Microbiology and Sterile/Compounded Preparations: Controlling Contamination
The chain of contamination
Fungal, bacterial and pyrogen contamination of compounded products comes from people, air, surfaces, equipment and ingredients. Control it with hand hygiene and gowning, a unidirectional airflow cabinet with high-efficiency particulate air filtration, environmental monitoring, disinfection, and validated transfer procedures. Grade A / ISO 5 critical zones sit inside progressively cleaner surrounding areas; the operator is the biggest risk.
Sterilisation methods and their limits
- Moist heat (autoclave): commonly 121 °C, ~15 minutes at 15 psi for aqueous solutions and heat-stable devices.
- Dry heat: typically 160 °C for 2 hours, for glassware and oils.
- Filtration: 0.22 micrometre membrane for heat-labile solutions; it removes organisms but not endotoxins or viruses.
- Ionising radiation: gamma for single-use devices and some powders. Sterility testing uses validated methods; pyrogen/endotoxin testing matters for parenterals and is mandatory thinking for intrathecal products.
Preservatives and beyond-use dates
Do not add preservatives to products going into the eye during surgery, the intrathecal space, or for neonates - the risk of toxicity outweighs the benefit. Beyond-use dates depend on the risk category of the preparation (low, medium, high) and on storage temperature; the numbers differ between standards, so learn the framework and check the current local standard rather than memorising a table from an obsolete edition.
Traps
Filtering a pyrogen-contaminated solution and calling it safe; assuming an autoclave indicator strip proves sterilisation; reusing single-dose vials; using a preservative-containing diluent for a neonatal infusion.
Worked example
A heat-labile antibiotic eye drop is prepared aseptically: filter sterilise the drug solution, prepare in a Grade A zone, use a sterile preservative-free vehicle, and assign a beyond-use date consistent with the risk category and refrigeration.