Overview
This lecture explains allegation, a pharmacy calculation method for mixing two concentrations to prepare a desired concentration, and covers both the ratio and tic-tac-toe methods with an example.
What is Allegation?
- Allegation is used to calculate how much of two different concentrations of the same drug are needed to make a new, desired concentration.
- This method is commonly used in compounding liquids or creams when only certain strengths are available.
- In all allegation problems, one available concentration is stronger and one is weaker than the desired concentration.
Recognizing Allegation Problems
- There will be exactly two concentrations (strengths) available.
- A different, specified concentration is required using the available ones.
Allegation Ratio Method
- Step 1: Identify the high concentration (HC), low concentration (LC), and desired concentration (DC).
- Step 2: Calculate ratios: HC part = DC - LC; LC part = HC - DC.
- Step 3: Add the two ratio values to get the total number of parts.
- Step 4: Set up fractions (each part/total parts) and multiply by the quantity needed to find the amount of each concentration.
Allegation Tic-Tac-Toe Method
- Step 1: Place HC in the top left, DC in the center, and LC in the bottom left of a tic-tac-toe grid.
- Step 2: Find diagonal differences: top right = |LC - DC|, bottom right = |HC - DC|.
- Step 3: Add the differences to find total parts.
- Step 4: Create fractions (each part/total parts) and multiply by the total quantity needed.
Worked Example (applies to both methods)
- Given: Need 100 mL of a 20% solution, have 50% (HC) and 10% (LC) available.
- Ratio values: HC part = 20-10=10, LC part = 50-20=30; total parts = 10+30=40.
- Amounts needed: (10/40)×100mL = 25 mL of 50%; (30/40)×100mL = 75 mL of 10%.
Key Terms & Definitions
- Allegation — A method for mixing two concentrations to achieve a desired third concentration.
- High Concentration (HC) — The stronger available solution.
- Low Concentration (LC) — The weaker available solution.
- Desired Concentration (DC) — The target concentration to prepare.
Action Items / Next Steps
- Practice solving allegation problems using both the ratio and tic-tac-toe methods.
- Review calculation steps and repeat the example for mastery.