# Confused! Dosage calculations

1. I'm having difficulties completing this problem.

Ordered: Retrovir 200 mg/kg/Day IV q4 hr in 60 mL LR over 90 mins.
On Hand: Retrovir 3 g vials. Each gram is reconstituted with 4 mL of diluent
patient weighs 182 lb. Use macrotubing 20 gtt/mL
Dilute the reconstituted Retrovir in 48 mL of LR

calculate flow rate:

I know the formula is:

I'm having troubles differentiating what is "filler" and what is not.
So far I have:

1. Convert lbs to Kg

182 lbs (1 kg/2.2 lbs) = 182/ 2.2= 82.73 kg

1. Find ordered dose in mg/day

200mg/kg/day
(200mg/kg/day) X 82.73 kg = 16,545.45 mg/day

1. Find ordered dose in mg/dose

Q4hrs therefore give 6 doses in one day
(16,545.45 mg/day) X (6 doses/1 day) = 99272. 73 mg/dose

1. Find concentration of medication on hand:

3 g is 3000 mg
Each gram is reconstituted in 4 mL
3000 mg/12 mL = 250 mg/mL

From here I'm completely lost, am I even in the right directions? What further steps do I need to take to solve or understand this problem?
•

Joined: May '13; Posts: 7; Likes: 3

3. All you need to do is multiply 60 by 20 is 1200 and divide by 90 and your answer is 13.333333 rounded is 13gtt/min
4. thank you for your help
5. nice job on a long problem..... however....you ordered dose daily:
This patient weighs 82.73kg x 200mg/kg/day= 16546mg/day.
Order is for q4h: that's 6 doses. 16546mg/6 doses= 2757mg per dose...so recalculate
Last edit by Esme12 on May 10, '13
6. Hi, I'm just a student myself so anyone correct me if I'm wrong....

To get the ordered dose daily:
Pt weighs 82.73kg x 200mg/kg/day= 16546mg/day.
Order is for q4h: that's 6 doses. 16546mg/6 doses= 2757mg per dose.

You have Retrovir 3000mg/12ml equivalent to 250mg/ml
2757mg/ 250mg= 11.028ml is what would be added into the 48ml of LR so approx 60mL of LR to be infused over 90 minutes.
So you've got 60ml/90 minutes= 0.6666666667 x20gtts/min= 13.33 round to 13gtts/min

7. Q4hrs therefore give 6 doses in one day
(16,545.45 mg/day) X (6 doses/1 day) = 99272. 73 mg/dose
Shouldn't this be (16,545.45 mg/day) / (6 doses/1 day) = 2757.58 mg/dose?

The way I am reading it here, you would be giving 595,636.38 mg per day to the patient. Or am I missing something?
8. you're right, instead of dividing, I multiplied. thank you for catching my error!
9. Quote from lclopez86
I'm having difficulties completing this problem.

Ordered: Retrovir 200 mg/kg/Day IV q4 hr in 60 mL LR over 90 mins.
On Hand: Retrovir 3 g vials. Each gram is reconstituted with 4 mL of diluent
patient weighs 182 lb. Use macrotubing 20 gtt/mL
Dilute the reconstituted Retrovir in 48 mL of LR

calculate flow rate:

I know the formula is:

I'm having troubles differentiating what is "filler" and what is not.
So far I have:

1. Convert lbs to Kg

182 lbs (1 kg/2.2 lbs) = 182/ 2.2= 82.73 kg

1. Find ordered dose in mg/day

200mg/kg/day
(200mg/kg/day) X 82.73 kg = 16,545.45 mg/day

You're perfect to here. Good thinking.

1. Find ordered dose in mg/dose

Q4hrs therefore give 6 doses in one day
(16,545.45 mg/day) X (6 doses/1 day) = 99272. 73 mg/dose

Um, no. That's a dose for the whole day, so for six doses you'd divide that by six. Look at what you have. You've determined that you're going to give 16,545.45 mg/day...so why would ONE dose be a whole lot bigger than that?

1. Find concentration of medication on hand:

3 g is 3000 mg
Each gram is reconstituted in 4 mL
3000 mg/12 mL = 250 mg/mL
So one dose is 2757mg. Now you should be able to figure how many reconstituted cc of the med will go in the minibag to make a total of 60cc, and then you can figure out how fast to run it.
10. Ok I get it now too! LOL
11. I would try to look up how to do dimensional analysis. It really helped me with dosage calculation and I got a 100% on my test this past semester. This website has a powerpoint that shows you how to do it:

Dosage Calculation Using Dimensional Ana
12. Great post on dosage calc help! The site is very helpful thank you.
13. We try!