Storage of extemporaneously prepared ophthalmic antimicrobial solutions

The feasibility of long-term storage of commonly used ophthalmic antimicrobial solutions was studied. Solutions of tobramycin 15 mg/mL (as the sulfate salt), cefazolin 33 mg/mL (as the sodium salt), and vancomycin 50 mg/mL (as the hydrochloride salt), each in artificial tears, were prepared with aseptic technique. Ten 15-mL portions of each solution were prepared; five of each were stored at 4 degrees C and the other five at 25 degrees C. Samples of each portion were tested before storage and 7, 14, 21, and 28 days after preparation for osmolality, pH, and antimicrobial activity. For the tobramycin solution there were no differences in osmolality or the zone of inhibition associated with temperature or time. The pH dropped between days 0 and 7 at both temperatures. For the cefazolin solution there were no differences in osmolality associated with temperature or time. The pH was higher in portions stored at 25 degrees C than at 4 degrees C and increased over time in portions stored at either temperature. The zone of inhibition was larger for portions stored at 4 degrees C than at 25 degrees C but did not change over time. For the vancomycin solution there were no differences in osmolality associated with temperature or time. The pH did not differ between portions stored at 4 and 25 degrees C but dropped sharply at both temperatures between days 0 and 7. The zone of inhibition did not differ with temperature or time. The tobramycin solution could be stored for 28 days at room temperature and the cefazolin solution for 28 days under refrigeration. The pH of the vancomycin solution changed too quickly for storage to be recommended.