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Case Study: Preparing a TPN Admixture
A Certified Sterile Compounding Pharmacist (CSCP) Case Study
The Scenario
You are a Certified Sterile Compounding Pharmacist in a hospital cleanroom. You have received a physician's order for a total parenteral nutrition (TPN) solution for a 70-year-old patient who has an ileus and cannot receive enteral nutrition. Your role is to verify the order, perform necessary calculations, and ensure the TPN is compounded safely according to USP <797> standards, paying close attention to stability and compatibility.
Data Sets
Table 1: TPN Order Components
| Component | Amount to Add |
|---|---|
| Dextrose 70% in Water | 500 mL |
| Amino Acids 10% | 1000 mL |
| Sterile Water for Injection | 500 mL |
| Electrolytes & Vitamins | Small volumes |
| Total Final Volume | 2000 mL |
Table 2: Excerpt from USP Chapter <797>
Risk Levels & Beyond-Use Dating (BUDs):
- Category 2 Compounded Sterile Products (CSPs): Aseptically processed using only sterile ingredients. TPNs fall into this category.
- BUD for Category 2 CSPs (No Sterility Testing):
- Room Temperature: ≤ 4 days
- Refrigerator: ≤ 10 days
- Freezer: ≤ 45 days
- Aseptic Technique Note: When compounding TPNs, phosphate salts should be added early in the mixing sequence, and calcium salts should be added near the end to minimize the risk of precipitation.
Your Task
1. What is the final concentration (as a percentage) of dextrose in the compounded TPN solution?
Answer:
The final concentration of dextrose is 17.5%.
Methodology:
- Calculate the total grams of dextrose added: $$500 \text{ mL} \times 70\% \text{ (or 0.70 g/mL)} = 350 \text{ grams}$$.
- Divide the total grams of dextrose by the total final volume: $$350 \text{ g} \div 2000 \text{ mL} = 0.175 \text{ g/mL}$$.
- Convert the decimal to a percentage: $$0.175 \times 100 = 17.5\%$$.
Insight:
Accurate calculations are paramount in sterile compounding, especially for high-alert preparations like TPNs. An error in dextrose concentration can lead to severe hyperglycemia or hypoglycemia, causing significant patient harm.
2. Based on USP <797>, what is the appropriate risk level and beyond-use date (BUD) for this TPN if it is to be stored in the refrigerator?
Answer:
It is a **Category 2** CSP with a BUD of **10 days** under refrigeration.
Methodology:
- Identify the risk level from Table 2: TPNs are Category 2 CSPs.
- Identify the BUD for a Category 2 CSP stored in the refrigerator from Table 2: ≤ 10 days.
Insight:
Assigning the correct BUD is a critical safety function governed by USP <797>. It ensures that the sterile product is administered to the patient before there is a significant risk of microbial growth, protecting the patient from bloodstream infections.
3. What is the most critical chemical compatibility issue that must be assessed when compounding a TPN, and how is it managed?
Answer:
The most critical compatibility issue is the potential for **calcium and phosphate to precipitate** as calcium phosphate. This is managed by adding the phosphate salts early in the compounding process and the calcium salts near the end, with adequate mixing in between, to minimize their time in a concentrated solution together.
Insight:
The formation of a calcium phosphate precipitate is a life-threatening complication, as the particles can form an embolus in the patient's lungs. A CSCP must be an expert in TPN compatibility and follow strict aseptic technique, including the proper order of addition, to prevent this from occurring.
4. What piece of equipment is typically used to compound a multi-ingredient TPN like this, and what is its primary function?
Answer:
An **automated compounding device (ACD)**, often referred to as a "TPN compounder." Its primary function is to accurately and precisely pump the specific volumes of the different base solutions (dextrose, amino acids, water, etc.) into the final TPN bag according to the programmed formula.
Insight:
While manual compounding is possible, the use of ACDs is the standard of care for complex, multi-ingredient sterile products like TPNs. A CSCP must be proficient in the setup, calibration, and operation of these devices to ensure the accuracy and safety of the final preparation. The ACD improves accuracy and efficiency compared to manually drawing up each large volume with syringes.
