Evaluation of disk approximation and agar dilution induction tests for demonstration of in vitro antagonism of cefotaxime by cefoxitin in Enterobacter species.

Cefoxitin-resistant clinical isolates of Enterobacter spp. demonstrated inducible beta-lactamase activity in 31 of 47 (65.9%) disk approximation tests and in 32 of 52 (63.5%) agar dilution induction tests. Agreement between the two results was only 72.3%, and 80.9% of the strains were positive in at least one induction test. Lack of valid interpretive criteria preclude their routine use in the clinical laboratory.