So we had a dimensional analysis quiz last night. Ah yes, the dreaded med math—what some nursing students have nightmares about. Every semester we have quizzes on drug calculations. In my program we have to get a 90% or higher on them—at some schools it’s 100%—or you will fail the clinical portion of the course (which means you’ll just fail the course—you can’t fail clinical and pass lecture). Hence why many students get their panties (or boxers/briefs) all up in a bunch. I’m not sure what the old-school method was, but dimensional analysis is really quite simple. The biggest challenge is remembering all of the conversion factors. Some are well-known such as 1000 mg = 1 g, whereas some like 1 gr = 60 mg (what the heck is a grain anyway?) are a little less so. But once you have the conversions factors under your belt, it’s pretty much all downhill from there.
Let’s look at an example. Let’s say the physician has ordered a patient to receive Erythromycin 150 mg PO TID. The pharmacy sends the medication in a bottle labeled 0.75 g per fluid ounce. How many ml should be given at each dose?
I always like to start with the amount ordered (150 mg). Next I look at the dose on hand (as if the pharmacy could ever send it to you the way you actually need it), which is 0.75 g per fluid ounce. The monkeywrench is that I need to administer the medication in milliliters. So I’ll need two conversion factors to make this work—1 gram = 1000 mg and 30 ml = 1 ounce. If you set up the problem correctly the unnecessary measurements will cancel out and you’ll be left with what you need—milliliters. Keep in mind that the conversion factors can always be flipped so that the unwanted measurements cancel out properly—notice how I wrote 1 oz / 0.75 g instead of 0.75 g / 1 oz. Once you have it all lined up, multiply across the top, multiply across the bottom, simplify, and voilà! Piece of cake.
There are a few miscellaneous rules to remember with med math. For instance, always use leading zeros and never use trailing zeros. This helps to reduce possible medication errors. People’s handwriting often sucks and so if a nurse transcribes an order into the MAR for 1.0 grams of a medication but the next nurse doesn’t see the barely-there decimal sign and misinterprets it as 10 grams—she just gave your patient ten times the medication they were supposed to get. Same thing with leading zeros—.1 g can be misinterpreted as 1g. Watch those zeros! Rounding answers can also be tricky. In general, you’ll want to round to a whole number when your measurement is a capsule, gelcap, unscored tablet, etc. Nothing like trying to administer 0.92 of a tablet to a patient…
We finally had the dreaded “math/drug calculation exam”—the one you need to get an A on or fail the course (though they do give you 3 tries before they cut you loose). I’ve always heard nursing students making such a big deal out of this exam, claiming to have lost sleep (and even hair) over it. It did take memorizing some of the lesser known conversions like 1 grain = 60 milligrams and 30 milliliters = 1 ounce, but it really wasn’t that bad. Aced it—100%. I think a lot of the stress nursing students experience comes from all of the melodrama and sensationalism they attach to everything. Chillax already—when one of us gets all worked up about something it is contagious—it spreads through the class faster than a fire in a grease pit. Not that I don’t have my own fair share of stress and worry, but the level of agitation among some of these students gives me flashbacks of my days working in inpatient psych—sheesh.
