Dosage Calculation Workbook
Chapter 1: The One Method: Dimensional Analysis Every dosage calculation on the exam can be solved with a single method called dimensional analysis, and learning it frees you from memorizing a different formula for every problem type. The idea is simple: you start with what you want to find, multiply by fractions that cancel units you do not want, and keep the units you do want. If the units cancel correctly and leave only the unit you are solving for, the math is almost certainly right. If the units do not cancel, the setup is wrong no matter how reasonable the number looks. The setup always follows the same skeleton. Write the desired unit on the left side of an equals sign. On the right, begin with the given quantity, then multiply by conversion fractions arranged so each unwanted unit appears once in a numerator and once in a denominator, cancelling out. For example, to convert an order of 500 milligrams into tablets of 250 milligrams each, you write: tablets equals 500 milligrams times one tablet over 250 milligrams. Milligrams cancel, leaving two tablets. The same skeleton scales up to the hardest IV and weight-based problems.
Safety habits before any math Three habits prevent most medication errors before calculation even begins. First, read the order three times and question anything that differs wildly from a typical dose; a tenfold error is the most common kind. Second, never let a trailing zero appear, write 5 mg never 5.0 mg, and always use a leading zero, write 0.5 mg never .5 mg, because a missed decimal point multiplies or divides the dose by ten. Third, estimate the answer before calculating; if the order is half the strength on hand, the answer should be about half a tablet, and a calculated answer of fifty tablets tells you instantly that something went wrong. An order reads 250 mg and the tablets available are 125 mg each. How many tablets are given? A. One-half tablet B. One tablet C. Two tablets D. Four tablets Answer: C. Two hundred fifty milligrams divided by one hundred twenty-five milligrams per tablet equals two tablets. The estimate confirms it: the order is twice the tablet strength, so two tablets.
Which written dose follows safe notation rules? A. 5.0 mg B. .5 mg C. 0.5 mg D. 5.00 mg Answer: C. Safe notation uses a leading zero before a decimal point and never a trailing zero after one. Writing 0.5 mg prevents the decimal from being missed, which would turn the dose into 5 mg.; Chapter 2: Conversions You Must Know Cold
Dosage Calculation Workbook; Dosage Calculation Workbook
An order reads 2 teaspoons of a liquid medication. How many milliliters is this? A. 2 mL B. 5 mL C. 10 mL D. 15 mL Answer: C. One teaspoon equals five milliliters, so two teaspoons equal ten milliliters. An oral syringe marked in milliliters is the accurate way to measure it.; Dosage Calculation Workbook
When mixing regular and NPH insulin in one syringe, which is drawn up first? A. NPH first B. Regular first C. Either order is fine D. They must never be mixed Answer: B. Regular, the clear insulin, is drawn before NPH, the cloudy insulin, to prevent contaminating the regular insulin vial with the longer-acting suspension, which would alter its rapid onset.
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