Effect of drug interactions on outcomes of patients receiving warfarin or theophylline.

The effect of drug interactions on costs and other outcomes for hospitalized patients receiving warfarin or theophylline was studied. Data were collected from medical records during a one-year period in two community teaching hospitals in Maryland. The data included demographic information, the duration of study-drug therapy, the number of days spent in the intensive care unit, the length of stay (LOS) in the hospital, the number of prothrombin-time tests and serum theophylline assays, and the test results. A multiple-regression procedure was used to compare outcomes of patients who were prescribed specific drugs interacting with warfarin or theophylline with those of patients who were not. Among warfarin-treated patients, there was a significant difference in each outcome measure (LOS, number of laboratory tests, and test results) between those who received an interacting drug and those who did not; mean LOS was 3.14 days longer in patients given an interacting drug. No significant differences in outcome measures were observed between the two groups of theophylline-treated patients. The cost of the increased LOS attributed to the presence of a drug interacting with warfarin was estimated to range from $779 to $1005 per hospitalization. The cost of additional prothrombin-time tests was estimated at $19-$50. Patients who received warfarin and an interacting drug had an increased LOS, required more laboratory tests, and had longer prothrombin times than patients given warfarin alone; these differences probably led to higher costs.