Necrotizing Fasciitis Workup

Updated: Oct 12, 2022
  • Author: Steven A Schulz, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Approach Considerations

Laboratory tests, along with appropriate imaging studies, may facilitate the diagnosis of necrotizing fasciitis. [26, 27] Laboratory evaluation should include the following:

  • Complete blood count with differential

  • Serum chemistry studies

  • Arterial blood gas measurement

  • Urinalysis

  • Blood and tissue cultures

Skin and superficial tissue cultures may be inaccurate because samples may not contain the infected tissue. Deeper tissue samples, obtained at the time of surgical debridement, are needed to obtain proper cultures for microorganisms. (See Biopsy, below.) New techniques include rapid streptococcal diagnostic kits and a polymerase chain reaction (PCR) assay for tissue specimens that tests for the genes for streptococcal pyrogenic exotoxin (SPE; eg, SPE-B) produced by group A streptococci.

B-mode and possibly color Doppler ultrasonography, contrast-enhanced computed tomography (CT) scanning, or magnetic resonance imaging (MRI) can promote early diagnosis of necrotizing infections. [61] In addition, these studies permit visualization of the location of the rapidly spreading infection. More importantly, MRI or CT scan delineation of the extent of necrotizing fasciitis may be useful in directing rapid surgical debridement.

However, when the patient is seriously ill, necrotizing fasciitis is a surgical emergency with high mortality. Therefore, laboratory tests and imaging studies should not delay surgical intervention. [62]

Most fluid collections in the tissue, especially in the musculoskeletal system, can be localized and aspirated under ultrasonographic guidance. Whether fluid is infected cannot be determined on the basis of its ultrasonographic characteristics; however, laboratory analysis of the aspirated fluid can help in identifying the pathogen. [63]

In a study of 13 patients with thoracic and abdominal wall infections, Sharif et al reported that CT and MRI were superior to sonography, scintigraphy, and plain radiography in providing useful information about the nature and extent of infections. [64] Furthermore, they point out that while CT compares favorably with MRI in accurate diagnosis of soft tissue infection, multiplanar MRI images can be obtained without ionizing radiation and the use of intravenous contrast agents.

Although the laboratory results may vary in a given clinical setting, the following may be associated with necrotizing fasciitis:

  • Elevated white blood cell (WBC count), possibly to more than 14,000/µL

  • Elevated blood urea nitrogen (BUN) level, possibly to greater than 15 mg/mL

  • Reduced serum sodium level, possibly to less than 135 mmol/L

A study by Sandner et al indicated that the laboratory risk indicator for necrotizing fasciitis (LRINEC) is an effective tool for early detection of cervical necrotizing fasciitis. The investigators, who used a cutoff score of 6, reported that the LRINEC had a sensitivity and specificity for cervical necrotizing fasciitis of 94%, as well as a positive predictive value of 29% and a negative predictive value of 99%. The study included 16 patients with the disease and 595 patients with severe nonnecrotizing neck infections. [65]  A retrospective study by Narasimhan et al reported that an LRINEC score of 5 or above had a sensitivity of 76.3% and a specificity of 93.1% in the diagnosis of necrotizing fasciitis, with positive and negative predictive values being 95.5% and 88.1%, respectively. [66]

In contrast, however, a literature review by Fernando et al indicated that an LRINEC score of 6 or above has a sensitivity and specificity of 68.2% and 84.8%, respectively, in the detection of necrotizing soft tissue infection, while a score of 8 or above has a sensitivity and specificity of 40.8% and 94.9%, respectively. [67]

Nonetheless, a retrospective study by El-Menyar et al suggested that the LRINEC score is not only useful as a diagnostic tool but also as a prognostic one. In the study, patients with an LRINEC score of 6 or above had a greater tendency toward diabetes mellitus, Pseudomonas aeruginosa infection, and a higher Sequential Organ Failure Assessment (SOFA) score. They also tended to have a longer period of intensive care, a longer hospital stay, and higher septic shock and mortality rates. [68]

However, a retrospective study by Neeki et al indicated that the LRINEC score may prove inaccurate when used in the emergency department for necrotizing fasciitis risk stratification and the differentiation of cellulitis from necrotizing fasciitis. The investigators found that in emergency department patients with confirmed cellulitis, the LRINEC score had a high false-positive rate, classifying 10.8% of these patients as being at moderate or high risk for necrotizing fasciitis. In emergency department patients with confirmed necrotizing fasciitis, however, the LRINEC score had a high false-negative rate, classifying 63.8% of these patients as being at low risk for the disease. The study also found that in both the cellulitis and necrotizing fasciitis patients, the misclassification rate was higher for nondiabetic patients than for patients with diabetes. The results of the report indicate that a high index of clinical suspicion is warranted in the use of the LRINEC score in the emergency department. [69]


Imaging Studies


Plain radiographs, often obtained to detect soft-tissue gas that is sometimes present in polymicrobial or clostridial necrotizing fasciitis, are of no value in the diagnosis of necrotizing infections. [70] Indeed, nondiagnostic plain radiographs may even hinder the diagnosis of necrotizing infection. [61] In their study of 29 patients with necrotizing soft tissue infections, Lille et al reported that nondiagnostic radiographs correlate with a delay in operative intervention and consequent increased morbidity and mortality. [71]

The presence of subcutaneous gas in a radiograph does not necessarily indicate a clostridial infection, as Escherichia coli, Peptostreptococcus species, and Bacteroides species may produce gas under appropriate conditions. Misleading subcutaneous gas can also result from the undermining of tissue planes during surgical debridement. Perforations of the esophagus, the respiratory tract, or the GI tract related to endoscopy or chest tube insertion can result in the radiographic appearance of gas.


Bedside ultrasonography may be useful in patients with necrotizing fasciitis, as well as other soft-tissue infections, including cellulitis, cutaneous abscess, and peritonsillar abscess. It may be superior to clinical judgment alone in determining the presence or the absence of occult abscess formation. [72]

Sonography may reveal subcutaneous emphysema spreading along the deep fascia, swelling, and increased echogenicity of the overlying fatty tissue with interlacing fluid collections, allowing for early surgical debridement and parenteral antibiotics. [28]

Parenti et al retrospectively reviewed the ultrasonographic appearances of 32 pathologically proven cases of necrotizing fasciitis. [73] Ultrasonography revealed changes in the subcutaneous fat (28 of 32 patients), investing fascia (18 of 32 patients), and muscle (15 of 32 patients), which correlated well with histological findings. However, in some cases, ultrasonography missed histologically apparent inflammation in the subcutaneous tissues (3 of 32 patients) or muscle (8 of 32 patients). [73]

CT and MRI

CT scanning can pinpoint the anatomic site of involvement by demonstrating necrosis with asymmetrical fascial thickening and the presence of gas in the tissues. However, note that early on, CT scan findings may be minimal.

While no published, well-controlled, clinical trial has compared the efficacy of various diagnostic imaging modalities in the diagnosis of necrotizing infections, MRI is the preferred technique to detect soft tissue infection because of its unsurpassed soft tissue contrast and sensitivity in detecting soft tissue fluid, its spatial resolution, and its multiplanar capabilities. [29, 30]

The usefulness of MRI in the diagnosis of necrotizing fasciitis has been supported in a study by Rahmouni et al, who were able to differentiate nonnecrotizing cellulitis that would respond to medical treatment from severe necrotizing infections that required rapid life-saving surgery. [74] In necrotizing fasciitis, MRI can provide dramatic evidence of an inflammatory process infiltrating the fascial planes. [61]

Craig notes that the combined use of MRI and aspiration under ultrasonographic guidance is very useful in complicated infections (eg, septic arthritis and osteomyelitis) and that its role in the diagnosis of necrotizing fasciitis should be considered. [75] Early muscle necrosis may be apparent.

Absence of gadolinium contrast enhancement in T1 images reliably detects fascial necrosis in those requiring operative debridement. Combined with clinical assessment, MRI can determine the presence of necrosis and the need for surgical debridement. T2-weighted MRI may show well-defined regions of high signal intensity in the deep tissues. However, the sensitivity of MRI exceeds its specificity. [76]


Finger Test and Biopsy

The finger test should be used in the diagnosis of patients who present with necrotizing fasciitis. [31, 32] The area of suspected involvement is first infiltrated with local anesthesia. A 2-cm incision is made in the skin down to the deep fascia. Lack of bleeding is a sign of necrotizing fasciitis. On some occasions, a dishwater-colored fluid is noticed seeping from the wound.

A gentle, probing maneuver with the index finger covered by a sterile powder-free surgical double glove puncture indication system is then performed at the level of the deep fascia. If the tissues dissect with minimal resistance, the finger test is positive.

Tissue biopsies are then sent for frozen section analysis. The characteristic histologic findings are obliterative vasculitis of the subcutaneous vessels, acute inflammation, and subcutaneous tissue necrosis. If either the finger test or rapid frozen section analysis is positive, or if the patient has progressive clinical findings consistent with necrotizing fascia, immediate operative treatment must be initiated.

Excisional deep skin biopsy

Excisional deep skin biopsy may be helpful in diagnosing and identifying the causative organisms. [77] Specimens can be taken from the spreading periphery of the necrotizing infection or the deeper tissues, reached only in surgical debridement, to obtain proper cultures for microorganisms.

Avoid doing this procedure from the actual necrosis or granulating center, as many bacteria that neither cause nor add to the infection would be detected.


Aspiration and Gram Stain

Uman et al recommended percutaneous needle aspiration followed by prompt Gram staining and culture for a rapid bacteriologic diagnosis in soft-tissue infections. [78] A needle aspirate should be taken on the advancing edge of the infection, where group A beta-hemolytic Streptococcus (GABS) is plentiful. [79]

The Gram stain usually shows a polymicrobial flora with aerobic gram-negative rods and positive cocci when polymicrobial infection is present. However, in many cases, a single organism (eg, GABS, methicillin-resistant Staphylococcus aureus [MRSA], Clostridium) may be causing the infection, while cultures, including blood cultures, may spuriously reveal a polymicrobial infection. The presence of plentiful cocci on the Gram stain is characteristic of necrotizing infection, whereas cocci are rarely identified in erysipelas. [79]

Polymicrobial infections are often associated with previous surgical procedures, pressure ulcers, penetrating trauma, perianal abscesses, and intravenous drug use. In the study by Andreasen et al, 71% of their patients had polymicrobial infections. [37]


Histologic Findings

Sections from necrotizing fasciitis tissue show superficial fascial necrosis with blood vessels occluded by thrombi. A dense infiltration of neutrophils may be observed in deeper parts of the subcutaneous tissue and fascia. Subcutaneous fat necrosis and vasculitis are also evident. Eccrine glands and ducts may be necrotic. Alcian blue or periodic acid-Schiff staining with diastase may show clusters of bacteria and fungi (see the image below).

Photomicrograph of Fournier gangrene (necrotizing Photomicrograph of Fournier gangrene (necrotizing fasciitis), oil immersion at 1000X magnification. Note the acute inflammatory cells in the necrotic tissue. Bacteria are located in the haziness of their cytoplasm. Courtesy of Billie Fife, MD, and Thomas A. Santora, MD.