What is the shelf life of physician-mixed antibiotic-impregnated calcium sulfate pellets?

This pilot study was undertaken to evaluate the short-term in vitro antimicrobial stability of both vancomycin- and tobramycin-impregnated calcium sulfate pellets mixed and stored in a clinical setting. Powdered tobramycin sulfate (500 mg) and vancomycin hydrochloride (500 mg) were blended into separate basins containing 25 g of surgical-grade calcium sulfate powder, then mixed with 8 mL of sterile saline. From this admixture, 6.0-mm pellets were produced. These were removed from the sterile container (stored at room temperature) at 1, 7, 30, 60, 90, and 120 days and tested against a variety of pathogenic bacterial isolates by using a modification of the standardized Kirby-Bauer test. Control pellets containing no antibiotic were also evaluated. There was no inhibition of bacterial growth by the non-antibiotic-impregnated (control) pellets. There was no appreciable difference in the zones of inhibition for any of the organisms with pellets stored for 1, 7, 30, 60, 90, or 120 days. Zones of inhibition for the various antibiotics to the strain of organism tested ranged from 17 mm to 30 mm, depending on the pathogenic isolate and the antibiotic evaluated. The zones of inhibition observed were similar to those designating antibiotic susceptibility in the Kirby-Bauer test. The results of this preliminary study suggest that clinician-mixed calcium sulfate pellets containing either vancomycin or tobramycin, when stored under normal room temperature and ambient humidity, appear to maintain their antimicrobial characteristics for at least 120 days.