had a patient bring in an obviously empty inhaler and floated it, the demonstration ‘worked’. Medicine has several of these ‘things everybody knows’ that are wrong.

So: no more float tests, and tell the patient taking albuterol (salbutamol for the non-US part of the world) to count backward from 200 doses (and remember!), then throw the canister away, and get a refill.


  1. If the canister is painted metal, just make a hash mark on it every time you use it.

  2. Thanks for this. Now I know why my Salbutamol inhaler didn’t work the other night…. :(