# Copyright © 2008 Thomson Delmar Learning Conversions: Between and Within Systems Revised KBurger0808 Textbook Assignment: Pickar, G. (2007). Dosage calculations:

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Copyright © 2008 Thomson Delmar Learning Conversions: Between and Within Systems Revised KBurger0808 Textbook Assignment: Pickar, G. (2007). Dosage calculations: A ratio-proportion approach. (2nd ed.) Chapter 4

Copyright © 2008 Thomson Delmar Learning Equivalents 1 grain (gr) = 60 milligrams (mg) 1 teaspoon (t) = 5 milliliters (mL) 1 tablespoon (T) = 3 teaspoons (t) 1 ounce (oz) = 30 milliliters (mL) 1 cup = 8 ounces (oz) 1 Kilogram (Kg) = 2.2 pounds (lbs) 1 liter (L) = 1000 milliliters (mL) 1 gram (g) = 1000 milligrams (mg) 1 milligram (mg) = 1000 micrograms (mcg) The equivalents listed in blue are only considered approximate equivalents

Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion Rule –Recall equivalents –Set up a proportion of two equivalent ratios –Cross-multiply to solve for an unknown quantity, X

Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion Remember –Each ratio in a proportion must have the same relationship and follow the same sequence –A proportion compares like things to like things

Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion Remember –The units of measurement in both numerators and denominators must match –ALWAYS, ALWAYS, ALWAYS label the measurement units in each ratio INCLUDING your unknown quantity X

Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion Example –How many feet are in 36 inches?

Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion Recall equivalent Set up a proportion of two equivalent ratios Cross multiply to solve for “X” Label units to match the unknown “X”

Copyright © 2008 Thomson Delmar Learning Using Ratio Proportion to Convert Within Metric System Recall equivalent Set up a proportion of two equivalent ratios Cross multiply to solve for “X” Label units to match unknown “X” EXAMPLE: Convert 5 grams to milligrams

Copyright © 2008 Thomson Delmar Learning Converting Within the Metric System Short Cut Medication conversions within the metric system most often occur between: mg and mcg [ mg are larger than mcg ] g and mg [ g are larger than mg ] L and mL [ L are larger than mL] These are all 3 decimal place differences [ a difference of 1000 ] To use this Short Cut you will need to remember -which unit is larger -to always move 3 decimal places

Copyright © 2008 Thomson Delmar Learning Conversion Slide Keep this visual in mind when converting within the metric system kg g mg mcg Move decimal point three places between each unit

Copyright © 2008 Thomson Delmar Learning Converting Within Metric System Short Cut continued Write out the desired equivalent in this format 5 mg = ______ mcg Then draw an arrow that starts at the larger unit and points toward the smaller unit Larger to Smaller Move the decimal point in the direction of the arrow by three places.

Copyright © 2008 Thomson Delmar Learning Calculating a Drug Dosage that requires Conversion between Systems Drug order reads Codeine sulfate gr ¾ p.o. q.4h p.r.n., pain Drug supplied is Codeine sulfate 30 mg per tablet Calculate one dose

Copyright © 2008 Thomson Delmar Learning Converting to Same System Drug order reads Codeine sulfate gr ¾ p.o. q.4h p.r.n., pain Drug supplied is Codeine sulfate 30 mg per tablet What do you notice? –Different system –Needs to be converted

Copyright © 2008 Thomson Delmar Learning Approximate Equivalent: gr i = 60 mg Step 1. Convert – Convert to equivalent units in the same system of measurement. Convert gr to mg. – Approximate equivalent: gr i = 60 mg.

Copyright © 2008 Thomson Delmar Learning Convert using Ratio Proportion Method Start by writing a known ratio: 1 grain = 60 mg [ the known equivalent ] Then fill in the rest of the proportion Solve for X 1 gr ¾ gr 60 mg = X mg 1X = 60 x ¾ (0.75) X = 45 mg Codeine gr ¾ = 45 mg

Copyright © 2008 Thomson Delmar Learning Think Step 2 Stop and think carefully about what a reasonable dosage should be: You have just figured out that the doctor ordered 45 mg. The drug label indicates that each tablet = 30 mg. Will you be giving more or less than 1 tablet? MORE

Copyright © 2008 Thomson Delmar Learning Step 3: Calculate using Ratio Proportion Method Start by writing known ratio from the problem Complete the proportion with other information you have [doctor’s order ] Check for matching units. Cross multiply and solve for X 30mg 45mg 30X = 45 1 tablet = X tablet X = 45 = 1 15 = 1 ½ tablets 30 30