Prediction of inventory sustainability during a drug shortage

Purpose A methodology for predicting how long the on-hand inventory of a given medication will last during a supply shortage is described; a practical example of application of the methodology is provided. Methods Single-site data on consumption of i.v. tobramycin over an eight-month evaluation period were collected using commercial software that tabulates barcode-assisted medication administration (BCMA) events; administered doses were standardized as 1200-mg “vial-equivalents” and summed over the review period. The total number of vial-equivalents consumed was divided by the number of “non-zero weeks of consumption” (i.e., weeks during which any tobramycin use occurred) to obtain a mean ± S.D. weekly consumption rate; this rate was multiplied by the total i.v. tobramycin on-hand supply (in vial-equivalents) to determine the mean number of potentially sustainable weeks of therapy in the event a shortage were to restrict the future supply of the drug. Results Overall, 99.6 vial-equivalents of i.v. tobramycin were used during the evaluation period. The mean ± S.D. number of vial-equivalents used per non-zero week of consumption was 3.11 ± 1.26. A manual count of pharmacy inventory revealed that 102.9 vial-equivalents were available at the time of analysis. The mean predicted duration of supply was 33 weeks (95% confidence interval, –126 to 192 weeks). Conclusion Available BCMA data on tobramycin consumption over eight months were used to calculate the mean number of weeks the on-hand supply of the drug could be expected to last during a persistent drug shortage.