Understanding Reconstitution Better

By Diane Rhodes, used by permission, copyright 2019

     At times, medications are reconstituted for use with a patient. The medicine may come as a powder or as a liquid available for reconstitution.

Any time you are working with a reconstitution problem, you will need to know the concentration of what you have to work with (or what is desired)—that is, what is available. This will be your starting point. But let’s clarify our terms here: what is a concentration? It is a solution, with so much medication in so much fluid, e.g. 100 mg/ml (read the / as “per”).

      It can be confusing to know if the vial size the medication is in is relevant or not, so let’s examine that first.

When does the vial size matter?

1. Suppose the problem tells you the medication comes in a 10-ml vial with a concentration of 500 mg/ml. Because the concentration of the medicine in the vial is given to you, the vial size is irrelevant here.

2. Suppose the problem tells you the medication comes as 1 g in a 10-ml vial, from which you are to withdraw 300 mg. Here, there is no concentration given to you, and the medicine is not stated to be a powder, so the vial size becomes part of the concentration: it is 1 g/10 ml.

3. Suppose you are working with a powdered medication in a vial: the medicine comes in a powdered form, 1 g in a 10-ml vial. (Note that powdered medication is useless to us as an inert substance in a vial; it cannot be given to a patient in this form.) The medicine is a powder, and we have said a concentration only describes a solution of medicine in fluid, which does not exist here, so the vial size is irrelevant. When you add fluid to the powder, you will get a concentration of the amount of medication in the amount of fluid you added. Say you have a 10-ml vial containing 1 g of powdered medication, and you add 8 ml of sterile water to the vial. The concentration in that vial is now 1 g/8 ml. Note that the powder does not change the total volume of fluid in the vial.

Reconstitution of a powdered medication

     We have said that a powdered medication in a vial has no concentration until we add fluid to it, so it is in a solution. The problem might therefore ask you how much fluid you would need to create a desired concentration – a concentration that has not previously existed. In this case, you need only two pieces of information: the desired concentration and the amount of powdered medication you have. Your answer will be in ml: how much fluid do you need to add to get the desired concentration? Let’s look at an example:

     A medication is available as a powder 1 g per vial. You are to add enough fluid to create a 200 mg/ml concentration. Desired concentration: 200 mg/ml. Available medication: 1 g (or 1000 mg). So how much fluid must you add?

ml = 1 ml/200 mg x 1000 mg = 5 ml

Reconstitution of a liquid medication

     Sometimes a medication is already in solution, but not at the concentration you want, so it requires reconstitution. Let’s consider an example of this:

     The available medication comes as 2 g in 2 ml in a large vial, and you want to create a concentration of 500 mg/ml. Initially, you act as if the medication is a powder. As with a powdered medication, you need only two pieces of information: the desired concentration and the amount of medication in the vial, simply ignoring how much fluid it’s in at this point. So how much fluid would you need to add to create the desired concentration?

     Please note that for these problems, you start with the desired concentration.

     ml = 1 ml/500 mg x 2000 mg = 4 ml

Note that you solved this in the same way as for the powdered medication, ignoring the fluid that was already in the vial. But you are not finished! It would require 4 ml of fluid to give the 2000 mg a 1 ml/500 mg concentration – that’s what you just showed with your calculations. But you already have 2 ml in the vial. Thus, you just have to add two more ml to get the 4 ml total: 4 ml – 2 ml = 2 ml.

     Let’s look at another example of this. You have a medication which is available 3g in 5 ml. The desired concentration is 500 mg/ml. How much fluid must you add to create the desired concentration?

ml = 1 ml/500 mg x 3000 mg = 6 ml – the desired concentration times the amount of medicine in the vial, as if it were a powder. Once you have that answer, you look at the fact that there are already 5 ml in the vial, so you add just 1 ml of fluid for the new/desired concentration.

Dilution of an existing concentration

      This one is easy. Suppose you have a medication which is available 1 g in 2 ml. The package directions tell you to dilute it with 10 ml of sterile water. Once you have done so, only the amount of fluid in your concentration has changed. The vial now contains a medication with a concentration of 1 g/12 ml. (Note the increased fluid portion of the concentration.)

Careful, though. If the instructions say something like, “You are to add 50 ml of fluid to create a concentration of 200 mg/4 ml,” the 50 ml is irrelevant, because you have a resulting concentration given to you: 200 mg/4 ml. This is true only if the problem states that by adding fluid you have created a given concentration. This created concentration is what you work with from there on.

Reconstituting IV medications

At times you will need to determine how much of an available solution must be added to an IV bag to give the dose a doctor ordered. You will again need only two pieces of information: the concentration of the available medication, and the amount of medicine the doctor ordered. Note that the bag size it is going into does not enter your calculations here.

Let’s look at an example:

You have a medication which comes 2 g/10 ml. The doctor wants 800 mg added to a 100 ml bag of NS, and to infuse it over 30 minutes. How much will you add? You are looking for ml, as you can only add fluid to fluid. Available: 2 g/10 ml. Doctor’s order: 800 mg.

 ml = 10 ml/2000 mg x 800 mg = 4 ml

With IV math, you may then be asked at what rate to set the pump to infuse the medication. Remember, you have added 4 ml of the medication to the 100 ml bag, so you will now be infusing 104 ml:

     100 ml NS + 4 ml medicine = 104 ml

     ml/hr = 104 ml/30 min x 60 min/hr = 208 ml/hr

What if the doctor’s order is not in ml/hr?

With some medications, the doctor may order it given as so many mg/hr, rather than ml/hr. Don’t let this throw you off. You use your dimensional analysis to multiply the concentration of the medicine you have x the doctor’s order, as in the example below:

Available medication comes as 2.5 g (or 2500 mg) in 2 ml NS in a 10-ml vial. (Note vial size is irrelevant because you have a concentration given: 2.5g/2 ml.) You are to add the medication to a 250 ml bag of NS, and then administer it at the rate of 90 mg/hr. (Note the doctor’s order is in mg/hr, not ml/hr.) What flow rate would you set to infuse it?

First, you add the medication to the bag: 2 ml + 250 ml = 252 ml. You now have 252 ml containing 2500 mg of medication. So, to calculate the flow rate:

ml/hr = 252 ml/2500 mg x 90 mg/hr = 9.1 ml/hr

Sample problems

Example 1

The medicine ordered comes in a powdered form with 20 mg in a large vial. You are to add 30 ml of sterile water and then give a 12 mg PO dose of the medicine. How many ml will you give?

     ml = 30 ml/20 mg x 12 mg = 18 ml

Example 2

     A medication is available as a powder 250 mg per vial. You are to add enough fluid to create a 25 mg/ml strength, and then you are to give 15 mg of the reconstituted medication.

     a. How much fluid should you add to the vial to create the desired concentration? (Desired concentration x amount of medication available)

         ml = 1 ml/25 mg x 250 mg = 10 ml

     a. How much should you give to follow the doctor’s order? (Available concentration x the             doctor’s order)

          ml = 1 ml/25 mg x 15 mg = 0.6 ml

Notice that in both parts, you are using the same concentration of medicine: the 1 ml/25 mg.

Example 3

The medicine to be reconstituted is available in a 5-ml vial with a concentration of 500 mg/ml. The doctor has ordered 2.3 g of the medicine, which is to be added to 150 mlof ½ NS and infused over 45 minutes. What hourly rate will you set?

Note that this must be solved in two steps. You must first determine how much fluid you must add to the 150 ml of ½ NS to create the concentration the doctor has ordered.

ml = 1 ml/500 mg x 2300 mg = 4.6 ml

ml/hr = 154.6 ml/45 min x 60 min/hr = 206.1 ml/hr

Example 4

     The medication ordered is available in a 10-ml vial with a concentration of 200 mg/ml. The doctor has ordered 1.2 g in 100 ml of D5NS, to be infused over one hour.

     a. How much will you add to the D5NS?

         ml = 1 ml/200 mg x 1200 mg = 6 ml

     b. At what rate will you set the pump?

         100 ml D5NS + 6 ml medicine = 106 ml, infused over 1 hour, so 106 ml/hr

Example 5

      You are to reconstitute a medication to create a concentration of 1.5 g in 100 ml of ½ NS. The 5-ml vial from the pharmacy contains the medication in a concentration of 200 mg/ml. The bag is to infuse in 30 minutes. Calculate the flow rate.

     ml = 1 ml/200 mg x 1500 mg = 7.5 ml to add to the 100 ml bag

     ml/hr = 107.5 ml/30 min x 60 min/hr = 215 ml/hr

Example 6

The available medication is 2 g in 5 ml in a 10-ml vial. You are to create a concentration of 250 mg/ml and add 1g to a 100 ml bag of NS. It is to be given at 70 mg/hr. Flow rate?

     This must be solved in two steps. First, how much will you add to the bag?

      ml = 1 ml/250 mg x 1000 mg = 4 ml.

Note that is is unnecessary to figure out how much fluid you would need to create the new concentration! All you need to know is you have a resulting concentration of 250 mg/ml, and want 1 g (1000 mg) to be added to the 100 ml bag.

     Now the second step: at what flow rate will it infuse?

     ml/hr = 104 ml/1000 mg x 70 mg/hr = 7.3 ml/hr

Example 7

     The available medication comes as 3g in 5 ml. The ordered concentration is 250 mg/ml. How much fluid must you add to create the desired concentration.?

      ml = 1 ml/250 mg x 3000 mg = 12 ml

      12 ml needed – 5 ml already in the vial = 7 ml to be added to the vial