Some medications are available in powder form and are too unstable to mix until ready for use. Mixing the powder is called reconstituting the powder. Sometimes a med will come with its own liquid for mixing and if this is the case, that liquid must be used. If a liquid is not provided with the medication, the label will advise if there are any restrictions regarding the liquid to use. Most commonly for parenteral medications the liquid used is either sterile normal saline or sterile bacteriostatic water and the pharmacy will provide you with a vial of one of these to use. Some powders designed for oral use may be able to be reconstituted with tap water.
The manufacturer will provide instructions regarding the type and amount of diluent (the liquid) to add and information regarding the concentration of the medication if it is mixed according to those directions. The instructions may appear on the vial/bottle of powder of they may be given as a package insert. For safe administration of these medications, it is essential that you follow these manufacturer's instructions.
Example instructions: add 3.8 mL sterile water to yield 500 mg/mL.; add 10 mL normal saline, results in 200,000 U per 5 mL.
To calculate a dosage using a reconstituted powder, the left hand side of your ratio should always be the final concentration of medication after the powder is mixed.
Example problem #1: The physician orders 150,000 units of Penicillin. You have a 10 cc vial of powder labeled Penicillin 500,000 units. Instructions indicate to add 4.8 mL sterile saline or sterile water to give 100,000 units per mL. Reconstitute with ________ and give ________.
In this example problem, it does not matter that it is a 10 cc vial; it does not matter that the vial contains a total of 500,000 units of penicillin. The information that you will use is that you will reconstitute with 4.8 mL (which goes in the 1st blank). Following these instructions would give you 100,000 units in each mL. This becomes the left-hand side of the ratio.
100,000 units = 150,000 units
1 mL X mL
100,000 X = 150,000
100,000 X = 150,000
100,000 100,000
X = 1.5 mL
HINT: Once you have your problem set up, reduce numbers as possible so that your arithmatic work is easier, especially when you are working problem without a calculator. In the above problem, crossing out equal numbers of zero's from the top and bottom, you are left with 15 divided by 10. This will be much easier arithmatic.
When reviewing the reconstitution instructions, sometimes the manufacturer will give you only one way to mix the medication (as in the above example, add 4.8 mL to mix); however, frequently the manufacturer will give several options on the amount of dilutent to add. In this situation, you choose one of the options and proceed with working the problem. The only restrictions on the choice will depend on the final answer and the route of administration for the medication. RULES: For an intramuscular (IM) injection in an adult, the final dose amount should not exceed 3 mL for a single injection in the buttocks and 1 mL in the arm. For a subcutaneous (SC) injection, the maximum amount is 1 mL, preferably less. The maximim amounts in children will vary based on the age/size of the child. If you chose a dilutent amount that gave you an answer in excess of the maximum amount allowed, you would need to choose a more concentrated solution that would give you a smaller amount to give.
Example problem#2: Orderd is penicillin 600,000 units IM. You have a vial of penicillin powder containing twenty million units. Reconstitute with _________ and give ___________.
Instructions on the vial include the following:
mL diluent added approx units per mL of solution
75 mL 250,000 u/mL
33 mL 500,000 u/mL
11.5 mL 1,000,000 u/mL
Choose one of the above options and set up the ratio and solve.
If you choose to reconstitute with 75 mL, you would have the following ratio:
250,000 units = 600,000 units
1 mL X mL
250,000 X = 600,000
X = 2.4 mL
This is less than 3 mL so it is an acceptable choice.
In the blanks in the problem you would fill in as follows: Reconstitute with 75 mL and give 2.4 mL.
{For this problem, any of the given reconstitution options would be acceptable because each option would give you an amount to administer of less than 3 mL.}
NOTE: You can reconstitute only with a liquid. On a dosage test, if you put 250.000 units/mL in the "reconstitute with" blank, it will be wrong. The 250,000 units/mL is the final concentration you end up with after you have followed the instructions of adding 75 mL of liquid to mix the powder.
Chapter 9 in the drug dosage workbook relates to reconstitution of powdered drugs. You are strongly encouraged to use the workbook and practice with these problems as students commonly have a little more difficulty with reconstitution problems.