Laboratory Studies
Diagnosis of wound infection should focus on a careful physical examination that is performed frequently by personnel trained in the management of burns.
Laboratory tests or changes in laboratory values such as white blood cell (WBC) count, neutrophil percentage, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) level are of low yield in detecting or predicting burn infections because of the inflammatory response associated with the burn itself. [25]
Laboratory examinations are useful for the initial risk assessment. Low prealbumin levels (100-150 mg/L) in burned patients are associated with a higher incidence of sepsis and organ dysfunction, lengthier stays, decreased ability of wound healing, and a higher mortality rate. [9, 26, 27]
In patients with suspected wound infections, procalcitonin (PCT) levels of 0.56 ng/mL have a reported sensitivity of 75% and a specificity of 80% when compared with quantitative swab culture. [28] Although these levels cannot be considered diagnostic, they should prompt the physician to start searching for an infectious source. In the only study that has compared PCT to American Burn Association (ABA) criteria for diagnosing sepsis, PCT was found to have potential utility in diagnosing sepsis in the burned patient. A threshold of 1.5 ng/mL had a sensitivity of 88% and specificity of 92% to diagnose septic complications in burned patients, with high levels found to be an independent predictor of mortality. [29] Without strong evidence to support a PCT threshold in burned patients, one recommendation is that PCT be obtained at admission and then monitored routinely to detect acute changes to guide the start of empirical antibiotic therapy. [28] PCT may also have a role in monitoring response to antimicrobial therapy, as appropriate antimicrobial therapy is associated with a reduction in PCT levels by the third day of treatment. In contrast, patients who died of multiple organ failure secondary to sepsis had PCT levels that remained elevated despite treatment. [28]
Diagnosis of a burn wound infection relies on clinical examination as outlined above (see Clinical) and culture data, including the following [30] :
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Quantitative biopsy can be used to confirm infection but is not reliable. This procedure is useful in identifying the infecting pathogen.
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Quantitative swab is of limited value but may aid in identifying the infecting pathogen.
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Tissue histopathology allows for quantification and evaluation of infection depth and involvement of non-burned skin.
The use of routine wound cultures as part of surveillance procedures has been proposed to provide early identification of organisms colonizing the wound, to monitor response to therapy, to guide empiric therapy, and to evaluate for nosocomial transmission. However, this has not been shown to improve patient outcomes, and routine application has been brought into question.
Ono et al (2015) demonstrated that, among cases of local infection with Staphylococcus species, the burn wound exudate pH rises prior to the onset of clinical signs of local infection. [31] Based off this discovery, Nischwitz et al (2019) engineered an advanced wound dressing capable of monitoring incremental changes in pH and, thereby, tracking appropriate wound healing. [32]
Imaging Studies
No imaging studies have been identified as useful for detecting wound infections.
Procedures
Multiple biopsy samples from several areas of the burn wound should be obtained and sent for histopathology and microbiological workup of the pathogens and their resistance profiles.
After cleaning the wound with isopropyl alcohol, 2 parallel incisions 1-2 cm in length and 1.5 cm apart with a depth to obtain a portion of the underlying fat are made in the skin. Alternatively, biopsy samples typically weighing 0.02-0.5 g may be obtained with a 3-mm punch-biopsy technique.
Biopsy is a commonly bypassed procedure because of technical difficulty within the microbiology section working up these types of samples or a lack of local histopathological expertise.
Histologic Findings
Bacteria are most commonly detected using Gram or hematoxylin and eosin (H&E) stains.
Fungi may be visualized on periodic acid-Schiff (PAS) or Gomori methenamine silver (GMS) staining, but recovery in culture is required for definitive identification. [13] Note the following:
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Aspergillus-like morphology - Presence of parallel-walled, branching, septate hyphae may indicate Aspergillus species, Fusarium species, phaeohyphomycetes, or other molds
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Mucor-like morphology (mucormycosis) - Presence of wide, ribbonlike, rarely septate hyphae most commonly denotes Mucorales (eg, Rhizopus, Mucor, Rhizomucor, Absidia)
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Yeast-like morphology - Presence of budding yeasts or rounded yeast-like structures; many yeasts, including most Candida species, also produce hyphae and pseudohyphae in tissue
Herpes simplex virus can be isolated via identification of inclusions on light or electron microscopy or other viral particles on biopsy specimen or lesion scraping.
Staging
Various stages are used to diagnose burn wound infections. [3]
Stage I is colonization and includes the following substages:
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Superficial - Microorganisms present only on burn wound surface
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Penetrating - Variable depth of microbial penetration of eschar
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Proliferating - Variable level of microbial proliferation at nonviable–viable tissue interface (subeschar space)
Stage II is invasion and includes the following substages:
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Microinvasion - Microorganisms present in viable tissue immediately subjacent to subeschar space
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Deep invasion - Penetration of microorganisms to variable depth and expanse within viable subcutaneous tissue
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Microvascular involvement - Microorganisms within small blood vessels and lymphatics (thrombosis of vessels is common)
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Second-degree burns often are red, wet, and very painful. Their depth, ability to heal, and tendency to result in hypertrophic scar formation vary enormously.
