If the drug is in liquid form in the ampule or vial, the desired amount is withdrawn from the ampule or vial. In some instances, the entire amount is used; in others, only part of the total amount is withdrawn from the ampule or vial and administered.
Whenever the dose to be administered is different from that listed on the label, the volume to be administered must be calculated. To determine the volume to be administered, the formula for liquid preparations is used. The calculations are the same as those given in the preceding section for parenteral drugs in disposable syringes or cartridges.
The physician orders chlorpromazine 12.5 mg IM.
The drug is available as chlorpromazine 25 mg/mL in a 1-mL ampule.
2 to be administered.
The physician orders hydroxyzine 12.5 mg. The drug is available as hydroxyzine 25 mg/mL in 10-mL vials.
25 mg 2
Therefore, 0.5 mL is withdrawn from the 10-mL multidose vial and administered. In this example, the amount in this or any multidose vial is not entered into the equation. What is entered into the equation as quantity (Q) is the amount of the available drug that is contained in a specific volume.
When the dose is less than 1 mL, it may be necessary, in some instances, to convert the answer to minims. A conversion factor of 15 or 16 minims/mL may be used.
The physician orders chlorpromazine 10 mg IM. The drug is available as chlorpromazine 25 mg/mL.
10 mg 25 mg
X 15 minims = 6 minims
In this example 15 minims = 1 mL is used because 15 can be divided by 5.
The physician's order reads methadone 2.5 mg IM. The drug is available as methadone 10 mg/mL.
Because 16 (and not 15) minims can be divided by 4, the conversion factor of 16 is used.
WARNING: ALWAYS CHECK DRUG LABELS CAREFULLY. Some may be labeled in a manner different from others.
a 2-mL ampule labeled: 2 mL = 0.25 mg a 2-mL ampule labeled: 1 mL = 5 mg
In these two examples, one manufacturer states the entire dose contained in the ampule: 2 mL = 0.25 mg. The other manufacturer gives the dose per milliliter: 1 mL = 5 mg. In this 2-mL ampule, there is a total of 10 mg.
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