Effect of previous antimicrobial therapy on blood culture results
Whenever possible, take blood samples before starting antimicrobial treatment to avoid inhibiting culture growth of pathogens that are sensitive to the antimicrobial. Many cases of culture-negative endocarditis are due to prior antibiotic therapy and are not truly ‘culture-negative’.
In patients with sepsis or septic shock, the key priority is to obtain blood cultures as soon as possible, regardless of the timing of recent antibiotic administration. Whereas, if a patient is currently receiving antimicrobial therapy and is not unwell or deteriorating, blood samples should be collected just before the next antimicrobial dose is due (ie when antimicrobial concentrations are likely to be lowest).