20 Alligations

State Standard 22) Calculate correct doses required when given a simulated prescription for a pediatric dose, adult dose, and geriatric dose based on weight (if applicable), length of administration, times per day of administration, and presence of other diseases/disorders.**

Objectives Students will be able to… Identify when to use Alligations calculations Calculate dosages using Alligations.

Alligations Principle relating to the solution of questions concerning the compounding or mixing of different ingredients Used when mixing two products with different percent strengths of the same active ingredient Strength of final product falls between the strengths of each original product.

Solving Alligations Determine how many parts of the same product, with different strengths, you need to create the final strength requested. Calculate exactly how many milliliters or grams you need of each constituent (beginning) product.

Solving Alligations Critical points Solvents and diluents, such as water, vanishing cream base, and white petrolatum, are considered to be a percent strength of zero. Liquids, including solutions, syrups, elixirs, and even lotions, are expressed in milliliters.

Solving Alligations Critical points Solids, including powders, creams, and ointments, are expressed in grams. The alligation formula requires that you express the strength as a percentage when setting up the problem. You must convert any ratio strength in the initial question to a percent strength.

Solving Alligations Critical points When writing percents or using decimals, always use a leading zero (e.g., 0.25%). This helps prevent errors in interpretation. It would be a terrible error—possibly even fatal—to dispense something in 25% strength that was really supposed to be 0.25% strength.

Solving Alligations Critical points 1 fl. oz = 29.57 mL (commonly rounded to 30 mL) 1 avoirdupois oz. = 28.35 g (used for solids, commonly rounded to 30 g)

Solving Alligations The Alligation Grid "Tic-tac-toe board" Tool to easily solve pharmacy alligation problems You will need to know: Higher strength: top left box Lower strength: bottom left box Desired strength: center box Desired quantity

Figure 20-1 The alligation grid.

Solving Alligations The Alligation Grid To calculate the number for the top and bottom right boxes, work diagonally and take the difference between the two numbers already in place Only positive numbers are used in the alligation grid (remove minus sign from any negative numbers) The procedure is the same for weights or volumes

Fill in the Alligation Grid

Calculate the Numbers

Solving Alligations The Alligation Grid Subtract the center number from the top left number to find the bottom right number

Calculate the Numbers

Calculate the Numbers

Solving Alligations The Alligation Grid Subtract the center number from the bottom left number to find the top right number

Calculate the Numbers

Calculate the Numbers

Solving Alligations The Alligation Grid The numbers on the right-hand side represent the number of parts per ingredient, when read straight across. Add the numbers in the right column to determine the total number of parts necessary

Calculate the Numbers

Calculate the Numbers

Solving Alligations Set up this proportion. parts needed total parts For a 2.5% ointment 1 part/1.5 parts or 1/1.5 For a 1% ointment 0.5 parts/1.5 parts or 0.5/1.15

Solving Alligations Multiply the proportion of parts needed of each ointment by the desired quantity to determine the quantity of each needed. Example 2.5% ointment: 120 g/1.5 × 1 = 80 g 1% ointment: 120 g/1.5 × 0.5 = 40 g

Summary In certain situations, a pharmacy must use the alligation method to combine two varying strengths of a drug, or combine a drug with a base or diluent, to achieve the prescribed strength. Once you master the alligation grid, the calculations can be performed easily.

Activity Complete drug cards for the following… Dexlansoprazole (Dexilant) Glyburide (Diabeta) Olanzapine (Zyprexa) Individually complete the provided Alligations Activity.