# Help with dosage calculations

1. Hello! I will be entering my fourth and final semester on August 9th and have just received an email from our instructor with several practice problems for our upcoming dosage exam. We always have a dosage exam the first day of class and must pass with 100%. This semester we have a new instructor (she has never taught at my school before) and the way she asks questions is completely different than we have had previously. I know at some point in my scholastic career I have learned this, but boy right now I can not remember how to solve for 'X' for the life of me! Usually we just have an instructor tell us this is what the MD ordered, this is what you have on hand, how much do you give? Any help you can offer would be much appreciated. Okay here it goes....

"The patient is receiving a Levophed drip at 3.7 ml/hr on the IV pump. The concentration of the drug is 8 mg/250 ml of D5W. What is the dose of the medication the patient is receiving? Please respond with the nearest whole number. Mcg/min?"

This is how I have set it up (I think it's right?) but where do I go from here?

_ X_ x 250 ml = 3.7 ml/hr
8 mg
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Joined: Jul '13; Posts: 1

3. 8mg x
---- = ----
250ml 3.7 ml
cross multiply
8*3.7 = 250x
(8*3.7)/250 = x
x=mg/hr
multiply 1000 to get mcg and divide by 60 to get min
then round it to the nearest whole number
4. AN is fiddling with the spacing
8mg/250=x/3.7
5. 3.7mL/1 hr X 8mg/250mL X 1 hr /60 min X 1000 mcg/ 1 mg = 2 mcg/min

Hope this helps!!!
6. It's often helpful to think these things through in words before ever embarking on any equations or formulas. Your instructor knows that, so she has given you a problem in a different format than you are used to, to see if you can figure it out. Rather than just launching into multiplication and division, think about what you're looking at hanging up on the IV pole. My goal here is not just to show you I know how to do the math but to show you how to think about how you're going to do the math.

So lessee here, what have we got?

8 mg = (how many) mcg? Right, 8000.

That number of mcg is in 250cc. So how many mcg are there in 1cc? That would be 8000 divided by 250 = 32 mcg in 1 cc.

If you have that number, how many mcg are in 3.7 cc? -- that gives you the mcg in an hour. That's 118.4 mcg per hour.

Divide by 60 to see how many mcg in a minute.(round to nearest whole number). That's 118.8 / 60 = 1.97, or 2 mcg/minute.

Does that make sense?
Last edit by nurseprnRN on Jul 18, '13
7. Solving for "x" is very simple and relies on a couple of basic concepts:

1) Don't fork (cuz AN gets mad at me when my sailor talk comes out) with the equal sign... Whatever is done on one side must be done exactly the same on the other side.

Think of it like a teeter-totter (if you're old enough to remember those...) or a lever on a fulcrum... in order to maintain stasis, a weight added, subtracted, or relocated on one side of the fulcrum must be offset by an equivalent action on the opposite side (remember when some jerk would jump off the teeter-totter and let you go crashing down?)

2) Multiplication is the inverse of division and subtraction is the opposite of addition.

3) Units can be canceled via multiplication/division just as can numbers.

4) Anything divided by itself equals one

You're trying to get 'x' all by itself (isolate it). Anywhere that 'x' is being multiplied (by either a number or a unit) calls for division by the same value (coefficient) on each side of the equal and anything being added to 'x' needs to be offset by an identical subtraction on each side of the equal.