Investigations
1st investigations to order
full blood count
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
In patients who are systemically ill and/or have comorbidities, send blood for a WBC count. Although non-specific, nearly all patients have a raised WBC count (but it may be low in patients with immunocompromising factors or sepsis). A normal result makes cellulitis or erysipelas unlikely.[23]
Result
elevated or low
erythrocyte sedimentation rate (ESR)
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
In patients who are systemically ill and/or have comorbidities, send blood for inflammatory markers. Although non-specific, nearly all patients have a raised ESR. A normal result makes cellulitis or erysipelas unlikely.[23]
Result
elevated
CRP
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
In patients who are systemically ill and/or have comorbidities, send blood for inflammatory markers. Although non-specific, nearly all patients have a raised CRP. A normal result makes cellulitis or erysipelas unlikely.[23]
Result
elevated
urea and electrolytes
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
In patients who are systemically ill and/or have comorbidities, send blood for urea and electrolytes as these provide baseline parameters and help assess for end-organ damage, which may influence antibiotic choice.
Result
may be normal or abnormal
blood culture and sensitivities
Test
Take blood for culture if the patient needs admission.
Guidelines differ in nuance regarding blood cultures.[20][21][23] In UK practice, general principles are as follows.
Take blood for culture if the patient needs admission. Typically, this will include patients who:
Are systemically unwell, with hypotension, tachycardia, or pyrexia
May have unusual organisms (e.g., immersion injury)
Are immunocompromised
Have a lower threshold for admitting these patients.
Blood culture is positive in 2% to 4% unless infection is severe.[15][20][44]
Draw blood for cultures before starting antibiotic therapy.[32][33][34]
Result
growth of typical pathogen; antibiotic sensitivity
Investigations to consider
skin swab
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
Guidelines differ in nuance regarding swab cultures.[20][21][23] In UK practice, general principles are as follows.
Take a swab when there is an obvious open wet wound, skin break, or ulcer. This culture can be useful in identifying MRSA infection.
Have a lower threshold for taking a swab if the patient has:
Diabetes
Immunocompromising factors.
Practical tip
Bear in mind that the National Institute for Health and Care Excellence in the UK specifically recommends that you consider taking a swab for microbiological testing only if the skin is broken and:[21]
There is a penetrating injury, or
There has been exposure to water-borne organisms, or
The infection was acquired outside the UK.
If taking samples for culture and sensitivities, do so before giving antibiotics.[32][33][34]
Result
growth of typical pathogen; antibiotic sensitivity
skin aspirate
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
Guidelines differ in nuance regarding aspirate cultures.[20][21][23] In UK practice, general principles are as follows.
Take an aspirate if there is a collection of fluid. This can be useful in identifying MRSA infection.
Have a lower threshold for taking an aspirate if the patient has:
Diabetes
Immunocompromising factors.
Practical tip
Bear in mind that the National Institute for Health and Care Excellence in the UK specifically recommends that you consider taking a swab for microbiological testing only if the skin is broken and:[21]
There is a penetrating injury, or
There has been exposure to water-borne organisms, or
The infection was acquired outside the UK.
Culture is positive in ≤5% to 40% of samples.[3][6][7][8][10][15][20][45][46]
Take the sample before administering antibiotics.[32][33][34]
Result
growth of typical pathogen; antibiotic sensitivity
skin biopsy
Test
Guidelines differ in nuance regarding biopsy cultures.[20][21][23] In UK practice, general principles are as follows.
Do not take a simple biopsy just for the purposes of identifying an organism.
Biopsies are rarely performed unless there is an indication for surgery, such as underlying osteomyelitis, or an alternative diagnosis is suspected in a patient with immunocompromising factors or malignancy.
Request histology in patients with a differential diagnosis that includes non-infectious inflammatory lesions.[20]
Culture is positive in approximately 20% of samples.[15]
Take the sample before administering antibiotics.[32][33][34]
Result
growth of typical pathogen; antibiotic sensitivity
histology may show infectious or other inflammatory features
molecular diagnostic procedures
plain x-ray
Test
Only consider if you suspect osteomyelitis or necrotising fasciitis. In practice, do not delay administering antibiotics while waiting for an x-ray.
Practical tip
You need to exclude osteomyelitis.[21] This can be challenging because osteomyelitis and cellulitis can often overlap. In practice, the pain may not reliably distinguish between the two conditions, and examining deep to an area of cellulitis would be painful for the patient. Have a low threshold for x-ray, and proceed to MRI if there is any suspicion of a bony infection on x-ray. The x-ray might need to be repeated, as early x-rays may not demonstrate osteomyelitis. See our topic Osteomyelitis.
Result
may reveal underlying osteomyelitis or gas in subcutaneous tissue
MRI
Test
Only arrange an MRI if you are uncertain of the diagnosis, or if you suspect osteomyelitis, necrotising fasciitis, or an abscess underlying the cellulitis.[25]
Practical tip
You need to exclude osteomyelitis.[21] This can be challenging because osteomyelitis and cellulitis can often overlap. In practice, the pain may not reliably distinguish between the two conditions, and examining deep to an area of cellulitis would be painful for the patient. Have a low threshold for x-ray, and proceed to MRI if there is any suspicion of a bony infection on x-ray. The x-ray might need to be repeated, as early x-rays may not demonstrate osteomyelitis. See our topic Osteomyelitis.
Result
may show evidence of associated osteomyelitis, gas in the subcutaneous tissue, or an abscess
ultrasound
Test
Only arrange if you are uncertain of the diagnosis, or if you suspect necrotising fasciitis or an abscess underlying the cellulitis.[25]
Result
fluid collection seen if abscess is present
liver function tests
Test
Do not routinely request laboratory investigations for patients without signs of systemic illness and without any comorbidities.[23]
These provide baseline parameters in patients who are systemically unwell or have comorbidities, and help assess end-organ damage, which may influence antibiotic choice.[31]
Result
may be normal or abnormal
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