In this regard, please refer the Culture-Negative Endocarditis section of this statement and the related Table 6 for additional details. Results of clinical efficacy studies support the use of most treatment regimens described in these guidelines. Other recommendations listed in this section are based largely on in vitro data and consensus opinion and include the following management considerations. It is reasonable for the counting of days for the duration of therapy to begin on the first day on which blood cultures are negative in cases in which blood cultures were initially positive. It is reasonable to obtain 2 sets of blood cultures every 24 to 48 hours until bloodstream infection is cleared. However, if a patient undergoes valve surgery and the resected valve tissue is culture positive or a perivalvular abscess is found, then an entire course of antimicrobial therapy is reasonable after valve surgery. If the resected tissue is culture negative, then it may be reasonable for the duration of postoperative treatment given less the number of days of treatment admin- istered for native valve infection before valve replacement. This, however, has been challenged by retrospectively col- lected data from 2 different medical centers116,117 that sug- gest that 2 weeks of antibiotic therapy may be sufficient in patients who undergo valve surgery and have negative valve tissue cultures, particularly in IE cases caused by VGS or Streptococcus gallolyticus (bovis). 