Your next patient has a swollen, very painful leg with spreading redness that has worsened over the last 24 hours. Here's how the necrotising fasciitis case unfolds and what to pick up on the way.

Your next patient has a swollen, very painful leg with spreading redness that’s worsened over the last 24 hours. In a necrotising fasciitis case the trap is how easily this can be filed under ‘cellulitis’, while the patient looks far sicker than the skin suggests and says, “A swollen, extremely painful leg that has become much worse overnight.” The key is pain out of proportion to the visible changes, then the ominous evolution: redness racing beyond yesterday’s line, dusky or violaceous skin with haemorrhagic blisters, islands of numb skin, and rapid decline with rigors and confusion, often on a background of type 2 diabetes. This page walks through how the case presents, what to pick up in the history, the tests, including LRINEC and imaging signs like soft-tissue gas or deep fascial oedema, and the teaching points.

How the case presents

Your next patient has a swollen, very painful leg with spreading redness that has worsened over the last 24 hours. In their own words: "A swollen, extremely painful leg that has become much worse overnight."

Observations on arrival:

Observation	On arrival
Heart rate	128 beats per minute
Blood pressure	88/54 mmHg
Respiratory rate	28 breaths per minute
Oxygen saturation	95%
Temperature	39.1 °C

What to pick up in the history

These are the findings the case is built around. Each one moves you towards or away from necrotising fasciitis, and the scorecard checks whether you found it.

Agonising pain far out of proportion to the leg: pain disproportionate to appearances is the cardinal discriminator from cellulitis and mandates surgical exploration.
Redness spreading visibly beyond yesterday's marked line: advance measurable in hours indicates necrotising infection rather than cellulitis.
Dusky purple skin with haemorrhagic blisters: bullae and violaceous change signal fascial necrosis and demand immediate theatre.
Patches of numb skin despite the surrounding agony: cutaneous anaesthesia from nerve destruction is near specific for necrotising fasciitis.
Rapid deterioration with rigors and confusion: systemic toxicity out of keeping with a skin infection points to necrotising infection and septic shock.
Type 2 diabetes: diabetes is the leading predisposing factor and raises the probability of polymicrobial necrotising infection.
Minor scratch to the shin four days ago: identifies the portal of entry and fixes the timeline of a trivial breach becoming a limb-threatening infection.

Red flags you must not miss

The case marks these as must-elicit. They only come out if you ask.

Agonising pain far out of proportion to the leg.
Redness spreading visibly beyond yesterday's marked line.
Dusky purple skin with haemorrhagic blisters.
Patches of numb skin despite the surrounding agony.
Rapid deterioration with rigors and confusion.

Investigations and what they showed

C-reactive protein: Markedly raised (>150), the highest-weighted component of the LRINEC score.
Full blood count: High white cell count with neutrophilia; haemoglobin may fall. Why it helps: WCC is an LRINEC component and reflects the severity of the infection.
Sodium: Low (hyponatraemia), characteristic and an LRINEC component. Why it helps: hyponatraemia in a 'cellulitis' that is deteriorating is a recognised warning feature of necrotising fasciitis.
Urea and electrolytes: Rising creatinine/urea (acute kidney injury from sepsis). Why it helps: creatinine is an LRINEC component and reflects organ dysfunction from septic shock.
Lactate: Raised, reflecting tissue hypoperfusion and septic shock.
Creatine kinase: Raised, indicating muscle/deep soft-tissue involvement rather than simple skin infection. Why it helps: A raised CK suggests necrosis extending into muscle/deep tissue, arguing strongly against simple cellulitis.
Random glucose: Elevated (poorly controlled diabetes). Why it helps: identifies and quantifies the diabetic risk factor and guides perioperative glycaemic control.
Blood culture: May grow Group A Streptococcus, Staphylococcus aureus or mixed anaerobes/coliforms (type I polymicrobial vs type II monomicrobial).
X-ray tibia and fibula: Soft-tissue GAS, linear lucencies tracking along the fascial planes.
CT abdomen and pelvis: CT is more sensitive than plain film for fascial gas and can define extent, but it must not delay theatre in a deteriorating patient.
MRI knee: MRI is the most sensitive imaging test for deep fascial involvement, but it is slow, and in a septic, deteriorating patient the correct response is immediate surgical exploration, not an MRI.
Inspect the calves and shins: An ill-defined dusky violaceous-red patch on the calf/shin with a blurred, indistinct margin that fades into normal skin. There is no fluctuance, no abscess and no obvious pus, the leg looks far less dramatic than the agony he is in. Why it helps: this is the image that carries the whole case. Waiting for the skin to catch up costs the limb and the life.
Inspect the skin: The border of the erythema has been outlined with a marker pen; the redness now visibly extends past that line at the upper edge, having advanced in a matter of hours. Why it helps: marking the border and re-checking it is a free bedside test with a huge yield.
Inspect the lower limbs: The whole lower limb is swollen and mottled dusky purple-brown, and carries several tense haemorrhagic bullae, dark blood-filled blisters sitting proudly on the skin, with more livid patches tracking proximally beyond them. Why it helps: haemorrhagic bullae are the cardinal alarming sign of necrotising fasciitis: they mean thrombosis of the subdermal vessels and the skin above dying.
Inspect the feet: Grey-black skin necrosis with haemorrhagic bullae is late necrotising fasciitis, the fascia beneath is already dead and will not bleed when cut.
Inspect the thighs: The erythema has swept proximally onto the thigh, which is angry red and studded with confluent purple-black necrotic patches; in places the epidermis has already lifted and sloughed away, leaving raw dermis. Why it helps: it shows the speed and the plane of the disease: necrotising fasciitis spreads along the deep fascia, so a shin infection reaches the thigh within hours.
Inspect the patient's level of alertness and demeanour: He is grey, sweating and shivering with rigors, drifting in and out of coherence so that his wife has to supply the history. He looks profoundly, systemically unwell, far sicker than a skin infection should make a man, and says he feels like he is dying. Why it helps: systemic toxicity out of keeping with the appearance of the skin is one of the discriminators between cellulitis and a necrotising infection. New confusion in this setting is organ dysfunction from septic shock and should trigger the Sepsis Six, critical-care escalation and an immediate surgical call, all in parallel.
Inspect the work of breathing: He is breathing fast and shallow at 28 breaths per minute at rest, with a dry mouth and cool, clammy peripheries; his blood pressure is 88/54 and his heart rate 128. Why it helps: tachypnoea is the earliest and most reliable sign of a compensating metabolic acidosis and is doing the work of a raised lactate before the blood gas comes back. In a 'cellulitis' patient, a respiratory rate of 28 with hypotension means septic shock and reframes the whole problem as a surgical emergency needing source control.
Inspect a wound: On the shin there is a trivial, four-day-old linear scratch, barely more than a scab, with no pus and no surrounding collection, but it sits at the centre of the spreading dusky erythema. Why it helps: it is the portal of entry, and its triviality is the point: necrotising fasciitis follows minor breaches of the skin, particularly in a diabetic. Dismissing the leg because 'it's only a scratch' is exactly how this diagnosis gets missed.

Why the diagnosis is necrotising fasciitis

The decisive diagnosis is necrotising fasciitis, a rapidly progressive, limb- and life-threatening infection of the deep fascia. The LRINEC score (CRP, WCC, haemoglobin, sodium, creatinine, glucose) supports the diagnosis but a low score does not exclude it, the decision to explore is clinical. 'Cellulitis' is the under-call this case exists to punish.

Necrotising fasciitis at a glance

What it is

Necrotising fasciitis is a rapidly progressive infection of the deep soft tissue, the fascia and subcutaneous tissue, that looks deceptively like ordinary cellulitis at first but is a true surgical emergency.

Pathogenesis

Bacteria, often group A Streptococcus alone or mixed aerobic and anaerobic organisms, enter through a breach in the skin, sometimes trivial, and spread rapidly along the poorly vascularised fascial planes at a rate of several centimetres an hour. Bacterial toxins destroy tissue and thrombose the small vessels supplying it, which is why the overlying skin can look almost normal even while the fascia beneath it is already dying, explaining the classic pain out of proportion to appearance.

Who it affects

4 per 100,000 people per year, but affects anyone with a breach in the skin, even a trivial scratch.

Causes and risk factors

Any breach of the skin can be the entry point, including surgical wounds, minor trauma, insect bites, or even no identifiable wound at all.

Investigations and lab tests

The LRINEC score (CRP, white cell count, haemoglobin, sodium, creatinine, glucose) supports the diagnosis, but a low score never excludes it, the decision to explore surgically remains clinical. Imaging may show soft-tissue gas on X-ray or CT, or deep fascial oedema on MRI, but gas is present in only a minority of cases and imaging must never delay theatre.

At a glance: Supports (never excludes): LRINEC score. May show gas/oedema: X-ray, CT or MRI, never delays theatre. Confirms: surgical exploration, not a test.

Teaching points

The cardinal clue is pain out of proportion to the skin appearance, with a patient who looks far sicker than a simple skin infection should make them.
Late signs, dusky/violaceous skin, haemorrhagic bullae, crepitus and skin anaesthesia, are highly suggestive, but waiting for them costs limbs and lives.
The LRINEC score (CRP, WCC, Hb, sodium, creatinine, glucose) supports the diagnosis, a low score does not exclude it. The decision to explore is clinical.
Imaging may show soft-tissue GAS (X-ray/CT) or deep fascial oedema (MRI), but gas is present in only a minority, and imaging must never delay theatre.

Practise a similar case

Reading that the pain was out of proportion and the redness had marched past yesterday’s marker is easy when it is laid out for you. Finding that detail yourself, under a little pressure, with a confused patient and a clock ticking, is different. Try a new surgical history with another patient and test your questioning.

Here's one from the same specialty, with the diagnosis left for you to find. Your next patient is a 72-year-old man with sudden severe pain across the middle of his tummy since this morning. Take this patient's history in Diagnosica. It's free to start, no card needed.

Necrotising fasciitis case presentation: history, findings and tests