The amount of medication remaining in a metered-dose inhaler canister is impossible to determine just by looking at it. A variety of methods are available to determine when it is necessary to obtain a new canister.

Methods:

(1) float testing

(2) tracking the number of doses administered

(3) tracking the weight of the canister

In float testing, the canister is placed in a glass of water or jar that is filled with water to a depth greater than the length of the canister and with a width greater than the length of the canister. If and how the canister floats reflects the relative weight and center of gravity for the remaining drug solution.

Appearance of Canister |
Estimate of Drug Remaining |

sitting on the bottom of the glass |
66 – 100% |

floating with nozzle pointing up and out of the water |
50% |

floating with the nozzle point down and under the water, with base near the water surface |
33% |

floating with the nozzle point down and under the water, with base a distance above the water surface |
25% |

floating with nozzle down, but at a slight angle to the vertical axis |
10% |

canister on its side, with the nozzle point to the side of the glass |
0% |

Disadvantages of float testing:

(1) It is somewhat inaccurate and insensitive.

(2) It requires immersing the canister in water, which could cause problems with contamination.

Tracking the number of doses administered can be done by hand, or by the use of an automatic counter that tracks the number of uses.

In tracking the weight of the canister, you need to know:

(1) the weight of the canister when filled

(2) the weight of the empty canister

(3) the number of doses in the canister, or an estimate of the change in weight per dose

An estimate of the change in weight per dose can be achieved by measuring the weight of the canister before and after discharging 10 or 20 doses, then dividing the change in weight by the number of doses.

Method 1:

(1) weight per dose = ((weight of canister full) – (weight empty)) / (total number of doses in canister)

(2) number of doses left = ((weight now) – (weight empty)) / (weight per dose)

Method 2:

(1) weight per dose = ((weight of canister full) – (weight after x doses)) / (x doses)

(2) number of doses left = ((weight now) – (weight empty)) / (weight per dose)

Method 3:

(1) percent drug left = ((slope of line) * (weight of canister now)) + (intercept)

(2) slope = 100 / ((weight of canister new) – (weight of canister empty))

(3) intercept = 100 * (weight of canister empty) / ((weight of canister empty) – (weight of canister full)), which is a negative number.

Limitations:

• The number of doses in the canister may be greater than that stated.

• The weighing method requires an accurate scale.

**Purpose: **To estimate the number of doses left in a metered-dose inhaler.

**Specialty: **Pharmacology, clinical, Pulmonology

**Objective: **dosage calculations, dosage adjustments

**ICD-10: **J45,