Single Dosage Calc. Question! Please Help!

Published

Specializes in Operating Room.

Okay so this is my question:

Order: Demerol 50 mg IM

Supply: 75mg/mL in single use syringe

______ mL to waste with witness

Now I know that it is 0.66mL needed to fulfill the order. Why is it 0.33 to waste? Why not 0.34??? In a 1.0mL syringe what happens to that 0.1mL? Just curious. Sorry for asking a silly question I should know the answer to but my first day of NS is tomorrow and we have a dosage calculations pre-test! Thanks all!

Up Vote Up Vote ×

When I took 50/75 x 1 ml I got 0.666667 so your answer would actually round to 0.67 which would leave you an amount of 0.33 to waste

Up Vote Up Vote ×
Specializes in Operating Room.

I just double checked my calculation! Thanks so much! Hope rounding errors don't catch up with me tomorrow! Thanks again!

Up Vote Up Vote ×
I just double checked my calculation! Thanks so much! Hope rounding errors don't catch up with me tomorrow! Thanks again!

No problem. Just take your time and check all of your answers twice...maybe even three times, just to be safe. :) Good Luck!

Up Vote Up Vote ×
Specializes in Operating Room.

Definitely will - I feel so silly not doing it before I even posted here! Haha :cheeky:

Up Vote Up Vote ×

Hi there :)

The math works out to 0.66666mL as the required dose, so that would (according to rounding rules!) be rounded to the nearest hundredth since the dose is less than 1mL, therefore it would be 0.67mL.

1mL-0.67mL=0.33mL

(And the difference in between 0.33mL and 0.34mL is 0.01mL, not 0.1mL...watch your decimal spacings!!)

Good luck on your comp!!! :D

Up Vote Up Vote ×
Specializes in Operating Room.

I knew there was a reason I disliked decimals and fractions! They always trip me up! I am trying to memorize them as much as possible. Placement and rounding seem to be my biggest struggle and they are such simple mistakes. Makes me feel senseless.

Up Vote Up Vote ×

Don't feel senseless...attack it like you're gonna womp on it! It's good to be cautious of your calculations, but there's a difference between being cautious and scared senseless. Being cautious gives you that healthy respect for making sure you're calculating correctly, being scared senseless puts you in that "I can't do this" mindset-you don't want that ;)

Go through your exam, do all the problems. Any you are struggling with, mark and save for after you've completed the others (don't spend all of your time on one problem). After you're done, cover your answers and go back and rework the problems, and see if you come up with the same answers you did the first time. If you're questioning an answer, work it backwards to see if it's right. Every problem can be worked backward once you have an answer (whether your answer is right or wrong working it back will let you know if your answer is correct...or not).

Reeeelllllaaaaaxxxx...:D I think the psychology of the dosage calc exams are students' worst enemy. Go in there and knock it out of the park!! :)

Up Vote Up Vote ×
Specializes in Hospice, Palliative Care.

Good day, SweetSouthernLove:

DosageHelp.com - Helping Nursing Students Learn Dosage Calculations is a great place to learn and practice dosage calculations. In addition to learning Desire / Have * Quantity = Dosage formula, I also recommend learning dimensional analysis (DA). DA is fantastic at making sure you always are using the right units for the answer, and is very accurate (there's no pre-conversions to do before applying the formula; pre-conversions can be an area of making a mistake).

is an excelent video for learning DA.

0.33 mL is waste because the answer is 0.67 mL. 1.00 mL - 0.67 mL = 0.33 mL

While rounding rules do vary from time to time for the school, the "general rule of thumb" is as follows:

When rounding final answers…If the answer is less than 1 such as 0.456 calculate to the thousandth place and use that last number to round to the hundredth place….so 0.456 would round to 0.46 If your final answer is greater than 1 such as 1.78….calculate to the hundredth place and use that last number to round to the tenth place….so 1.78 would round to 1.8.

Exceptions include (but are not limited to) such things as drop (gtts) rates which are expressed in whole numbers.

Thank you.

Up Vote Up Vote ×
Specializes in Operating Room.

This is wonderful! I was looking for something else to practice with for my specific problems besides the books and this is amazing. Thank you so much!

Up Vote Up Vote ×
Specializes in NICU, ICU, PICU, Academia.

Just to lighten things up a bit: Good luck finding a syringe that will be accurate to the 1/100 of a mL! :)

Up Vote Up Vote ×
Specializes in Pediatric Hematology/Oncology.

I struggled with the rounding, too, but then realized I was making it more complicated than it was.

Think about it this way:

When you've got a number like .66666666667ml and the order is for 50 mg/ml, you can safely say that your number is going to have two digits in it and, since the 6 is bigger than 5, round up - hence, .67 and not .66. If your answer was .6611 or something like that, then definitely round down to .66 (since 1 is less than 5, 5 being the cut off for rounding up or down). Usually your numbers are going to look more like .66666667 than but .6611 but you obviously see where the error was and I feel suddenly as though I'm rambling...anyway, good luck! :)

Up Vote Up Vote ×
+ Join the Discussion