Chapter 5: Dosage Calculations

Mastery of accurate drug calculation is a fundamental responsibility for the practicing nurse, ensuring patient safety and preventing medication errors, as the nurse is legally and professionally accountable for the administered dose. This comprehensive chapter establishes the mathematical methods necessary to convert drug orders into the appropriate available dose forms. It begins by outlining the four primary systems of measurement currently used in drug preparation and delivery: the metric system, which is the most widely adopted and is based on multiples of ten; the older apothecary system, which uses the minim and the grain; the household system, which utilizes common units like the teaspoon and the pound; and the less frequent avoirdupois system. Because the U.S. Pharmacopeia Convention mandates that all drugs be dispensed in the metric system, nurses must be proficient in conversion, or finding equivalent values between these systems. The simplest and most reliable method for achieving conversions and calculating doses is setting up a ratio and proportion equation. This algebraic technique is applied universally to determine the number of tablets or capsules required for oral doses, the volume of liquid needed for oral or parenteral (injectable) administration, and the flow rate for intravenous (IV) solutions. IV calculations require determining the number of drops per minute, factoring in the prescribed volume, the administration time, and the specific drop factor of the tubing, such as 60 drops per milliliter for a microdrip system or 15 drops per milliliter for a macrodrip system. Furthermore, the chapter addresses the necessity of different drug doses for children compared to adults, whose bodies handle drugs differently across all areas of pharmacokinetics. While older, less precise methods like the Fried rule (for children younger than 2 years), the Young rule (for children 1 to 12 years of age), and the Clark rule (based on the child’s weight) can estimate a dose based on the average adult dose, contemporary practice favors more accurate methods. These include basing the dose on Body Surface Area (BSA) determined via a nomogram, or prescribing the dose directly in milligrams per kilogram (mg/kg) of body weight, which is especially important for drugs with a narrow margin of safety.