Calculate with Confidence5th edition

Gray Morris

Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Dosage Calculation Using the Ratio and Proportion Method

Unit Three: Chapter 14

Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Dosage Calculation Using Ratio and Proportion Method: Objectives

After reviewing this chapter, you should be

able to:1. State a ratio and proportion to solve

a given dosage calculation problem2. Solve simple calculation problems

using the ratio and proportion method

Use of Ratio and Proportion

in Dosage Calculation Often need to find a single “unknown” in med calculations Example: Available dose is 50 mg in 1

mL, ordered dose is 25 mg—one “unknown”: x mL

State known first, then unknown Sequence must match

Example: mg : mL = mg : mL (left sequence matches right sequence)

Using Means and Extremes50 mg : 1 mL = 25 mg : mL (known) (unknown) 50 = 25(1) 50 = 25

50 25

50 50 = 0.5 mL

x

x

x

x

x

Stated as a Fraction50 mg 25 mg

(known) = (unknown)1 mL mL

50 = 25(1) 50 = 25

50 25

50 50 = 0.5 mL

xx

x

x

x

Ratio and Proportion: Key Points

Convert everything into same unit of measure before starting calculations

Convert to the unit of measure in which the medication is available

Convert in the direction that eliminates decimals whenever possible—decimals are sources for error

Ratio and Proportion: Key Points (cont’d)

State “known” or “available” first State “unknown” or “desired” second Keep left and right sides in matching

sequence mg : mL = mg : mL

Label all terms, including x Make mental “estimate” of logical

answer first

Convert to Same System FirstOrder: Phenobarbital gr i p.o. at bedtime

WRONG!60 mg : 1 tab = gr i : x tab

RIGHTConversion: gr i = 60 mg60 mg : 1 tab = 60 mg : x

tabx = 1 tab

Convert Toward No Decimals

Order: Augmentin 0.25 g p.o. q8hr

WRONG!500 mg : 1 tab = 0.25 g : x tab

RIGHTConversion Factor: 1 g = 1,000

mgTherefore – 0.25 g = 250 mg

500 mg : 1 tab = 250 mg : x tabX = ½ tab

Ratio and Proportion: Key Points

Label final answer for x in appropriate unit for delivering med (e.g. mL, tabs, caps)

Double-check work Be consistent in set-up and

performance of calculations