What The Evidence Reveals About Wound Cultures

Should you use deep tissue cultures or swab cultures in your evaluation of wounds? These authors review the literature on wound cultures and explore the early emergence of more advanced technologies to detect wound infection.

Diabetes mellitus is the most common metabolic disease worldwide and 2013 estimates show that diabetes affects 382 million people with type 2 diabetes accounting for 90 percent of the cases. This is equal to 8.3 percent of the population with an equal incidence between men and women.¹ Foot ulceration occurs in 15 to 25 percent of patients with diabetes in their lifetime. It is the most common cause for lower limb amputation and hospitalization for those with diabetes.² Readmission rates for those with diabetic foot infections are approximately 40 percent and nearly one in six patients die within one year of their infection.³

Aerobic Gram-positive cocci (especially Staph aureus) are the predominant pathogens in diabetic foot infections.⁴ Gram-negative rods may also be a factor for patients with infected chronic wounds or those who have had recent antibiotic therapy. The wounds of patients with ischemic limbs or those with associated gangrenous changes may also include some anaerobic pathogens.⁴ The Infectious Diseases Society of America recommends routine culture of all diabetic foot infections before initiating empiric antibiotic therapy.⁴

Many clinicians often obtain cultures from the surface of wound beds as an initial step in the wound evaluation process. This may allow for some appropriate antibiotic treatment in the initial phase of care. Some studies, particularly a 2012 study by Spears, suggest that surface wound cultures are not ideal for defining the etiology of acute wound infections and that deep tissue cultures should serve as the gold standard for directing therapy.⁵ Conversely, there are also studies that indicate swab cultures are valuable for identifying pathogens when circumstances prevent or delay debridement and deep tissue cultures.⁶

The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) continues to rise and is associated with slower ulcer healing and an increased risk of lower extremity amputation. Methicillin-resistant Staphylococcus aureus is often associated with previous hospitalizations and the use of recent antibiotics. Broader coverage to include MRSA is often the first choice empirically when doctors initially face diabetic foot ulcerations in the clinical and hospital setting.^2,7 Diabetic foot ulcers are often complicated by infection. Among pathogens, Staphylococcus aureus predominates with a prevalence of 79 percent in diabetic foot ulcers, according to a study by Eleftheriadou and colleagues.2 In 2010, Dang and colleagues estimated the prevalence of MRSA in infected foot ulcers at 15 to 30 percent with “an alarming trend for increase in many countries.”⁷

It is important to keep in mind that wound culture results and sensitivities may take up to 24 to 48 hours, and may not always identify pathogens for many different reasons. New techniques for rapid identification of causative pathogens include the polymerase chain reaction (PCR) assay for organisms and the oligonucleotide assay for virulent gene identification.⁸

A Closer Look At The Research On Wound Cultures
Regardless of the many studies that weigh in on whether swab cultures are beneficial or not, it seems rational that swabbing wounds when one initially encounters them can only help to begin the infectious treatment process. Accordingly, let us take a closer look at the literature both against and in favor of using culture swabs, the importance of MRSA coverage and the emergence of new technology utilizing DNA assays for deciphering colonization versus infection.

A study by Gardner and colleagues examined associations between ulcer bioburden and ulcer outcomes in uninfected neuropathic DFUs in 77 patients.⁹ The authors found that culturing DFUs with no clinical signs of infection had no predictive value for outcomes of DFUs managed with total contact casts and routine debridement. The study authors note their findings support Infectious Diseases Society of America recommendations that one should avoid culturing and antibiotics for DFUs that show no clinical signs of infection.⁴

In 2010, in the Journal of Hospital Medicine, researchers out of Tulane University and George Washington University published a systematic review of articles between January 1960 and August 2009 that involved superficial wound cultures and deep tissue cultures.⁶ Two independent reviewers performed this data extraction and synthesis, and used a random effects meta-analysis model. Out of 9,032 unique citations, eight total studies with 615 patients or samples met all inclusion criteria. Despite some inherent limitations including prior antibiotics, culturing methods, technology in labs and organism classifications, the results, using a meta-analytic approach, showed superficial wound cultures provided mediocre sensitivity (49 percent) and specificity (62 percent). This systematic review does not support the use of culture swabs for guiding initial antibiotic therapy.⁶

However, a 2004 study by Slater and colleagues suggested that swab cultures before a debridement are valuable in the identification of pathogens in diabetic foot ulcers.¹⁰ This study focused on a total of 60 wounds from 56 patients and found that 62 percent of the wounds had identical superficial and deep tissue culture results. The researchers discovered that both swab cultures and deep tissue cultures identified the same organisms and bacterial yield for many wounds not involving bone.

However, as the depth of the wound extended to the level of bone, the study authors found that the percentage of swab cultures identifying microorganisms decreased.10 In regard to Wagner grade 1 and 2 ulcers, swab cultures identified all pathogens in the corresponding deep tissue specimen 90 percent of the time. However, this was true for only 65 percent of Wagner grade 3 ulcers. The study authors argued that clinicians can confidently select appropriate antibiotic therapy for infected diabetic foot ulcers not involving bone based on swab culture results with under-treatment occurring in 8 percent of cases and over-treatment occurring in 13 percent of the cases.

In a prospective follow-up study by Pellizzer and colleagues out of Italy and published in 2001 in Diabetic Medicine, the authors found equality among swabbing and deep tissue cultures for initial antimicrobial treatment in severe diabetic foot infections.¹¹ The study included a total of 29 patients with Wagner grade 3 or 4 ulcers and associated clinical and/or radiological signs of osteomyelitis. The number of bacterial isolates yielded originally from swabbing and deep tissue sampling were 68 and 60 respectively. Swab cultures resulted in a 2.34 mean number of species per patient and deep tissue cultures uncovered a mean of 2.07 species per patient. The species were also equally represented for the most part up until day 14. At day 30, deep tissue sampling found positive cultures in 18 out of the 21 ongoing infected ulcers in comparison to just 11 out of 21 with swabbing.

Accordingly, these study authors found that in determining an empiric antibiotic treatment course, swab cultures and deep tissue cultures had similar results. However, for wounds that are still active 30 days after treatment, the authors suggested that deep tissue cultures provided more sensitivity.¹¹

Demetriou and colleagues evaluated the diagnostic performance of swab cultures versus tissue cultures.¹² In 28 patients with diabetic neuropathy, the authors found that swab cultures had 100 percent sensitivity, 40 percent specificity, an 88.5 percent positive predictive value and 100 percent negative predictive value for the diagnosis of infection. In 22 patients with diabetic neuroischemic foot ulcers, the study authors noted that swab cultures had 100 percent sensitivity, 22 percent specificity, a 65 percent positive predictive value and a 100 percent negative predictive value for the diagnosis of infection. The authors concluded that swab cultures are highly sensitive but not as specific, and have an excellent negative predictive value both for patients with diabetic neuropathy and neuroischemic foot ulcers.

Why There Is A Need For Faster Evaluation Of Wound Infection
As we noted earlier, researchers have found MRSA present in 15 to 30 percent of infected diabetic foot ulcers.¹⁰ Furthermore, Eleftheriadou and colleagues noted that MRSA is associated with an increased need for surgical debridement and amputation.² Risk factors for MRSA include prior antibiotic use and hospitalization, larger ulcers with chronic components, osteomyelitis and nasal carriage colonization. Physicians often cover MRSA with empiric therapy for wounds at risk but antibiotics may not always be indicated. Additionally, these drugs can be quite costly, potentially cause adverse drug reactions and may lead to the formation of drug-resistant bacteria.

Faster identification of specific organisms with new rapid diagnostic methods including the polymerase chain reaction assay as well as the oligonucleotide assay may be the wave of the future with process times within one day.

In a 2007 prospective study published in Diabetes Care, Sotto and colleagues reported that DNA assays may help distinguish colonized Wagner grade 1 ulcers from infected grade 2 wounds as well as predict ulcer outcome and help with narrowing antibiotic coverage for Staph aureus.¹³ The study included a total of 72 patients with 38 patients having their first ulceration. Researchers used the oligonucleotide array for validation of mecA genes and virulence genes. All the study patients had Staph aureus as the sole pathogen and had not received antibiotic coverage within the last six months. In this study, researchers were able to determine the genus, species and antibiotic susceptibilities, and classified wounds as antibiotic sensitive or intermediately resistant.

The study results demonstrated that 20 of the 22 grade 1 ulcers had no evidence of virulence genes present.¹³ Among the patients with grade 1 ulcers, 13 (59 percent) had persistent colonization with Staph aureus on follow-up cultures. Of the two ulcers that healed, virulence factors were not present and the opposite was true of those that did not heal. When it came to patients with clinically uninfected ulcers, they did not receive antibiotics and only 36 percent healed their ulcer. Forty-nine of 50 grade 2 to 4 ulcers had virulence genes present with the enterotoxin A gene present in severe grade 3 and grade 4 ulcers. Of the 72 total patient wounds, 35 were methicillin resistant and 37 were methicillin sensitive. The MRSA isolates were present in almost half the patients with the majority as recurrent wounds that were colonized and unlikely to heal.

This data suggests that the identification of virulence genes can be an indicator for infected wounds and help predict an unfavorable clinical result. This rapid genotype identification technology can help identify pathogens and susceptibilities. Indeed, this technology may help reduce the exposure of patients to unnecessary antibiotics and accordingly help decrease the development of antibiotic resistance.

In Conclusion
There is still an ongoing debate of whether superficial swabs or deep tissue cultures are more beneficial for organism identification and consequently diabetic limb salvage. Perhaps newer technologies such as DNA assay can help to eliminate some of the ambiguity. Identification of these complex multi-drug resistant organisms will help us treat and eradicate limb-threatening ulcers and associated infections. This will ultimately lead to longer and more active lives for patients while eliminating the perpetuated resistance developed by these organisms.

Dr. Pupp is the Program Director of the Podiatric Residency at Providence Hospital and Medical Center in Southfield, Mich. He is a Fellow of the American College of Foot and Ankle Surgeons, and is board certified in foot and ankle surgery by the American Board of Podiatric Surgery. He is the Chairman of the Board for the Save A Leg, Save A Life Foundation.

Dr. Black is a resident at Providence Hospital in Southfield, Mich.

References

1.     Shi Y, Hu FB. The global implications of diabetes and cancer. Lancet. 2014; 383(9933):1947-8.
2.     Eleftheriadou I, Tentolouris N, Argiana V, Jude E, Boulton A. Methicillin-resistant Staphylococcus aureus in diabetic foot infections. Drugs. 2010; 70(14):1785-97.
3.     Frykberg RG, Wittmayer B, Zgonis T. Surgical management of diabetic foot infections and osteomyelitis. Clin Podiar Med Surg. 2007; 24(3):469-82.
4.     Lipsky BA, Berendt, AR, Cornia PB, et al. 2012 Infectious Diseases Society of America Clinical Practice Guideline for the Diagnosis and Treatment of Diabetic Foot Infections. Clin Infect Dis. 2012; 54(12):132-173.
5.     Spears M. Best technique for obtaining wound cultures. Plast Surg Nurs. 2012; 32(1):34-36.
6.     Chakraborti C, Le C, Yanofsky A. Sensitivity of superficial cultures in lower extremity wounds. J Hosp Med. 2010; 5(7):415-20.
7.     Dang CN, Prasad YDM, Boulton AJM, Jude EB. Methicillin-resistant Staphylococcus aureus in the diabetic foot clinic: a worsening problem. Diabetic Med. 2003; 20(2):159-61.
8.     Lipsky BA. New developments in diagnosing and treating diabetic foot infections. Diabetes Metab Res Rev. 2008; 24(Suppl 1):S66-71.
9.     Gardner SE, Haleem A, Jao YL, et al. Cultures of diabetic foot ulcers without clinical signs of infection do not predict outcomes. Diabetes Care. 2014; 37(10):2693-2701.
10.     Slater RA, Lazarovitch T, Boldur I, Ramot Y, Buchs A, Weiss M, Hindi A, Rapoport MJ. Swab cultures accurately identify bacterial pathogens in diabetic foot wounds not involving bone. Diabetic Med. 2004; 21(7):705-09.
11.     Pellizzer G, Strazzabosco M, Presi S, Furlan F, Lora L, Benedetti P, Bonato M, Erle G, de Lalla F. Deep tissue biopsy vs. superficial swab culture monitoring in the microbiologic assessment of limb-threatening diabetic foot infection. Diabetic Med. 2001; 18(10):822-27.
12.     Demetriou M, Papanas N, Panopoulou M, et al. Tissue and swab culture in diabetic foot infections: neuropathic versus neuroischemic ulcers. Int J Low Extrem Wounds. 2013; 12(2):87-93.
13.     Sotto A, Richard JL, Jourdan N, Combescure C, Bouziges N, Lavigne JP. Miniaturized oligoonucleotide assays. Diabetes Care. 2007; 30:2051-6.
14.     Lipsky BA. Diabetic foot infections: microbiology made modern? Diabetes Care. 2007; 30(8):2171-2.

Editor’s note: For further reading, see “How To Differentiate Between Infected Wounds And Colonized Wounds” in the July 2005 issue of Podiatry Today.