The treatment guidelines are generally decided on the basis of either percent resistant (%R) or percent sensitive (%S) bacterial population tested with a given antimicrobial that vary geographically and represent only a part of total bacterial population existing in response to the antimicrobial used. The isolates with intermediate sensitivity (%I) are either not reported or clubbed with resistant isolates though the two may differ in clinical response. Sensitivity Index (SI) of an antimicrobial is sensitive to change in any of the three co-existing bacterial population and may be a better criterion for rational use of antimicrobial.