A 2018 study evaluated the physical and chemical stability of hydromorphone 15 mg/mL and bupivacaine 10 mg/mL in 0.9% sodium chloride. Solutions were all aseptically prepared, and test samples of the mixture were stored at 37°C for 90 days in 20 mL plastic syringes. Physical stability was assessed by visual observation, while chemical stability was evaluated with stability-indicating high-performance liquid chromatographic (HPLC) analytical technique; pH and osmolarity were measured electronically. After 3 months, no visible haze, particulate formation, or gas evolution was discovered. The pH and osmolality were maintained stable. The HPLC revealed no significant change in concentration of either hydromorphone or bupivacaine, with less than 5% variability over the 3 months. 
A 2000 stability study evaluated the microbiological stability of fentanyl, bupivacaine, and clonidine diluted in a 0.9% sodium chloride to a total of 1,200 mL and a final concentration of 35 mcg/mL for fentanyl, 1 mg/mL bupivacaine, and 9 mcg/mL for clonidine. The solutions were prepared in eight syringes (polypropylene syringes, 50 mL, B. Braun, Germany) and eight reservoirs (Pharmacia Deltec PVC medication cassettes, 50 mL, Pharmacia, Sweden) and stored at 4, 21, and 36°C. After 12 hours and 28 days of sample incubation, the samples were clear, and no precipitation or change in color was observed during the 28-day storage period. Although there was also no change in pH of the mixtures stored in the PVC medication cassettes, a substantial increase in pH (from pH 4.60 to pH 6.06) was seen when bupivacaine was mixed with fentanyl and clonidine and stored in polypropylene syringes at 36°C after 28 days. The change in pH is likely due to the polypropylene syringes leaching from the natural rubber material of the piston. Overall, there was no in situ antimicrobial activity during the seven days of incubation, and all mixtures retained the acceptable microbiological stability for at least 28 days at 4, 21, and 36°C when prepared under aseptic conditions.