Investigations
1st investigations to order
surgical exploration
Test
If you suspect necrotising fasciitis clinically, refer the patient immediately for inspection, exploration, and debridement of infected tissue.
The ‘finger test’ is a surgical method that can be performed under local anaesthesia at the bedside for the diagnosis of necrotising fasciitis.[3] It involves making a 2 cm incision down to the deep fascia. Findings that suggest necrotising fasciitis following incision include:[3]
Minimal resistance to finger dissection (a ‘positive’ finger test)
Absence of bleeding
Presence of necrotic tissue
Murky or greyish ‘dishwater’ fluid.
Result
necrotising soft-tissue infection on surgical exploration
positive finger test, absence of bleeding, presence of necrotic tissue, murky or greyish ‘dishwater’ fluid following incision
blood and tissue cultures
Test
Definitive bacteriological diagnosis is best made using tissue specimens obtained from surgical debridement and blood cultures.[2]
Result
positive; may indicate polymicrobial or monomicrobial aetiology
Gram stain
Test
Staining of clinically affected tissue may provide early indication of causative organism(s). For example, small chains of gram-positive cocci suggest a streptococcal infection; clumps of large cocci suggest Staphylococcus aureus.
Result
variable
full blood count and differential
Test
High WBC count is a non-specific finding that may be seen in any systemic infection or circulatory collapse. A low WBC count may be a sign of severe sepsis.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
abnormally high or low WBC count with or without a left shift (elevated percentage of polymorphonuclear leukocytes and/or bands)
serum electrolytes
Test
Hyponatraemia is a non-specific finding that may be seen in any systemic infection or circulatory collapse. If a spreading soft-tissue infection is present, necrotising fasciitis should be suspected.[2][3][4][5]
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
sodium may be decreased
serum urea and creatinine
Test
Elevated urea and creatinine may be seen due to intracellular volume depletion, and in any systemic infection or circulatory collapse.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
serum urea and creatinine may be elevated
serum CRP
Test
Elevated CRP is a non-specific finding that may be seen in a range of systemic infections.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
usually elevated
serum creatine kinase
Test
A non-specific finding suggestive of systemic infection or circulatory collapse.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
may be elevated
liver function tests
Test
May be elevated if there is organ dysfunction due to sepsis.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
may be elevated
serum lactate
clotting screen
Test
Use to determine whether the patient has established coagulopathy in the presence of sepsis. This is associated with a worse prognosis.[55]
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
may show coagulopathy
blood gas (venous or arterial)
Test
Acidosis may be present in the setting of sepsis. Obtain an arterial blood gas if you are concerned about respiratory compromise in order to determine the patient's respiratory status.
If you suspect necrotising fasciitis, immediately refer the patient to the surgical team; do not wait for the results of investigations before referral.[2][3][4][5] Necrotising fasciitis is a clinical diagnosis. However, investigations can support the diagnosis if this is unclear.[3]
Result
acidosis may be present and lactate is usually elevated
arterial blood gas may show hypoxaemia
Investigations to consider
CT/MRI, x-ray, ultrasound
Test
Necrotising fasciitis is a clinical diagnosis.[4] However, investigations can support the diagnosis if this is unclear.[3]
Imaging may show soft-tissue gas, which is highly suggestive of the diagnosis; imaging may also demonstrate abnormalities in the involved soft tissue.[1][3][16]
Seek advice from a radiologist to determine the most appropriate imaging modality for your patient.
CT is the imaging of choice.[5]
Do not use a plain x-ray to rule out the diagnosis because x-ray is frequently normal during the early stages; subcutaneous gas may only be present as the disease progresses.[15]
Bedside ultrasound may be performed if the patient is clinically unstable.[3] In practice, however, bedside ultrasound is not widely used in all regions (including in the UK).
In one prospective study, ultrasound findings of diffuse thickening of the subcutaneous tissue, accompanied by fluid accumulation greater than 4 mm in depth, had a sensitivity of 88% and a specificity of 93%.[47]
Result
oedema extending along fascial plane and/or soft-tissue gas
fresh frozen section
Test
Early frozen-section soft-tissue biopsy can provide a definitive diagnosis and it may be used if the diagnosis is unclear clinically or radiologically.[3] However, frozen-section soft-tissue biopsy requires specialist pathology expertise, takes time to perform, and is not widely available in all regions, including in the UK.[3]
Result
evidence of bacteria and tissue necrosis
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