Dosage Calc, part 14: Reconstitution

  • 00:00 Intro
  • 1:15 Review of terms
  • 3:55 Example 1
  • 5:02 Example 1A Dimensional Analysis
  • 5:57 Example 1A Ratio & Proportion
  • 6:43 Example 1A Formula Method
  • 7:28 Example 1B

Full Transcript: Dosage Calc, part 14: Reconstitution

Hi. This is Cathy with Level Up RN. In this video, I'll be going over a dosage calculation problem involving reconstitution. Before I solve that problem on my whiteboard, however, I will explain the differences between concentration, reconstitution, and dilution so you can fully understand what those terms mean. And then I will solve the reconstitution problem on my board using three different methods including dimensional analysis, ratio and proportion, and the formula method. You can find all the information that I'll be covering in this video in our Level Up RN dosage calculation workbook. If you are in nursing school, then you know how important it is to master dosage calculations, and our workbook will help you do just that. In a nutshell, our workbook contains all different types of dosage calculation problems that you are likely to encounter in nursing school. And we demonstrate how to solve each problem using multiple methods so you can pick the way that makes the most sense to you.

But before I get into specific problems, I just want to do a review of some terminology. So when we are talking about a concentration, that's the amount of solute that is dissolved in a certain amount of fluid. So for example, if we have a vial of gentamicin that says 800 milligrams per 20 ml, the solute is the 800 milligrams, and the amount of solution is 20 ml. So the concentration of this vial is 800 milligrams per 20 ml of solution. When we are talking about reconstitution, that involves adding a diluent, a fluid, to a powder to achieve a specific concentration. So it's not liquid; it's a powder. And then on the label, it will give you the instructions on how many milliliters to add to reconstitute that powder. So for example, you might see instructions that say, "Add 106 milliliters water to the dry mixture in the bottle to achieve a final concentration of 125 milligrams per 5 ml." So these are reconstitution instructions, and they can be when you're given a dose calculation problem that involves reconstitution. If you are just being asked how many milliliters to give the patient, you can kind of ignore this part, right, like how it was reconstituted. Really, what you care about is the final concentration, but this is what you'll see when it comes to reconstitution instructions. And then we have dilution. This is where we add a diluent, so a fluid to a liquid concentration in order to reduce the concentration of that liquid. So for example, potassium chloride comes in a bottle with a concentration of 20 mEq per 15 ml, but we don't want to administer that concentrated liquid to the patient directly. We need to dilute it with at least four ounces of cold water before we give it to the patient. So the difference between reconstitution and dilution, with reconstitution, we're adding fluid to a powder to reconstitute it. With dilution, we are adding fluid to a liquid concentration to reduce the concentration of that liquid.

All right. So let's work through this reconstitution problem here on the board. With this problem, we have an order for azithromycin 80 milligrams PO daily. Reconstitution instruction state to tap the bottle to loosen the powder, add 9 milliliters of tap water to the bottle for a concentration of 100 milligrams in 5 mls of solution. The bottle contains 300 milligrams or 15 ml when mixed. So I know when you look at that, there are a lot of numbers going on, but we are only going to pay attention to the numbers we need to solve the question being asked. So what's being asked is how many milliliters should be administered per dose, and we need to round that to the nearest tenth. So we don't care how many milliliters of tap water were added to the bottle. All we care about is this final concentration. So we can solve part A using dimensional analysis, ratio and proportion, or the formula method. So using dimensional analysis first, we're going to write down what's ordered, 80 milligrams. We're going to make sure that our units of measure are the same between what's ordered and the available concentration, which they are. You've got milligrams here, milligrams here, so I don't have to do any converting. And then I'm going to multiply this times the available concentration. I'm going to put my 5 mls on top and my 100 milligrams on bottom such that my milligrams will cross off and I'll be left with milliliters. Again, I'm kind of ignoring the fact that the bottle contains however many milliliters or that 9 milliliters of water were added to get that concentration. This is the number I care about, what's ordered, and the available concentration. So I'm going to multiply that out, and I end up with 4 ml.

So we can solve this as well with ratio and proportion. So we would put the known ratio on one side. So that's our available concentration. We have 100 milligrams in 5 ml, and we want 80 milligrams, and we need to know how many milliliters that will take. So with ratio proportion, we're going to cross multiply. 100X equals 5 times 80. 100X equals 400. X equals 400 divided by 100, and that equals 4 ml. So basically, just solve for X there. We can also calculate this with the formula method. So the formula method is desired over have times vehicle. So we desire is 80 milligrams. What we have is 100 milligrams in 5 mls of solution. 5 mls is the vehicle for getting that 100 milligrams. Cross off our milligrams, calculate this out. We end up with 4 milliliters. So that is how much we are going to give the patient with each dose. And we need to round to the nearest tenth, and we're good to go. This is already rounded, actually, to the nearest whole number, so we don't need to do anything else there. Now we have to figure out how many full doses are in the bottle. That's when we can pay attention to this number. So according to this information, there are 15 mls in a bottle. And with each dose, we are giving 4 mls. So we can divide 15 mls by 4 mls to figure out how many doses are in the bottle. If we do this, we end up with 3.75. So we can't give 0.75 of a dose. So in this situation, we have three full doses that we can give the patient from this bottle. And that is the answer to part B.

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