Clinical considerations and costs associated with formulary conversion from tobramycin to gentamicin

The clinical and financial effects of replacing tobramycin with gentamicin on the formulary of a 550-bed teaching hospital were studied. On the recommendation of the pharmacy and therapeutics committee, the formulary aminoglycoside was changed from tobramycin to gentamicin in June 1985; the nonformulary status of amikacin was unchanged. Five weeks later, physician compliance was assessed and the reasons for prescribing nonformulary aminoglycosides were determined. Two four-month-long evaluations were done at 6 and 18 months after implementation to assess patterns of use of nonformulary aminoglycosides. The impact on costs was determined after one and two years by considering use patterns of formulary and nonformulary aminoglycosides, as well as those of third-generation cephalosporins and mezlocillin. Resistance patterns of two gram-negative organisms, Pseudomonas aeruginosa and Serratia marcescens, were assessed for 1982-1987. Finally, the rate of nephrotoxicity in gentamicin-treated patients was determined. During the first five weeks after the formulary conversion, 80.3% (106 of 132) of the aminoglycoside orders received were for gentamicin. After telephone follow-up by the pharmacy department, that figure rose to 93.9%. During the four-month reviews beginning at 6 and 18 months, nonformulary orders accounted for 10.9% and 7.4%, respectively, of the total number of courses of aminoglycosides prescribed. In the majority of these cases, tobramycin and amikacin were used to treat infections caused by organisms with documented resistance to gentamicin or to gentamicin and tobramycin, respectively. No clear-cut changes in resistance patterns for Ps. aeruginosa or S. marcescens could be associated with the formulary conversion.(ABSTRACT TRUNCATED AT 250 WORDS)