Cultures for Cellulitis

ABSTRACT & COMMENTARY

Synopsis: The method of obtaining a sample for culture from a focus of soft tissue infection was studied. Direct negative pressure aspiration resulted in significantly higher total protein recovery compared to the injection/aspiration method of first injecting 0.2 mL of sterile phosphate-buffered saline into the site followed by immediate aspiration. The direct aspiration method, without the injection of saline, yields higher amounts of aspirate material for culture.

Source: Traylor KK, Todd JK. Needle aspirate culture method in soft tissue infections: Injection of saline vs. direct aspiration. Pediatr Infect Dis J 1998;17:840-841.

A bovine animal model of soft tissue infection was used to study aspiration through a skin nick with a 20-gauge needle attached to a syringe. Each of 12 pairs consisting of one sample taken by direct negative pressure aspiration and one sample by aspiration following injection of 0.2 mL of sterile phosphate-buffered saline was studied for total protein recovered. The difference was significantly higher (P = 0.019) for the direct aspiration method.

Comment by Hal B. Jenson, MD, FAAP

The best technique to culture cellulitis and soft tissue infections has been controversial. This study showed that direct aspiration, compared to injection/aspiration followed by aspiration, yields a greater amount of total tissue protein for culture. This is supported by a review of 13 published reports of the direct or injection methods that found a significantly higher recovery of bacteria with direct aspiration (75 of 160, 47%) than with the injection/aspiration method (77 of 403, 19%) (P < 0.001). This may be because the core material cut during needle aspiration is not subsequently ejected. One study has shown a higher rate of positive bacterial culture results with aspiration from the point of maximal inflammation than with aspiration from the leading edge,1 although two subsequent studies have found both sites to be equivalent.2,3

Cultures of cellulitis are not always necessary. It is difficult for me to justify cultures of cellulitis on the face where even minimal scarring is undesirable, especially in patients prone to keloid formation. Even cases of periorbital cellulitis associated with minor trauma (e.g., insect bites, scratches) are almost always caused by Staphylococcus and/or Streptococcus rather than Haemophilus influenzae type b. This would be even more likely since the implementation of conjugate H. influenzae type b vaccines. Bringing a needle close to the eye in an uncooperative or anxious child for culture of periorbital cellulitis causes even more anxiety in me.

Another issue for children with cellulitis is the value of blood cultures. In immunocompetent children with uncomplicated cellulitis, blood cultures are almost always negative. In a cohort of 243 children with cellulitis, blood cultures were negative except in three children with varicella (who each grew group A-hemolytic streptococci) and two children with septic arthritis (one with Streptococcus pneumonia and one with Staphylococcus aureus).4 It is unnecessary to obtain a blood culture in an immunocompetent child with uncomplicated cellulitis (acute onset of cellulitis associated with a break in the skin). (Please realize that this is hard for an infectious disease specialist to say, where the dictum is usually to culture early and often.) The appearance of cellulitis without a break in the skin as a portal of entry should suggest the possibility of extension of underlying infection (e.g., osteomyelitis or septic arthritis) and the need for further evaluation before beginning empiric treatment.

Cultures of tissues and blood from immunocompetent patients with uncomplicated cellulitis without unusual exposure to soil or fresh- or saltwater are generally unnecessary prior to the initiation of empiric antibiotic therapy for staphylococcal and streptococcal bacteria. When a culture of soft tissue or cellulitis is desired, this study supports the use of simple direct aspiration for optimal recovery of material for bacterial culture. Cultures of tissues and blood of patients with cellulitis are indicated for immunocompromised persons, those with development or progression of inflammation while on antibiotics, or with unusual exposures such as soil, fresh- or saltwater, or animal feces or products.

References

1. Howe PM, et al. Etiologic diagnosis of cellulitis: Comparison of aspirates obtained from the leading edge and the point of maximal inflammation. Pediatr Infect Dis J 1987;6:685-686.

2. Newell PM, Norden CW. Value of needle aspiration in bacteriologic diagnosis of cellulitis in adults. J Clin Microbiol 1988;26:401-404.

3. Epperly TD. The value of needle aspiration in the management of cellulitis. J Fam Pract 1986;23: 337-340.

4. Sadow KB, Chamberlain JM. Blood cultures in the evaluation of children with cellulitis. Pediatrics 1998;101(3):e4. URL: http://www.pediatrics.org/cgi/content/full/101/3/e4.