Compounding Formulas Quick Reference
Compounding is the preparation of a customized medication to meet a specific patient need. This cheat sheet covers the essential formulas and concepts tested on the CPHT exam.
Key Compounding Terminology
| Term | Definition |
|---|---|
| Levigation | Reducing particle size of a powder by incorporating a small amount of liquid (levigating agent) and triturating |
| Trituration | Grinding or mixing powders together using a mortar and pestle |
| Geometric dilution | Technique for mixing a small amount of drug with a large amount of diluent by doubling the amount of diluent at each step |
| Spatulation | Mixing powders on an ointment slab using a spatula |
| Comminution | The process of reducing particle size |
| Emulsification | Creating a stable mixture of two immiscible liquids |
| Punch method | Method for filling capsules by hand |
| Excipient | An inactive ingredient in a compound (base, filler, binder, etc.) |
| Vehicle | The liquid carrier for a drug in a solution or suspension |
| BUD (Beyond-Use Date) | The date after which a compounded preparation should not be used |
Percentage Calculations for Compounding
Example 1: Prepare 60 g of 2% salicylic acid ointment.
Amount of salicylic acid = 2% x 60 g = 0.02 x 60 g = 1.2 g
Amount of ointment base = 60 g - 1.2 g = 58.8 g
Example 2: Prepare 120 mL of a 0.5% w/v solution.
Amount of drug = 0.5% x 120 mL = 0.005 x 120 = 0.6 g dissolved in sufficient vehicle to make 120 mL.
Dilution and Concentration
Formula: C1 x Q1 = C2 x Q2
Example: You have 10% povidone-iodine solution. How much is needed to make 500 mL of 2% solution?
10% x Q1 = 2% x 500 mL
Q1 = (2 x 500) / 10 = 100 mL of 10% solution. Add vehicle to bring total volume to 500 mL.
Alligation for Compounding
Example: Prepare 100 g of 3% hydrocortisone cream using 1% and 5% creams.
- Higher: 5% (upper left); Lower: 1% (lower left); Desired: 3% (center)
- Diagonals: 3 - 1 = 2 parts of 5%; 5 - 3 = 2 parts of 1%
- Total parts = 2 + 2 = 4
- Amount of 5% cream = (2/4) x 100 g = 50 g
- Amount of 1% cream = (2/4) x 100 g = 50 g
Enlarging and Reducing Formulas
To scale a formula, use the ratio of desired quantity to original quantity as a multiplier.
Example: A formula makes 30 capsules. You need 100 capsules.
Multiplier = 100 / 30 = 3.33
Multiply each ingredient amount by 3.33.
Least Weighable Quantity (LWQ)
The LWQ depends on the balance sensitivity and the acceptable error (typically 5%).
Formula: LWQ = (Sensitivity of balance x 100) / Acceptable error (%)
Example: A Class III balance has a sensitivity of 6 mg. At 5% error:
LWQ = (6 mg x 100) / 5 = 120 mg
If you need less than 120 mg, you must use an aliquot method or a more sensitive balance.
Aliquot Method
Used when the required amount of drug is below the LWQ of the available balance.
- Weigh an amount of drug that is at or above the LWQ.
- Mix with a known amount of inert diluent (e.g., lactose).
- Weigh an aliquot (portion) of the mixture that contains the desired amount of drug.
Example: You need 20 mg of drug. Your LWQ is 120 mg.
- Weigh 120 mg of drug.
- Mix with enough lactose to create a workable total (e.g., add 2880 mg lactose for 3000 mg total).
- Drug concentration in mixture = 120 mg / 3000 mg = 0.04 (4%)
- Aliquot needed = 20 mg / 0.04 = 500 mg of the mixture contains 20 mg drug.
Non-Sterile Compounding - USP 795 Key Points
- USP Chapter 795 governs non-sterile compounding.
- Compounders must follow the master formulation record and create a compounding record for each batch.
- Proper PPE includes gloves, lab coat, and sometimes a mask and eye protection.
- Equipment must be cleaned and calibrated regularly.
- Ingredients must be obtained from reliable, FDA-registered suppliers.
- A BUD must be assigned based on stability data or default guidelines.
Sterile Compounding - USP 797 Key Points
- USP Chapter 797 governs sterile compounding (CSPs - Compounded Sterile Preparations).
- Sterile compounding must be performed in a cleanroom environment with appropriate ISO-classified engineering controls (laminar airflow workbench or compounding aseptic isolator).
- The primary engineering control (PEC) provides ISO Class 5 air.
- Personnel must complete initial and ongoing training, including aseptic technique assessment (media-fill testing and gloved fingertip sampling).
- Garbing order: shoe covers, hair cover, face mask, hand washing, non-shedding gown, sterile gloves.
- All work surfaces must be cleaned with sterile 70% isopropyl alcohol.
- Items introduced into the PEC must be wiped with sterile 70% IPA.
Exam Tips
- Know how to use geometric dilution when mixing a potent drug with a large amount of filler.
- Practice aliquot method calculations - they test your understanding of LWQ and precision.
- Remember the garbing order for sterile compounding - it is frequently tested.
- Be able to distinguish between USP 795 (non-sterile) and USP 797 (sterile).
- USP 800 covers hazardous drug handling and overlaps with compounding regulations.