Reliability of a disk diffusion method using semiconfluent growth in the determination of aminoglycoside resistance

Susceptibility of 553 blood culture isolates to gentamicin, tobramycin, amikacin, and netilmicin was determined by a routine disk diffusion method with semiconfluent growth (AB Biodisk, Solna, Sweden) and a microtiter-method (Sensititre). The disk diffusion test gave false sensitive results in 9.4-71% of the 9-39 aminoglycoside resistant strains studied. False resistant results occurred in 1.0-7.6% of measurements among the over 500 aminoglycoside sensitive strains. Because of the frequent error rate, the disk method with semiconfluent growth should be replaced with other methods in the determination of aminoglycoside resistance in bacteriological laboratories.