The need for predicting bacteriological and clinical antibiotic activity appeared early after development of the first antibiotics. Antimicrobial efficacy is significant among many therapeutic management factors, including surgery, drug interactions and non-antibiotic therapies. Clinical efficacy is not only related to antimicrobial efficacy, but also to host characteristics and causative organism attributes. Because all these factors interact to achieve success or failure in the management of a given infected patient, to model an n-factor in vivo system from n'-factors in vitro may be difficult.