Predictive value of susceptibility tests for the outcome of antibacterial therapy

A total of 510 charts of patients who received antibacterial agents were examined for clinical outcome and microbiology findings. A total of 382 patients (75%) had one or more specimens submitted for culture and susceptibility tests before the administration of the drugs; 298 (78%) of these had positive cultures and susceptibility tests were done. A total of 18 species were isolated. Of the 298 patients with organisms of known susceptibilities, 271 (91%) received antibacterial agents to which the respective organisms were susceptible and 219 of these patients (81%) improved (P less than 0.05). This high rate of good infectious diseases practice is probably due to two factors: (1) susceptibility tests results were available in most cases the next day after the submission of a specimen; (2) the medical board distributed guidelines for the use of antibiotics and monitored the compliance closely. The patients treated with antibacterial agents to which the bacteria were resistant improved in 3% and did not improve in 82% (P less than 0.05) of the patients. This study shows that choosing an antibacterial agent in accordance to the susceptibility test resulted in a high rate of improvement. When the choice of agent disregarded bacterial resistance in vitro, therapy almost always ended in failure. Therefore, susceptibility tests in vitro have a good predictive value for the outcome of antibacterial therapy.