Pharm problem help? please

[btf4]

(question)1. An oral tablet has strength of 0.25 mg. The order is to give 0.125 mg. How many tablets will the nurse give?

Do I do it like this? "X" tablet= 0.25 mg * 1tab/0.125 = 2 tabs?

Or like this? "X" tablet =0.125 mg * 1tab/0.25mg= 0.5 tabs?

(question)2. A client is ordered 150mg of aspirin. 300 mg aspirin tablets are available. How many tablets would you give?

This question has similarities with question number 1, but where I get confused is where do I put my "X" under the strength? Or the order?

[seaofclouds21, BSN, RN]

For these types of problems, I always start by writing what I have on hand (mg over # of tablets) and then what I need to figure out for the ordered dose (mg over X). So, for question one, I have one tablet that is 0.25mg and I need to give 0.125mg, so I would write 0.25mg/1 tab = 0.125mg/x tablet(s). Then solve for X.

[Kuriin, BSN, RN]

Remember your basic Want/Have multiplied by the vehicle. So the second one would be 150/300*1. You can use the same for the first problem.

[Nightmaren, BSN, RN]

Also, (and I'm not trying to be facetious here), try to reason through it based on how much you have vs how much you want instead of trying to fuss about plugging in your numbers into a formula properly.

If you have smaller-dosed tablets and the order asks for a larger dose, of course you will be giving multiple tablets.

If you have larger-dosed tablets and the order asks for a smaller dose, you know that you'll be cutting up a tablet.

[KrCmommy522, BSN, RN]

For these problems you do the ordered dose in the numerator then you multiply it by the form available (i.e., tablets in this case)/dosage.

I always put the dose ordered first, it always goes in the numerator. Then, you write the rest of the equation to set up to cancel out unwanted units. *REMEMBER* The cancellations of units of measure are done either diagonally (cross) or vertical EVER horizontal so, in both of your problems, since mg is the unit in numerator of the ordered dose, you need mg to go in the denominator in fraction of the what's on hand. Another thing our instructor taught us was that when you look at the dose on hand (in this case 0.25 mg/tablet), you put what matches the unit of dose ordered in the denominator (in this case, 0.25 mg) and you ALWAYS put what units you want in the numerator (in this case, you want tablets, so that goes in the numerator)

Then, you cancel out the units (mgs) and you multiply across the numerators and across the denominators, then you divide the numerator and denominator, and you come up with the wanted quantity (in this case, the number of tablets to give your patient).

1. 0.125 mg goes first - in the numerator
2. 0.25 mg/tablet goes next
3. tablets in the numerator (because remember, you put what you want to know in the numerator - and you want to know how many tablets to give the patient)
4. 0.25 mg goes in the denominator, because you want to match up the units you can cancel diagnonally - in this case equation starts with mg in numerator, so you want this fraction to have mg in the denominator so they can cancel each other out)
5. Cancel out like units - cancel out both mg's.
6. Multiply across the numerators - 0.125 x tablets = 0.125. Then multiply across the denominators - 0.25 mg
7. Solve the problem - 0.125/0.25 = 0.5 tablets

So, you give your patient a 0.5 mg tablet

You do the same for your second problem

Ordered Dose = 150 mg

Dose on Hand = 300 mg/tablets (form = tablets, dosage = 300 mg)

Put ordered dose in numerator of first fraction (I forgot to mention this before, you can put a 1 in the denominator, or you can just leave it blank (whatever makes you comfortable). Since this is only a one-factor problem, there is only something in the numerator. If you were doing a dosage calculation based on weight, for example, then you would have ordered dose in numerator and kg or lb in denominator)

Then second fraction is what you have on hand – form goes in numerator (in this case tablets) (on a side note, if you were trying to figure out mL to give, then dosage in mL would go in numerator (e.g., 15 mL)). Then the dosage on hand goes in denominator (in this case 300 mg) – remember you want to be able to cancel out the mg which can only happen diagonally, so it needs to go in the denominator to cancel out the mg in the numerator of the first fraction.

Then multiply across the numerators: 150 x tablets = 150

Multiply across the denominators: 300 = 300

Divide numerator and denominator: 150/300 = 0.5 tablets

You would give the patient 0.5 tablets

Hope this helped some!

By the way, this is called dimensional analysis. Our instructor had us try all different types of equations to solve dosage calculations to find which one we felt most comfortable with. For me it was dimensional analysis.

Some Good Sources:

Uses Dimensional Analysis: Dosage calculations the easy way! - Straight A Nursing

Sources that Show Different Methods of Calculations (e.g., basic formula, ratio proportion, fractional method, dimensional analysis)

Brush up on Your Drug Calculation Skills

Sources That Show Simple to Harder Dosage Calculation Problems:

The nurse's quick guide to I.V. drug calculations : Nursing made Incredibly Easy

DosageHelp.com - Helping Nursing Students Learn Dosage Calculations

Hope this helps you!

[KrCmommy522, BSN, RN]

P.S. Sorry about the big thumbnail attached at the end of my post. I tried to get rid of it but couldn't figure out how. It didn't do it for the other image I posted of the second equation, so I don't know why it did it for the first? Sorry guys!

[flowirgyrl]

Yes what previous posted said about dimensional analysis- that is how we were taught and it makes it so much easier! Knowing you are setting up equation to be able to cross out units helps with worrying about getting ratios wrong. If any of her links help you understand it- I would recommend looking and teaching yourself that way it truly helps any confusion on ratios.

[KrCmommy522, BSN, RN]

flowirgyrl said:

Yes what previous posted said about dimensional analysis- that is how we were taught and it makes it so much easier! Knowing you are setting up equation to be able to cross out units helps with worrying about getting ratios wrong. If any of her links help you understand it- I would recommend looking and teaching yourself that way it truly helps any confusion on ratios.

I agree! Being able to cross out the units and follow the path to the units you need makes it so much easier and you don't have to worry about multiplying the wrong things (like the means and extremes with the ratio and proportion method). For me it just made it so much easier to be able to follow it along the path!