Problems arising in antimicrobial therapy due to false susceptibility testing.

In vitro antibiotic susceptibility testing provides clinicians with guidelines to assess microbial sensitivities to various antibiotics. However, susceptibility testing is a laboratory procedure which does not always reflect the in vivo situation. False susceptibility testing is often due to methodological/disk concentration problems. In vitro versus in vivo discrepancies may also occur because of problems with varying pH/antibiotic concentration at the infection site. Clinicians should use susceptibility testing as a guide, but must rely upon the patient's clinical response as the ultimate in vivo susceptibility test.