Dosages help!!

Shan, RN · Apr 21, 2023

Study up on how exactly dimensional analysis works. I went into nursing school with a background in engineering/physics, and dosages were a walk in the park because I knew how to set up dimensional analysis equations with ease. If you can get that down, you can solve any dosage problem. Always go into the problem by writing down the unit that your answer should be in. Then, cancel out the units until you're left with ONLY what you need. Go back over everything carefully to make sure you're not missing anything, or that you didn't accidentally incorporate something unnecessary (dosage problems love to give you info that you don't need). Then, re-write the equation with just the numbers and multiply/divide. Run everything through the calculator at least twice, because all it takes is one wrong button to get a really wrong answer.

Memorize your key conversions (ml to L, mg to g, lbs to kg, etc.). Do A LOT of practice questions--there's tons online. Start with easy ones and work your way up in complexity as you gain confidence. Look up tutorials on YouTube. Take time to really understand the solutions to problems that you get wrong. There's also some great workbooks out there that focus solely on nursing dosage calculations.

Best of luck! ?

SciGuy27 · Apr 21, 2023

I have worked as a chemist for many years, and I think Shan gives great advice. One thing I always instill in new hires and assistants is to write out the dimensional analysis to see it. Never skip steps even if you start to feel confident because skipping over one unit can screw the whole thing up.

Oh and always make sure the numerator of one side and the denominator of the other side have matching units to cross cancel!

chare · Apr 21, 2023

I agree with both the above posts; learn and use dimensional analysis (DA) and none of these problems will be difficult.

If you are unfamiliar with DA you might find the attachment to the first post in this thread helpful.

Best wishes.

bluedreamerstu · Apr 23, 2023

Shan said:

Study up on how exactly dimensional analysis works. I went into nursing school with a background in engineering/physics, and dosages were a walk in the park because I knew how to set up dimensional analysis equations with ease. If you can get that down, you can solve any dosage problem. Always go into the problem by writing down the unit that your answer should be in. Then, cancel out the units until you're left with ONLY what you need. Go back over everything carefully to make sure you're not missing anything, or that you didn't accidentally incorporate something unnecessary (dosage problems love to give you info that you don't need). Then, re-write the equation with just the numbers and multiply/divide. Run everything through the calculator at least twice, because all it takes is one wrong button to get a really wrong answer.

Memorize your key conversions (ml to L, mg to g, lbs to kg, etc.). Do A LOT of practice questions--there's tons online. Start with easy ones and work your way up in complexity as you gain confidence. Look up tutorials on YouTube. Take time to really understand the solutions to problems that you get wrong. There's also some great workbooks out there that focus solely on nursing dosage calculations.

Best of luck! ?

Thank you so much for your tips. I have been practicing dimensional analysis everyday and it definitely is a matter of practice practice practice, and also running those numbers through calculator more than once!! great tips, they've helped a ton.

marienm, RN, CCRN · Apr 24, 2023

You can do this! The biggest thing to remember with dimensional analysis is that each "fraction" is equal to 1 (or equal to itself), which means you can flip it over if needed. You can write

12 inches 1 foot

----------- or -------------

1 foot 12 inches

They are the same (I hope all the spacing works out on the screen LOL!). Remember that the "units" in nursing might be something like "tablets."

Start by writing out what unit(s) you NEED. This will be the numerator and denominator units you won't cross out. How many tablets will you give a patient who has 975mg acetaminophen ordered? Each tablet is 325mg.

NEED: Tablets per dose.

1 tab 975mg 975 tab

------ X -------- = ---------- = 3 tab/dose

325mg 1 dose 325 dose

(mg is shown in italics because you will cross it out)

For a simple problem like this one it's easy to forget that you need 2 units (tablets and dose), but it becomes more obvious when you are trying to solve something more complicated like "how many mL/hour?" or "how many gtt/min?"

To solve mL/hr you need

*the volume of the container (mL/dose)

*the time it's supposed to run over (minutes or hours/dose)

and of course

*the number of minutes in 1 hour (60min/1hour).

You can flip each of those parts over as necessary so the only units you aren't crossing out are mL (numerator) and hour (denominator). A 25mL bag of lopressor is ordered to run over 15 minutes. So you have 25mL/1 dose, 1 dose/15 min, and 60 min /1 hour. Cross out the doses and the mins and you are left with mL/hour, right? (Then multiply the numbers straight across and divide--or reduce--the final fraction, of course!)

With practice, you'll get better. Many instructors will mark you off if you don't write out every step, even for an easy problem, so make sure you are doing that even if you KNOW the answer is "3 tablets" they want to see all your work. However, when you KNOW the answer is 3 tablets it's a good way to check your method and make sure it's giving you the right answer. The final fraction usually looks really dumb (like 975 tablets/325 doses, who wants to give that much tylenol?!) but divide out that fraction and you (should) see a more logical answer.

londonflo · Apr 25, 2023

marienm, RN, CCRN said:

For a simple problem like this one it's easy to forget that you need 2 units (tablets and dose),

Totally confused,,, you eventfully came up with 3 tablets in your math. But your answer is 2 tablets.

FiremedicMike, BSN, RN, EMT-P · Apr 25, 2023

For what worth, I didn't learn dimensional math growing up and thankfully my nursing program didn't require us to use it. I don't know what the old school version on of math is called, but I find it infinitely easier to navigate.

It may be worth your time to look at the way math was done prior to dimensional math, it may be easier for you.

vintagegal, BSN, DNP, RN, NP · Apr 25, 2023

Common sense it, don't get into the weeds of the problem or the set up. If the calculation seems off it probably is.

marienm, RN, CCRN · Apr 25, 2023

londonflo said:

Totally confused,,, you eventfully came up with 3 tablets in your math. But your answer is 2 tablets.

3 tablets is correct and is the answer I showed.

The point I was making about "2 units" is that the answer requires calculating "tablets per dose", where tablets and doses are both units, just like "mL per hour" or "gtt per minute." Students tend to forget that "dose" is a unit and aren't sure how to set up the equation.

To those who are saying that the OP shouldn't use dimensional analysis if another method works better: at my school, we were required to use DA (10 years ago, so it definitely could have changed but all my recent new grads use it too). Even for easy questions like my Tylenol question, we had to write out the whole thing or we'd get the question wrong. We had 3 chances each semester to pass a medication test and if we didn't succeed we would fail the entire semester. So, OP may need to use DA in this circumstance. If it's not required, I fully support the OP in writing it out the way that works best for them!

bluedreamerstu · Apr 25, 2023

marienm, RN, CCRN said:

3 tablets is correct and is the answer I showed.

The point I was making about "2 units" is that the answer requires calculating "tablets per dose", where tablets and doses are both units, just like "mL per hour" or "gtt per minute." Students tend to forget that "dose" is a unit and aren't sure how to set up the equation.

To those who are saying that the OP shouldn't use dimensional analysis if another method works better: at my school, we were required to use DA (10 years ago, so it definitely could have changed but all my recent new grads use it too). Even for easy questions like my Tylenol question, we had to write out the whole thing or we'd get the question wrong. We had 3 chances each semester to pass a medication test and if we didn't succeed we would fail the entire semester. So, OP may need to use DA in this circumstance. If it's not required, I fully support the OP in writing it out the way that works best for them!

Yes. Unfortunately dimensional analysis is required for our program. We do have to show our work and how we got everything and they collect our papers afterwards. We do have a dosage competency exam and have 2 attempts. If you fail the exam you fail the semester. These tips have worked great, however! A lot of helpful tips. Practicing everyday helped a lot, using outside resources. I got a 95 on my second exam hopefully moving forward it stays the same.

londonflo · Apr 25, 2023

marienm, RN, CCRN said:

To those who are saying that the OP shouldn't use dimensional analysis if another method works better: at my school, we were required to use DA (10 years ago,

so if someone solved a problem correctly but did not use DA they would be marked wrong?

DA may work for chemistry but it belabors the math for medications -- take a look

https://www.Google.com/search?rlz=1C1CHZN_enUS905US905&sxsrf=APwXEdcKPilo1TnUayd3aQpxyhrWHUGRkA:1682444695934&q=How+do+you+do+new+addition+in+math%3F&sa=X&ved=2ahUKEwim0dHzysX-AhWnkYkEHWy_BkMQzmd6BAhrEAY&biw=1280&bih=577&dpr=1.5#kpvalbx=_uxFIZJOpPIfdptQPh6O_0As_28