In short
Cellulitis is a bacterial skin infection that causes spreading redness, warmth, and swelling — it responds well to antibiotics, but delayed treatment lets it spread deeper or into the bloodstream.
Emergency if: Red, hot, swollen skin together with a very high temperature or shivering, a fast heartbeat or fast breathing, confusion, dizziness or fainting, cold, clammy or pale skin, purple or dusky patches, blisters, or pain far worse than the skin looks: call emergency services now or go to an emergency department now.
On your report
CRP, WBC / Total Leukocyte Count
On this page
01
How It Gets Started
Cellulitis is a bacterial infection of the deeper layers of the skin, usually entering through a break in the skin's surface — a cut, insect bite, blister, athlete's foot between the toes, or even skin that's just cracked from dryness or a chronic wound. Sometimes no entry point is ever identified.
It's more likely in skin that's already compromised: swelling from poor circulation or lymphedema, diabetes (which impairs wound healing and immune response), and obesity all raise the risk. Most cases involve one leg, though it can occur anywhere — cellulitis on the face, especially around the eye with eyelid swelling or pain moving the eye, is treated as more urgent regardless of how mild it looks, since it sits close to the eye socket and, less directly, the brain.
02
Recognizing It
The hallmark is an area of skin that's red, warm, swollen, and tender, with the redness typically having a less sharply defined edge than a rash — it tends to spread outward over hours to days rather than staying in one contained shape. Fever and feeling generally unwell can accompany more significant infections.
A red streak extending from the area toward the nearest lymph nodes (lymphangitic streaking), or swollen, tender lymph nodes nearby, suggest the infection is tracking through the lymphatic system and deserves prompt attention rather than a wait-and-see approach.
03
How It's Diagnosed
Cellulitis is usually diagnosed by its appearance and the story around it — a doctor examining the skin, checking for a fever, and often marking the current edge of the redness with a pen to track whether it's spreading despite treatment. Blood tests such as CRP or a white blood cell count aren't always needed for straightforward cases but are added when someone appears more unwell, has a fever, or has a wound that's actively draining, to gauge how significant the infection is.
Cellulitis can look similar to a blood clot (deep vein thrombosis) or a flare of an inflammatory skin condition, so a clinical exam helps make sure the right condition is being treated, especially since a DVT needs a completely different approach.
04
Treatment
Oral antibiotics for five to ten days treat most cases of cellulitis effectively, along with elevating the affected limb to reduce swelling and treating any underlying skin condition, like athlete's foot, that provided an entry point. Symptoms typically start improving within 48 hours of starting antibiotics.
More severe infections — with high fever, rapid spread, or in someone with a weakened immune system — may need intravenous antibiotics, sometimes requiring hospitalization. Marking and monitoring the border of redness helps confirm the infection is responding rather than continuing to spread despite treatment.
05
When to Seek Care Urgently
Redness that's rapidly spreading over hours, a high fever, confusion, a fast heart rate, intense pain out of proportion to how the skin looks, blistering, or skin turning dusky, purple, or numb are signs of a severe or deep infection or sepsis: call emergency services now or go to an emergency department now.
For typical cellulitis, symptoms not improving within 48 hours of starting antibiotics — or clearly getting worse — is a reason to be seen again promptly, since it may mean a different antibiotic is needed or the infection has spread deeper than initially thought.
How urgent is it?
Emergency — act now
Red, hot, swollen skin together with a very high temperature or shivering, a fast heartbeat or fast breathing, confusion, dizziness or fainting, cold, clammy or pale skin, purple or dusky patches, blisters, or pain far worse than the skin looks: call emergency services now or go to an emergency department now.
Same-day — call promptly
Skin that is painful, hot and swollen, a red streak spreading from the area, swelling around the eye or pain when moving the eye, or cellulitis that is not improving or is spreading 48 hours after starting antibiotics: see a doctor today.
Routine — see a doctor
Cellulitis that keeps coming back, or swollen legs, athlete's foot or cracked skin that keep letting infection in: book a doctor's visit.
Values mentioned in this story
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Sources
- Cellulitismedlineplus.gov
- Cellulitis — NHSnhs.uk
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