# Easy (Hard?) Math Question

- 0Aug 21, '11 by cheska_rnso, on wednesday i will be officially starting my third semester (rn-adn) and we will have our wonderful math exam. the first two semesters we could get a 90% and pass, but now it is 100% to pass.

in reviewing for this exam, i came across a math question that is different than others i have encountered, or have been tested on thus far. i have heard that one similar may be on our exam, so i want to make sure i understand how to calculate it.

example: calculate infusion time for an iv (1000ml) of d5w infusing at 25gtt/min, drop factor 10gtt/min.

so, is this like reversing the traditional problem?

1000ml x 10gtt/min divided by 25gtt/min = 400 minutes (??????????????)

i have honestly never had a problem that had me calculate to find the overall infusion time (we normally are looking for the drop factor), so i am really confused. i have looked through my med math books, my notes from the class, and various other resources to no avail. please help!!! -
- 0Aug 21, '11 by
*ParkerBC,MSN,RN*Drop factor is 10gtt/min, so every 10 drips equals 1ml. You are running 25gtt or 2.5ml/minute. 2.5 X 60 = 150ml/hr

If you have 1000ml, how long will it take to complete the entire bag? So, 1000ml/150ml/hr = 6.7 hours or 400 minutes. - 0Aug 21, '11 by cheska_rnQuote from ParkerBeanCurdRN,BSNWhen in my schooling did I fail to learn that drop factor = mL?? So if it is 15gtt/min does that equal 15 drips equaling 1mL?? Is it interchangeable with the both macro and micro?Drop factor is 10gtt/min, so every 10 drips equals 1ml. You are running 25gtt or 2.5ml/minute. 2.5 X 60 = 150ml/hr

If you have 1000ml, how long will it take to complete the entire bag? So, 1000ml/150ml/hr = 6.7 hours or 400 minutes.

I came up w/the same answer, but used a completely different formula. I wonder if it is just a coincidence???

Okay, so what abt this one:

An IV of 500mL is to run at 40gtt/min using a 10 gtt/mL set. What is the infusion time?

Doing it the way I did it before my answer would be: 125 minutes (2hrs and 5mins). Is that the same answer if you do it your way?

Thanks for your help!!! - 0Aug 21, '11 by Ashley, PICU RN15 gtt/min does NOT equal mLs. gtt/min is the rate of the infusion. The drop factor of the tubing tells you how many drops equal one mL. The units for drop factor are gtt/mL and I think you transcribed it wrong in your post. Macrodrip tubing can have a many different drip factors. 15 is usually standard, but it can also be 10, 20, 25, etc. Microdrip tubing always has a drop factor of 60 gtt/mL.

Yes, both formulas get the same answer. Think of it this way:

Total volume / (rate/drop factor) = infusion time

Your first step is to find out how many mLs you are infusing per minute. Do this by dividing the rate of the infusion byt the drop factor of the tubing. Then find out how long it will take to infuse the entire volume by dividing the total volume in the bag by the number of mLs that are infused per minute. - 0Aug 22, '11 by ♪♫ in my ♥I think there's a typo because the drip factor is a measure of how big are the drops... that is, how many drops make up one mL.

If it takes 10 drops to make 1 mL (10 gtt/mL) then it means that each drop has a volume of 0.1 mL (1 mL/10 gtt = 0.1 mL/gtt).

At the given rate (25 gtt/min) it means that 2.5 mL infuses over one minute (25 gtt/min x 0.1 mL/gtt = 2.5 mL/min) OR (25 gtt/min x [1 mL/10 gtt] = 2.5 mL/min).

At that very slow rate, it'll take a long time for the bag to empty. Specifically, (1,000 mL x [1 min/2.5 mL] = 400 minutes) - 0Aug 22, '11 by ♪♫ in my ♥Quote from cheska_rn2be- It doesn't. The drop factor defines the size of each drop (that is, how many drops make up 1 mL).When in my schooling did I fail to learn that drop factor = mL??So if it is 15gtt/min does that equal 15 drips equaling 1mL??

The rate (as in "per minute") is a function of the height of the bag, the size of the angiocath, the size and quality of the vein, the site of the IV, and the positioning of the roller clamp or setting on the flow restrictor.

You really need to try to wrap your head around this.Is it interchangeable with the both macro and micro?

I came up w/the same answer, but used a completely different formula. I wonder if it is just a coincidence??? - 0Aug 22, '11 by
*ParkerBC,MSN,RN*I am sorry if I confused you. When I do these calculations, I have my own method in mind. I need to remember that there is a formula that you are required to follow. I think so as long as we get the same answers, it is okay. I wasn’t trying to “show” you a different way, I was simply confirming that the answer you received is correct. - 0Aug 25, '11 by nowim cleanYou need to know your drip chamber factors it can make a difference. 10,15,60 drips a ml so if you know your chamber and how drips a minute you figure how many gtts or minutes for any bag.
- 0Aug 27, '11 by
*GrnTea*, BSN, MSN, RN"i have honestly never had a problem that had me calculate to find the overall infusion time (we normally are looking for the drop factor), so i am really confused."

now i'm the confused one. you ought never to be asked to calculate drop factor (which as all the above folks have mentioned, is not gtt/minute but gtt/cc), because that's a function of the iv tubing, it's written on the package, and you can't do anything to change it .

*(btw, "drop factor" does not equal "drip rate," which is where your misconception originates, i think. glad we cleared that up for ya!)*

you will always be asked to calculate how long a bag ought to last (so you can eyeball it and have a reasonable idea of when to plan to hang the next one), how many cc/hour (so you can set the pump properly or put those little lines on the bag in magic marker-- antiquated, but still possible), or gtts/minute.

those you can (and must) set accurately to match the medical plan of care as written.