24 Reconstituting Dry Powders

Some medications are provided to the pharmacy in powder form to be later reconstituted, or mixed, with diluents like water and other liquids to form different suspensions and solutions. These commercially made bottles and vials may also contain lyophilized, or freeze dried, powders and come as oral or parenteral products.

A common example is antibiotics. This is because they tend to lose their potency in a short period of time after being prepared in liquid dosage form. That's why it's important not to reconstitute them until it's time for the antibiotic to be dispensed. Additionally, most oral preparations are formulated so that after reconstitution 1 teaspoonful of the solution will contain the "normal" dose.

Dry powder reconstitution questions will typically ask you to find either the concentration of drug or volume of powder in the final volume of constituted product. You may also be asked how to make different concentrations or strengths of the constituted dosage form by varying the amount of diluent. Let's look at an example of each, respectively.

Let's say we have the active ingredient gentamicin sulfate, an antibiotic, reconstituted with 0.9% sodium chloride. If the concentration of drug in a 50 mL reconstituted solution is 800 mcg/mL, then what is the concentration and percent strength of drug in this solution?

$$\text{800 mcg} \times \frac{\text{1 mg}}{\text{1,000 mcg}} = \text{0.8 mg} \;\text{;}$$

$$\frac{\text{0.8 mg}}{\text{1 mL}} = \frac{\text{x mg}}{\text{50 mL}}$$

$$\frac{\text{50 mL} \times \text{0.8 mg}}{\text{1 mL}} = \text{40 mg}$$

$$\frac{\text{40 mg}}{\text{50 mL}} = \frac{\text{0.8 mg of gentamicin sulfate}}{\text{1 mL solution}} \;\text{;}$$

$$\text{0.8 mg} \times \frac{\text{1 g}}{\text{1,000 mg}} = \text{0.0008 g}$$

$$\frac{\text{0.0008 g}}{\text{1 mL}} = \frac{\text{x g}}{\text{100 mL}} = \text{x % gentamicin sulfate}$$

$$\frac{\text{100 mL} \times \text{0.0008 g}}{\text{100 mL}}$$

$$= \text{0.08 g = 0.08 % gentamicin sulfate}$$

You're a natural now! Okay, what is the volume of dry powder in a 170 mL reconstituted solution that contains 80 mL of diluent?

$$\text{170 mL - 80 mL = 90 mL dry powder}$$

Did you notice something? The amount and volume of drug from the manufacturer's bottle remains the same, no matter how much diluent is added, unless some medication is removed from the bottle.

Next, let's say the directions on a bottle of dry powder indicate that after reconstitution, every 10 mL of solution will contain 125 mg of drug. To achieve this, you're instructed to reconstitute the dry powder with 90 mL of diluent for a total 120 mL volume solution. However, you were not paying attention and added 50 mL of diluent. What is the concentration of drug in the solution at this point?

$$\text{120 mL - 90 mL = 30 mL dry powder}$$

$$\text{30 mL dry powder + 50 mL diluent}$$

$$\text{= 80 mL solution} \;\text{;}$$

$$\frac{\text{125 mg}}{\text{10 mL}} = \frac{\text{x mg}}{\text{120 mL}}$$

$$\frac{\text{120 mL} \times \text{125 mg}}{\text{10 mL}} = \text{1,500 mg of drug} \;\text{;}$$

$$\frac{\text{1,500 mg}}{\text{80 mL}} = \frac{\text{18.75 mg}}{\text{1 mL}}$$

Notice that after working backwards to find the volume of dry powder, we first set up a proportion to find out the weight of drug in the bottle before calculating the concentration of drug in the solution. Luckily, you can simply add another 40 mL of diluent to fix the problem and find out the correct concentration.

$$\frac{\text{1,500 mg}}{\text{120 mL}} = \frac{\text{12.5 mg}}{\text{1 mL}}$$

This makes sense of course because we'd expect a more diluted solution to be less concentrated.

Okay, now just one more. Let's say you were up all night studying for the pharmacy technician exam and didn't get enough sleep. You were going to call out but you decide to go in anyway. Now in the sterile compounding area, you crack open that same dry powder bottle and instead of adding 50 mL of diluent you accidentally add 150 mL -- YIKES! Now unfortunately you can't remove the diluent easily. However, all that studying panned out because your genius self can still figure out EXACTLY how much of this solution contains 125 mg of the drug!

$$\text{30 mL dry powder + 150 mL diluent}$$

$$\text{= 180 mL solution} \;\text{;}$$

$$\frac{\text{1,500 mg}}{\text{180 mL}} = \frac{\text{125 mg}}{\text{x mL}}$$

$$\frac{\text{125 mg} \times \text{180 mL}}{\text{1,500 mg}} = \text{15 mL of solution}$$

YOU SAVED THE DAY! But seriously, when you're too tired to work at your best always discuss it with your supervisor.

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