To include or not to include - perplexed about specific flow rate calc

[dpm621]

Good evening, I hope everyone is well.

I am about to starting the clinical rotation at my ADN program (BMCC in NYC) and as is normal with a nursing program, there will be a dosage competency exam. Now I consider myself to be very adept at dosage math, and have otherwise no problems with anything I've come across in preparing myself, but one of the exam practice questions was as follows:

Order: 3g calcium gluconate in 250mL D5W and infuse over 12hrs via IV pump. (The calcium gluconate drug label reads 5g/50mL, 100mg/mL).

So, 3g ordered of a drug where the concentration is 5g/50mL on the label means 30mL for the nurse to draw up, and add to the 250mL of D5W to get 280mL total liquid, and divide 280mL/12hrs to get an IV flow rate of 23.3mL/hr. This was the official correct answer; no problems.

However, in my Calculate With Confidence 5th edition (which BMCC uses), there is a similar problem on page 541 which reads:

Clindamycin 900 mg in 75 mL D5W over 30 minutes. Drop factor: 10 gtt/mL (the drug label states 150mg/mL)

a. how many mL of the solution must be added to the medication? 150mg/mL means a total of 6mL must be added to the solution to satisfy the doc's order of 900mg. So far, so good.

b. Calculate the rate in gtt/min at which the IV should infuse. Okay, so we have 75+6 = 81mL of solution to infuse over 30 min, yielding 2.7mL/min, which with a 10gtt/mL drop factor, converts to a drip rate of 27gtt/min. Right?

Wrong. The book states 25gtt/min is correct, and in its steps showed that they used 75mL for the volume, not 81mL. Am I missing something here? Why wouldn't the nurse use 81mL for the drip rate calculation? Is there a rule I need to know about when to include the volume of the drug added, and when to ignore it? I've read it varies from place to place, and I've also read a volume of 10mL or less can be ignored, but any insight would be appreciated. Thanks!

Dan

[Double-Helix, BSN, RN]

#1, thank you for being clear, specific, and including your work and thought process when you posted this question.

#2, unless the question specifically asks you to reconstitute or dilute the medication, I don't assume that I need to add additional volume. If both these questions were presented to me exactly as you worded them, I wouldn't add additional volume to either.

If you have a question this ambiguous on your actual exam, just ask the instructor to clarify. As long as you have a good understanding of the process (and it seems that you do), I don't think you'll be faulted either way.