[Refinement of presumptive antimicrobial therapy based on initial microbiological information on positive blood...

Positive blood culture represents either true bacteremia or contaminants of the normal skin flora. The number of positive bottles, rapidity with which blood culture turns positive, and appropriate interpretation of Gram-stain findings usually assist physicians or technologists in deciding whether it reflects true-positive results or contamination. In the case of true bacteremia, two aspects of the Gram-stain findings, Gram-positive or negative, cocci or rod, are important initial findings that safely guide physicians to select appropriate antimicrobial agents. Gram-positive cocci in clusters strongly suggest Staphylococci, and "in-chains" indicates Streptococci or Enterococci. Although distinction between the latter two organisms is occasionally difficult, glycopeptide should be the first choice, especially in critically ill patients. Gram-positive rods, when first reported, also require the empiric administration of glycopeptides, and sometimes their false Gram-negative staining could result in errors of pathogen identification, resulting in the inappropriate choice of antibiotics. The detection of gas production by Gram-negative rods, which indicates Enterobacteriaceae, is helpful initial information to start cephalosporin antibiotics, whereas the absence of gas would suggest nonfirmentative rod bacteremia, for which the administration of anti-pseudomonal agents is strongly warranted. Gram-negative cocci, such as Moraxella or Acinetobacter sp., may initially be reported as Gram-positive, so empiric antimicrobial drugs should be carefully selected taking into account these pitfalls and patients' conditions, and the situation regarding the development of diseases (community-acquired vs. nosocomial). The rapid and appropriate treatment of bacteremia thus requires careful interepretation of Gram-stain findings as described above, and should always be integrated with pathognomonic features of individual patients.