Burn wound infection: diagnosis, colonization versus invasive infection, and pathology

Overview¶

Infection is the leading cause of death after the patient survives the initial resuscitation. Most patients with severe burns who die do so from septic problems ^[25]. The burn wound itself is the central battleground. Severely burned skin loses its protective role and becomes a nidus and portal for bacterial invasion ^[23], and the systemic inflammatory state of a major burn makes the usual indicators of infection both insensitive and nonspecific ^[6].

The diagnostic problem is not detecting organisms. Every open burn wound becomes colonized. The problem is deciding when colonization has crossed into invasion of viable tissue, which is what distinguishes a wound that will heal from one that is killing the patient ^[1][5]. That distinction drives the decision to excise, to start systemic antimicrobials, and to escalate care. Differentiating colonization from infection remains a major diagnostic challenge, and although tissue biopsy is the gold standard for confirming infection, it is not applied uniformly across centers ^[5].

Epidemiology¶

Burn wound infection has historically been the most common and most serious complication in burn units ^[12]. The organisms recovered from burn wounds have shifted over decades of changing wound care, but a consistent core dominates. Pseudomonas aeruginosa and Staphylococcus aureus are the organisms most commonly isolated from the bloodstream of burned children ^[29], and Pseudomonas aeruginosa has been reported as the predominant burn wound pathogen ^[12].

Surveys of infected burn wounds show a mix weighted toward gram-positive organisms. In one series of 420 isolates from infected wounds, 68.2% were gram-positive and 31.8% gram-negative, with Staphylococcus aureus accounting for 38.3% of all isolates, coagulase-negative staphylococci 16.7%, and Pseudomonas 10.7% ^[27]. Resistant organisms have become prominent drivers of invasive disease, including methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus, Pseudomonas, Acinetobacter, non-albicans Candida species, and Aspergillus ^[28].

The organisms causing bacteremia, and their species-specific effect on mortality, have changed over time in concert with changes in wound flora ^[26]. Fungi are a distinct and dangerous subgroup. Candida frequently colonizes the eschar without progressing; in one series of 172 patients with candidal eschar colonization, only 20.7% went on to develop invasive candidal sepsis ^[30]. The microbial pattern also changes over the hospital course, which is why the timing of a positive culture carries clinical meaning, not just its identity.

Pathophysiology¶

The burn destroys the cutaneous mechanical barrier and leaves nonviable avascular eschar, and that combination, layered on impaired host defenses, is what renders the burn patient susceptible to both local and systemic infection ^[22]. Thermal injury destroys the physical skin barrier that normally prevents microbial invasion ^[24], and burned skin shows reduced expression of endogenous antimicrobial peptides, further weakening local defense ^[23]. The burned patient also has impaired host defenses, the backdrop against which septic complications develop ^[25].

Invasion is a sequence, not an event. Histologic studies of experimental burn wound infection show surface colonization of nonviable tissue followed by progressive deeper growth and then invasion of underlying viable tissue ^[14][15]. Once organisms cross into viable, perfused tissue, the wound becomes a source of systemic seeding. The likelihood that this sequence completes depends on patient factors such as age, burn extent, and burn depth combined with microbial factors such as the type and number of organisms, enzyme and toxin production, and motility ^[1]. The type and quantity of organisms colonizing the wound appear to influence the future risk of invasive infection ^[9].

Fungal invasion follows the same logic. A large wound surface, impaired local immunity, and broad-spectrum antibiotic therapy support overgrowth of opportunists such as Candida albicans, which can then invade ^[31].

Classification¶

Burn wound infections can be classified by the causative organism, the depth of invasion, and the tissue response ^[1]. A widely used surveillance framework, the Peck definitions, separates impetigo, surgical-wound-related infection, cellulitis, and invasive infection of unexcised wounds ^[13]. Histologic depth scoring formalizes the depth-of-invasion axis: in the McManus scheme, biopsy specimens were scored on a scale of 1 to 6, with values of 4 or greater indicating microbial invasion of viable tissue ^[2].

The bacterial-density threshold provides a separate, microbiology-based classification. Wounds with 10^4 or fewer organisms per gram of tissue are conventionally considered colonized, whereas 10^5 or more organisms per gram indicates wound infection ^[3], and quantitative increases beyond 10^5 per gram have been used to represent infection and burn wound sepsis ^[4]. The clinical weight of this colonization classification has shifted with practice. With universal adoption of early excision and grafting, classification of colonization in unexcised wounds has become less relevant, moving clinical attention toward open burn-related surgical wound infections ^[11].

Assessment¶

Assessment answers one question: has colonization become invasion? No single clinical sign is sufficient, because the systemic inflammatory response of a major burn mimics infection ^[6]. Diagnosis therefore rests on the wound itself, evaluated by biopsy, supported by quantitative culture and systemic criteria.

Histologic biopsy as the reference standard¶

Histologic examination of a biopsy specimen, which permits staging of the invasive process, is the only reliable means of differentiating wound colonization from invasive infection ^[1]. This is the central diagnostic fact of the topic. Histology directly visualizes whether organisms have crossed into viable tissue, which is the definition of invasion; nothing else does.

Rapid histologic methods exist to shorten the delay. A combination of rapid manual histology, acridine orange fluorescent staining, and quantitative culture on biopsy specimens allows a therapeutic decision to be made within hours while definitive quantitative culture and sensitivity results, which take 24 to 48 hours, are pending ^[19].

Quantitative culture and tissue counts¶

Quantitative culture of biopsy tissue counts organisms per gram and has been the standard microbiologic adjunct ^[10]. The McManus comparison of quantitative microbiology against histopathology on divided biopsy specimens is the key study showing why culture alone is insufficient: agreement of 96.1% was found between negative cultures (fewer than 5 logs per gram) and histologic absence of invasion, but histologic invasion occurred in only 36% of culture-positive specimens ^[2]. Low tissue counts are essentially synonymous with negative histology, so a negative quantitative culture reliably rules invasion out; high tissue counts commonly do not indicate invasion, so quantitative microbiology is not a diagnostic substitute for histology ^[2].

Semiquantitative culture techniques approximate the full quantitative method at lower cost ^[10]. One comparison reported a positive predictive value of 100% and a negative predictive value of 93.7% for wound sepsis ^[10]. Surface swab cultures, by contrast, track wound flora, and although swab and biopsy counts correlate, the count from one method predicts the other poorly ^[35]; full-thickness biopsy culture identifies the critical tissue load beyond which metastatic invasion occurs, which surface swabs do not ^[21]. Adjuncts on the biopsy specimen itself add speed: a Gram stain of the biopsy homogenate correlates with quantitative culture, though a negative Gram stain still grew significant organisms a fraction of the time ^[20].

Systemic sepsis criteria¶

Because the burn wound is not always accessible or unambiguous, systemic criteria attempt to flag infection from the patient's physiology. The American Burn Association published diagnostic criteria in 2007 to standardize the definition of sepsis in burn patients ^[6]. These criteria include temperature thresholds (>39°C or <36°C), progressive tachycardia (>110 beats per minute), progressive tachypnea, thrombocytopenia (<100,000/µL, not applied until 3 days after resuscitation), hyperglycemia, and feeding intolerance; meeting more than three of these is intended to trigger concern for infection ^[6]. Their performance has been disappointing: the area under the receiver operating characteristic curve for meeting more than three criteria was only 0.638 against bacteremia, and only heart rate and temperature correlated significantly with bacteremia on multivariate analysis ^[6]. The ABA criteria have demonstrated poor specificity for identifying sepsis and septic shock ^[7].

Biomarkers¶

Biomarkers are studied as a way to separate the inflammatory noise of the burn from true infection. Procalcitonin is the best-studied. A systematic review and meta-analysis found procalcitonin moderately sensitive (73%) and specific (75%) for sepsis in burn patients, while C-reactive protein was highly sensitive (86%) but poorly specific (54%) ^[8]. Other meta-analyses report a useful pooled diagnostic performance for serum procalcitonin, with pooled sensitivity and specificity of 0.74 and 0.88 ^[16], and conclude that procalcitonin has a strong ability to discriminate septic from non-septic burn patients ^[17]. Adult-specific meta-analysis supports procalcitonin as a useful early-diagnosis biomarker that is best combined with other indexes to improve sensitivity and specificity ^[18]. No biomarker replaces wound assessment; they are adjuncts to the clinical and histologic picture.

Special Considerations¶

The diagnosis of burn wound infection in the high-risk burned child is difficult and depends on a very high index of suspicion and daily clinical evaluation of the wound by consistent observers ^[32]. Traditional diagnostic criteria for sepsis are often unreliable in this population ^[33], which is the same insensitivity problem seen in adults, amplified. When the cause or site of infection in a severely ill child is unclear, wound biopsy for histologic examination and quantitative culture is the recommended step ^[32].

Controversies and Evidence Gaps¶

The evidence base on burn wound infection diagnosis is fragmented by a problem that is itself unsettled: how to define the disease. A systematic review of systematic reviews found that variation in the definition of burn wound infection limited the validity of review conclusions, with definition heterogeneity affecting whether a conclusion could be drawn at all ^[34]. This undermines pooling across studies.

The gold standard is contested in practice rather than in principle. Histology is accepted as the reference standard, yet tissue biopsy is not uniformly implemented across centers ^[5], so much real-world diagnosis rests on the less reliable culture and clinical criteria. The mismatch between quantitative culture and histology ^[2] means that the most available test is the least definitive.

Systemic detection remains weak. The ABA sepsis criteria show poor specificity ^[7] and only modest discrimination against bacteremia ^[6]. Biomarkers narrow but do not close the gap; procalcitonin performance is moderate at best ^[8]. Machine-learning methods for predicting sepsis and bloodstream infection in burn patients remain an underexplored field, and traditional sepsis criteria remain unreliable in this population ^[33]. The shift to early excision has also changed the question, moving clinical relevance from colonization of unexcised eschar toward surgical wound infection of excised and grafted sites ^[11], so the classical colonization-versus-invasion framework applies to a shrinking share of contemporary wounds.

References¶

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