Concentration After Reconstitution
When you add 1 mL of bacteriostatic water to a 10 IU vial, the resulting concentration is 10 IU per mL.
Calculation and Rationale
The concentration calculation follows a straightforward dilution principle used universally in pharmaceutical reconstitution 1:
- Total drug content: 10 IU (International Units) in the vial
- Diluent volume added: 1 mL bacteriostatic water
- Final concentration: 10 IU ÷ 1 mL = 10 IU/mL
This assumes the lyophilized powder contributes negligible volume to the final solution, which is standard practice for most pharmaceutical calculations unless the powder displacement volume is significant 1.
Practical Considerations for Multi-Dose Use
- Bacteriostatic water contains approximately 0.9% benzyl alcohol as a preservative, which permits multi-dose use for up to 28 days when refrigerated at 2-8°C (36-46°F) 1, 2
- The benzyl alcohol concentration of 0.9% provides adequate antimicrobial preservation for repeated withdrawals from the same vial 2, 3
- Label the reconstituted vial clearly with the concentration (10 IU/mL), reconstitution date, and beyond-use date (28 days from reconstitution when refrigerated) 1
Aseptic Technique Requirements
- Sanitize the vial stopper with 70% isopropanol before each needle insertion to prevent microbial contamination 1
- Inject the diluent slowly down the side of the vial to minimize foaming and preserve solution integrity 1
- Gently swirl (do not shake vigorously) until the powder is fully dissolved 1
- Perform visual inspection after mixing for clarity and absence of particulate matter 1
Common Pitfalls to Avoid
- Never use sterile water for injection without preservative for multi-dose vials, as this significantly increases contamination risk 1
- Do not assume extended stability beyond 28 days unless specific stability data supports it 1
- Discard the solution if it becomes discolored or contains particulate matter at any point during the 28-day use period 1
- Avoid reconstituting powder that has been exposed to moisture or appears discolored prior to mixing 1