# Drug Calculations - not sure what formula to use.

1. Please help. I am a LPN and bridging to my RN. I missed 1 question on my drug calculations competency test today and so I have to retest next week which my instructor informed us would be harder. I am a nervous wreck. Some of the questions she advised us to look over have to do with time. I am not sure what formula to use for these problems. One she had on a worksheet but didn't work out for us was this: A patient is to be given 1000 ml by IVI using a controller with a drip factor of 15 drops/ml. The infusion starts at 8.00am and every four hours the patient is given a 1hr rest. If the drip rate was set at 25 drops per minute, how much liquid would be left at 3.00pm?
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3. Quote from RE_EMT
Please help. I am a LPN and bridging to my RN. I missed 1 question on my drug calculations competency test today and so I have to retest next week which my instructor informed us would be harder. I am a nervous wreck. Some of the questions she advised us to look over have to do with time. I am not sure what formula to use for these problems. One she had on a worksheet but didn't work out for us was this: A patient is to be given 1000 ml by IVI using a controller with a drip factor of 15 drops/ml. The infusion starts at 8.00am and every four hours the patient is given a 1hr rest. If the drip rate was set at 25 drops per minute, how much liquid would be left at 3.00pm?
The way I do it is like this: the patient's IV will run from 8-12 and from 1-3, or 6 hours total. If you take 1000 ml and multiply by 15 drops/ml, you'll get 15000 drops, total # of drops in the bag. Multiply the 15000 drops by the drip rate to get the total minutes, then convert minutes to hours (15000 drops x 1 min/25 drops = 600 minutes x 1 hour/60 min = 10 hours). So the patient gets IV fluids for only 6 of the 10 possible hours, or 60% of the time. 60% of 1000 ml is 600 ml, which is the amount of fluid the patient received. The amount left is 400 ml, or your answer. If you're familiar with dimensional analysis, it's easy to work this problem. Hope my explanation helped!
4. Have you tried dimensional analysis method? You should view a few Youtube videos prior to your test and see if this works for you. Good Luck!!
5. Quote from jazzyrose
Have you tried dimensional analysis method? You should view a few Youtube videos prior to your test and see if this works for you. Good Luck!!
I totally agree! I had a friend who just graduated nursing school tell me from the beginning to learn dimensional analysis and it has helped! Youtube is also a GREAT suggestion. My math prof. never really taught so I had to teach myself everything! I would watch video after video and it made all the difference in the world! Good luck!
6. Although I'm an advocate for formulas, I am writing this in case you can't remember one for a test or quiz. Remember that dosages always uses simple math, always. There are no tricks and it can usually be answered without formulas if you forget them. So, if I forgot which formula to use I could still answer this question.

I would first start by figuring out how much time I'm infusing. That is the easy one. 0800 to 1500 is 7 hours. Subtract one hour for a break and you infuse for 6 hours total (or 360 min).

Next, I'd figure how many total drops fall during that period of 360 min. So, if 25 drops fall every minute then multiply 25 times 360 min which equals 9000 drops. Simple so far and no need to sweat.

Finally, it is important to understand what a drop factor is to keep going. It simply means that 15 drops equals 1 mL. It was given to you as 15drops/mL. So, if you divide 9000 drops by 15drops/mL you are left with 600mL.

Once again, I'm not advocating that you do this. My point is that this is simple math that can be used without any formulas if you forget them. In fact, the formulas really just do what I did above in a clear and concise way (I could argue that some formulas used are harder than what I just wrote.) If I have time, I check my formulas vs. simple math just to verify I chose the right formula.
7. I use dimensional analysis for everything, and love it! Makes everything simple to solve in the one format. I made a 100 on my med calculation exam last week.