Another Drug Calculation question

Dratz · Jul 25, 2005

The book says the answer to this question is 2.5 mL and to use a 3 mL syringe. But for the life of me I cannot figure out how they got that answer and to me I think the question is incomplete.

Doesn't anyone know? :crying2:

DusktilDawn · Jul 25, 2005

I learned to be able to calculate almost any med question in school by using a ratio method.

According to my drug book, there is 20mg/ml of lidocaine in 2%.

50mg:Xml = 20mg:1ml, multiply outside numbers together and inside numbers together (ie: 50 multiplied by 1, then 20 multiplied by X). Ignore mg & ml for time being.

20X = 50, now divide 50 by 20.

X=2.5, therefore 2.5mls is the correct.

Each semester while I was in the RN program, students were required to pass a pharmacology math test with 100% before they could administer medications for that rotation. We had 3 attempts to accomplish this. By utilizing this method, I always got 100% on the first attempt for every test.

Dratz · Jul 25, 2005

I learned to be able to calculate almost any med question in school by using a ratio method.
According to my drug book, there is 20mg/ml of lidocaine in 2%.
50mg:Xml = 20mg:1ml, multiply outside numbers together and inside numbers together (ie: 50 multiplied by 1, then 20 multiplied by X). Ignore mg & ml for time being.
20X = 50, now divide 50 by 20.
X=2.5, therefore 2.5mls is the correct.
Each semester while I was in the RN program, students were required to pass a pharmacology math test with 100% before they could administer medications for that rotation. We had 3 attempts to accomplish this. By utilizing this method, I always got 100% on the first attempt for every test.

Ok Thanks, had the book told me that lidocaine is 20mg/ml I would have figured out the answer. Thanks for your help.:)

DusktilDawn · Jul 25, 2005

Tricky question without that important tidbit of info. Never did have a similar question on a test. But now we both know what to do with %'s.

LadyT618, MSN, APRN, NP · Jul 26, 2005

Remember that x% solution is really "x" grams in 100 ml of solution. Once you remember that, you'll be ok with any "percent" solutions.

2% solution of Lidocaine = 2gm in 100ml solution = 2000mg in 100ml = 20mg/1ml

Then you can use: (Desired/Have) x Volume..... (50/20) x 1 = 2.5.

That's how I would do it. That's how I got 100s in my med math exams in school, first try all the time.

zambezi, BSN, RN · Jul 26, 2005

And if you are lucky, your lidocaine will come in a handy premixed syringe...100mg in 10 ml. So for 50 mg, just give 5 mL.

dave1117 · Jul 26, 2005

But according to all of my Pharm tests we would never asssume the 20mg/ml or have to look it up in a book. The drug would clearly be marked with more than just a 2% lidocaine. I believe her initial question was correct that enough info was not given.

ckh23, BSN, RN · Jul 26, 2005

But according to all of my Pharm tests we would never asssume the 20mg/ml or have to look it up in a book. The drug would clearly be marked with more than just a 2% lidocaine. I believe her initial question was correct that enough info was not given.

I agree. The first time you assume something about a med and not verify it, you are bound to have some type of med error.

heartICU · Jul 27, 2005

The drug would clearly be marked with more than just a 2% lidocaine. I believe her initial question was correct that enough info was not given.

Drug dosages are not always marked with more than their volume percent dosage. Often times, we have ampules with only their volume percent - 0.1% epinephrine, 2% lidocaine, etc. This is just another way of expressing the concentration of the drug.

X percent is the number of grams per 100ml of fluid. (An easy way to remember is just to move the decimal place to the right...2% lido is always 20mg/ml, 0.1% epi is always 1mg/mL, etc). Just a little shortcut - but remember to know how the calculation actually works too!