Are blood cultures necessary in adults with cellulitis?

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by Jose Vega
You’re leaving the room of a patient with uncomplicated cellulitis.  You’re just about to order blood cultures when you realize all the nurses are busy.
At that moment an email from your department chair concerning cutting costs and improved efficiency begins to resonate in your head.  If you order the cultures, it’s going to tie up a room for half an hour until someone can draw them.  So do you really need them?  Will they change your management?

Review by Jose Vega, MD
Column Organized by Evan Schwarz, MD
Division of Emergency Medicine
Washington Univeristy
Mills AM. Chen EH.  Are Blood Cultures Necessary in Adults with Cellulitis?  Ananls of Emergency Mediicne May 2005; 45 (5) 548-549.
Cellulitis is a common soft tissue infection that extends into the dermis and subcutaneous tissues.  Microorganisms, typically gram-positive bacteria, invade disrupted skin leading to clinically apparent infection.  The hallmark physical findings include pain, redness, swelling, warmth and lymphangitis if the infection is within the lymphatic system.  Systemic antibiotic therapy is routinely used to treat cellulitis. However, identifying a specific pathogen is often not possible on clinical grounds. Therefore, antibiotic therapy with activity against group A Streptococcus and Staphylococcus aureus is usually selected empirically.  Ideally, blood culture susceptibility would direct antimicrobial therapy.  Mills and Chen reviewed 5 original published research articles that specifically addressed the utility of blood cultures in adult cellulitis. 
Perl et al performed a retrospective study of 757 patients that were admitted for community acquired cellulitis, of which 553 had blood cultures performed.  A specific microbial strain was isolated in 11 cases and 9 cases grew gram-positive organisms and were already treated with appropriate antimicrobial therapy.  The other 2 cases had risk factors for a more complicated infection such as an indwelling catheter.  Contaminant organisms were seen in 20 cases.  Hook et al conducted a prospective study that evaluated the diagnostic value of cultures from primary site infections, aspirates from the advancing edge erythema, skin biopsy specimens and blood cultures from 50 adults with cellulitis.  Blood cultures were positive in 2 patients, Group A streptococci was isolated from one and staph from the other.  Bottom Line:  blood cultures are of little value in determining the microbial origin of acute cellulitis and no patient required change in antimicrobial therapy based on culture results in either study.
Kulthanan et al performed a retrospective study of 150 patient charts that were admitted for adult cellulitis.  Blood cultures were obtained from 116 patients and compared between immunocompetent and immunocompromised patients.  Blood cultures were positive in 3 immunocompetent patients and 17 immunocompromised patients; however, no statistical significance was shown between the two groups.  The organisms isolated in the immunocompetent group were gram-positive, while in the immunocompromised group only half of the organisms were gram-positive.  Ho et al performed a retrospective study of blood and wound cultures obtained from healthy adult patients admitted for acute cellulitis.  Of the 110 patients that were included, 66 had blood cultures and only 1 set of blood cultures was positive (Group A streptococcus).  Bottom Line:  both studies concluded that blood cultures are not necessary in acute adult cellulitis without serious underlying disease and no patient required a change in antimicrobial therapy based on culture results in either study.
Lutomski et al enrolled 25 nondiabetic adult patients with cellulitis.  All patients had 2 sets of blood cultures drawn before the initiation of antimicrobial therapy.  Blood cultures were positive in four and all contained gram-positive organisms.  The authors concluded that empiric therapy with penicillinase resisitant penicillins or 1st generation cephalosporins are adequate coverage for gram-positive organisms.  No mention was made if the culture results changed the initial antimicrobial therapy.  Of note concerning the antibiotic recommendations, three of the five studies were published before 1990 and all were published before the year 2000.  The antibiotic recommendations predate wide-spread community associated MRSA and are probably no longer adequate.
The LLSA article suggests that blood cultures rarely alter the treatment in the management of cellulitis in the normal host.  Furthermore, the Perl et al study demonstrated that contaminated blood cultures (false positives) were twice as likely as true positives.  These findings could lead to unnecessary tests and increased costs.  All five studies demonstrated that empiric treatment of uncomplicated cellulitis in the normal host should be directed against Group A streptococci and Staph aureus.  Blood cultures are of little value in determining the microbial origin of acute cellulitis in immunocompetent patients, unless the clinician suspects exposure to unusual organisms or potentially complicated cellulitis.

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