CSP stands for compounded sterile preparation. It refers to any medication that a pharmacy prepares in a sterile environment, typically because the exact product a patient needs isn’t available as a commercial, off-the-shelf drug. Common examples include custom IV admixtures, injectable medications, implants, and ophthalmic (eye) preparations. Because these products enter the body through routes that bypass its natural defenses, contamination can cause severe harm, so CSPs are governed by strict federal standards under USP General Chapter <797>.
Why CSPs Exist
Manufacturers can’t anticipate every combination of drug, dose, and delivery method that patients need. A hospitalized patient might require a specific concentration of an antibiotic mixed into an IV bag, or an ophthalmologist might need a preservative-free injection that no company sells in that form. Pharmacies fill these gaps by compounding the preparation themselves, combining ingredients under controlled conditions to produce a sterile, patient-ready product.
The stakes are high. A CDC review of outbreaks linked to contaminated compounded sterile preparations between 2001 and 2013 identified 19 separate incidents, resulting in at least 1,000 patient cases and multiple deaths. Nearly half of those outbreaks involved injectable steroids or intraocular injections. The most devastating was a nationwide fungal meningitis outbreak that exposed nearly 14,000 people and caused at least 751 infections and 64 deaths across 20 states. Contaminated CSPs have caused bloodstream infections, blindness from eye infections, meningitis, and life-threatening sepsis.
Category 1 vs. Category 2
USP <797> divides CSPs into categories based on the conditions under which they’re made and how long they can be stored before use. The category determines the beyond-use date (BUD), which is the deadline after which the preparation should no longer be administered.
Category 1 CSPs are made under simpler conditions and carry the shortest shelf life: no more than 12 hours at controlled room temperature, or up to 24 hours if refrigerated. These are typically prepared for immediate or near-immediate use.
Category 2 CSPs are prepared under more rigorous conditions that allow longer storage. When all starting ingredients are already sterile, the BUD extends to 4 days at room temperature, 10 days refrigerated, or 45 days frozen. If any starting ingredient is nonsterile, those windows shrink to 1 day at room temperature, 4 days refrigerated, or 45 days frozen.
The Cleanroom Environment
CSPs must be prepared in specially designed rooms that control airborne particles, because even microscopic contaminants can cause infection when injected into a patient. USP <797> requires facilities to meet ISO air quality classifications, which define how many particles of a given size are permitted per cubic meter of air.
The actual compounding happens inside a primary engineering control, such as a laminar airflow hood or an isolator, which must meet ISO Class 5, the strictest standard used in pharmacy. This workspace sits inside a buffer room that meets ISO Class 7. An anteroom, which serves as a transitional space where staff gown up and supplies are staged, meets ISO Class 8.
Air pressure differences between these rooms prevent contaminated air from drifting into cleaner spaces. For nonhazardous compounding, both the buffer room and anteroom maintain positive pressure relative to the surrounding hallway, pushing clean air outward. The reverse applies when compounding hazardous drugs like certain chemotherapy agents: the buffer room is kept at negative pressure so that drug particles are contained rather than escaping into adjacent areas.
Personnel Training and Testing
Equipment alone doesn’t guarantee sterility. The person doing the compounding is the most common source of contamination, so USP <797> requires rigorous initial and ongoing competency testing for every pharmacist and technician who prepares CSPs.
Before compounding any preparation for a patient, staff must complete a formal training program (typically 20 to 40 hours through an accredited provider) and pass three types of evaluation:
- Direct observation: A trained compounder watches the employee perform aseptic technique and identifies errors in real time.
- Media-fill testing: The employee prepares a simulated CSP using growth media instead of actual drugs. If bacteria grow in the finished product, the test fails, meaning the employee introduced contamination somewhere in the process.
- Gloved fingertip testing: After donning sterile garb, the employee presses all ten fingertips onto growth media plates. This is repeated three times during initial training to confirm that the gowning and gloving process itself doesn’t introduce organisms. Any microbial growth indicates a break in technique.
These evaluations are repeated on an ongoing basis, not just at hire. Continuing education requirements of 2 to 4 hours per renewal period keep staff current on evolving standards.
Labeling and Documentation
Every CSP must carry a label that allows any healthcare provider to identify exactly what’s in the preparation, how to store it, and when it expires. Required information typically includes the names and quantities of all ingredients, the intended route of administration (IV, injection, eye drop), the beyond-use date, storage conditions, and the identity of the compounder.
Behind the scenes, pharmacies maintain detailed records for each preparation. A master formulation record acts as the recipe, documenting ingredients, quantities, equipment, and step-by-step procedures. Each time that recipe is used, a compounding record captures the specific lot numbers of ingredients, the name of the person who prepared and verified it, and any environmental monitoring results from that session. This paper trail makes it possible to trace a problem back to its source if a patient develops an adverse reaction.
How Contamination Happens
The CDC’s analysis of pharmacy-related outbreaks found two practices most commonly linked to contaminated CSPs: compounding that converts nonsterile ingredients into a supposedly sterile final product, and repackaging of already-sterile products into new containers. Both steps create opportunities for bacteria or fungi to enter the preparation if technique, equipment, or environmental controls fall short.
The infections that result depend on how the contaminated product is used. Contaminated IV drugs most often cause bloodstream infections. Contaminated eye injections lead to endophthalmitis, a serious infection inside the eye that can cause permanent vision loss. Contaminated spinal injections have caused meningitis. In all of these cases, the root cause traced back to deviations from established sterilization procedures, whether that meant inadequate air filtration, poor aseptic technique, or failure to properly sterilize nonsterile starting materials.