Diabetic foot infection

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Diabetic foot infection develops when organisms invade a neuropathic, ulcerated or traumatised foot. Sensory loss delays recognition, motor neuropathy creates pressure deformity and autonomic dysfunction dries and fissures skin.

Peripheral arterial disease reduces oxygen, immune-cell and antibiotic delivery and can prevent healing even when antimicrobial selection is correct. Infection and ischaemia together require urgent vascular and surgical planning.

Diagnosis is clinical because all chronic ulcers contain microorganisms. Tissue culture identifies organisms only after examination establishes invasive infection; a positive surface swab cannot distinguish colonisation.

Depth matters. Infection may extend through tendon sheath, joint and bone along low-resistance planes, while neuropathy masks pain. Every assessment should therefore include probe depth, bone involvement, perfusion and systemic severity.

Key points

Diabetic foot infection is a clinical diagnosis: purulence or at least two local inflammatory features must be attributed to infection, not culture alone.
Do not give antibiotics to an uninfected ulcer; colonisation is expected and treatment does not promote healing.
Remove dressings, inspect between toes and examine depth, undermining, necrosis, fluctuance, deformity, sensation and perfusion.
Grade infection as mild, moderate or severe because extent, depth and systemic features determine setting, spectrum and urgency.
After cleaning and debridement, obtain deep tissue or curettage for culture; avoid using a superficial swab when a better sample is feasible.
Plain radiography is the initial bone study; MRI is the preferred advanced imaging test when osteomyelitis remains uncertain.
Antibiotics are only one component: drainage, debridement, offloading, perfusion restoration, glucose management and wound care determine outcome.
For a mild infection, NICE lists flucloxacillin 500 mg to 1 g four times daily for seven days as a first-choice adult regimen, with review and culture adjustment.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Ulcer barrier failure

Neuropathy, deformity and repetitive pressure cause skin breakdown that permits colonising staphylococci, streptococci and other organisms to invade tissue.

Deep and chronic flora

Long-standing, previously treated or ischaemic ulcers increasingly contain mixed Gram-positive, Gram-negative and anaerobic organisms, shaped by healthcare and antimicrobial exposure.

Procedure and trauma portals

Callus debridement injury, puncture, surgery, footwear friction and unnoticed thermal damage can create a portal in an insensate foot.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Neuropathic pressure injury
Sensory loss removes protective pain, motor imbalance deforms the foot and autonomic dysfunction dries skin, concentrating repetitive load over vulnerable sites.
2
Contiguous deep extension
Organisms spread from ulcer through subcutaneous tissue along tendon and fascial planes into joint and bone, particularly when pressure and necrosis persist.
3
Ischaemic immune failure
Peripheral arterial disease reduces oxygen tension, neutrophil function, tissue repair and antibiotic delivery, allowing minor infection to become limb threatening.
4
Metabolic amplification
Infection raises counter-regulatory hormones and glucose, while hyperglycaemia impairs phagocyte function and may precipitate ketoacidosis or hyperosmolar illness.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Clinical local infection

Purulent discharge or at least two of erythema, warmth, swelling, tenderness or induration supports infection after non-infective inflammation is considered.

Mild superficial disease

Local infection limited to skin and subcutaneous tissue with small surrounding erythema and no systemic features may be treated through an organised ambulatory pathway.

Moderate deep diseaseRed flag

Erythema extending more widely, lymphangitis, abscess, gangrene or involvement of tendon, muscle, joint or bone requires urgent multidisciplinary assessment.

Severe systemic diseaseRed flag

Temperature disturbance, tachycardia, tachypnoea, hypotension, confusion or metabolic deterioration makes infection severe and usually requires admission.

Ischaemic phenotypeRed flag

Cool pale or cyanosed skin, absent pulses, dependent rubor, rest pain, tissue loss or gangrene suggests inadequate perfusion and poor healing capacity.

Neuropathic maskingRed flag

Loss of protective sensation can make deep infection painless; callus, deformity and an unexpectedly deep ulcer are important danger clues.

Red flags requiring action

Hypotension, confusion, tachypnoea, lactate rise or oliguria indicates severe infection with organ dysfunction.
Crepitus, bullae, anaesthesia, necrosis or disproportionate pain suggests necrotising soft-tissue infection.
Fluctuance, purulent tracking or pain with passive movement suggests a deep abscess or compartment infection.
Cool skin, absent pulses, rest pain, gangrene or delayed capillary return indicates limb-threatening ischaemia.
Visible or probe-detectable bone, chronic deep ulcer or persistent inflammatory drainage raises osteomyelitis probability.
Neuropathy can remove pain, so an apparently comfortable patient may still have extensive infection.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Cleaned and debrided wound examinationFirst step
Why
Define ulcer size, depth, undermining, exposed structures, drainage and viable versus necrotic tissue.
Interpretation and limitations
Document location and dimensions reproducibly. Probe gently for sinus and bone; avoid assuming a small surface opening represents a small cavity.
02
Deep tissue or curettage culture
Why
Identify invasive organisms and susceptibility after surface contaminants and devitalised material are removed.
Interpretation and limitations
Obtain before antibiotics when safe. Tissue is preferred to superficial swab; bone culture is most specific when osteomyelitis treatment depends on organism definition.
03
Plain foot radiographs
Why
Assess bone destruction, gas, foreign body, deformity and a baseline for suspected osteomyelitis.
Interpretation and limitations
This is the initial imaging study, but early osteomyelitis may be radiographically normal and neuropathic Charcot change can mimic infection.
04
MRI of the foot
Why
Map marrow infection, abscess, sinus, tendon and joint involvement when diagnosis or operative extent remains uncertain.
Interpretation and limitations
MRI is the preferred advanced test for suspected osteomyelitis, interpreted with ulcer anatomy and Charcot changes; it should not delay urgent drainage.
05
Probe-to-bone assessment
Why
Refine osteomyelitis probability in a deep ulcer using a simple bedside anatomical test.
Interpretation and limitations
A hard gritty endpoint increases probability in a high-risk ulcer, while a negative result does not exclude bone infection when pre-test probability remains high.
06
Perfusion assessment
Why
Determine whether arterial disease threatens healing and whether revascularisation is needed.
Interpretation and limitations
Palpate pulses and use Doppler waveforms, ankle pressure, toe pressure or other specialist tests; calcified vessels can make ankle indices falsely reassuring.
07
Blood tests and cultures
Why
Assess inflammation, renal function, glycaemia, metabolic decompensation and bacteraemia in significant disease.
Interpretation and limitations
Check full blood count, CRP, renal profile, glucose and ketones as indicated; take blood cultures for systemic illness without delaying treatment.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Acute Charcot neuroarthropathy

A hot swollen neuropathic foot with bony collapse but no convincing ulcer-linked infection can reflect sterile joint and bone destruction.

Gout or inflammatory arthritis

Acute joint-centred pain and swelling may be crystal or inflammatory disease, requiring aspiration when septic arthritis remains possible.

Venous or lymphatic inflammation

Chronic oedema, stasis dermatitis and lipodermatosclerosis can produce bilateral foot or leg redness without clinically invasive ulcer infection.

Critical limb ischaemia

Rest pain, coolness, tissue loss and absent flow may cause sterile necrosis, although ischaemia and infection frequently coexist.

Pressure injury without infection

Callus, blister or ulcer can be inflamed from mechanical trauma yet lack purulence or sufficient attributable infection features.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ASSESSRemove dressings and grade threatFirst stepA person with diabetes has a new ulcer, drainage, redness, swelling or unexplained metabolic deterioration.

1Use ABCDE when systemically unwell, remove all dressings and inspect both feet, interdigital spaces, footwear and the entire affected limb.
2Clean and debride enough to assess ulcer depth, purulence, necrosis, fluctuance, exposed tendon or bone, sensation and deformity.
3Assess pulses, temperature, capillary return and Doppler signals and classify both infection severity and ischaemic urgency.
4Refer immediately through the diabetic-foot multidisciplinary pathway when moderate, severe, deep, ischaemic or rapidly progressive disease is present.

02SOURCE CONTROLDrain and restore viable tissueAbscess, necrosis, wet gangrene, infected compartment, devitalised bone or continuing purulent tracking is present.

1Obtain urgent surgical and podiatric review and drain pus, decompress involved spaces and excise non-viable soft tissue and bone as anatomy requires.
2Send separate deep tissue, fluid and bone samples during the procedure before antibiotic contamination when this does not delay lifesaving therapy.
3Involve vascular specialists at once when perfusion is inadequate and coordinate revascularisation timing with infection source control.
4Protect the foot from load using an appropriate offloading plan and maintain wound, pressure, nutrition and glycaemic care after surgery.

03ANTIMICROBIALMatch spectrum to severityClinical infection is present after examination and samples have been obtained when feasible.

1For mild infection, use narrow oral treatment such as flucloxacillin under NICE and local guidance, accounting for allergy, renal function and previous cultures.
2For moderate or severe infection, use the local intravenous or oral pathway based on systemic state, prior antibiotics, resistant isolates, ischaemia and deep anatomy.
3Review cultures and operative findings at 48 to 72 hours and narrow treatment; do not continue anti-pseudomonal or MRSA cover without a supported reason.
4Set duration from soft-tissue response, bone involvement and completeness of resection rather than copying a simple cellulitis course.

04UNINFECTEDHeal the ulcer without antibioticsAn ulcer lacks purulence and attributable local or systemic inflammatory features.

1Do not prescribe antibiotics or collect routine surveillance swabs solely because an ulcer is open, malodorous or culture-positive.
2Provide sharp or other appropriate debridement, moisture-balanced dressings, pressure offloading and regular podiatry review.
3Optimise perfusion, glycaemia, nutrition, smoking cessation, footwear and treatment of oedema or fungal skin disease.
4Teach daily foot inspection and give rapid-access instructions for new warmth, swelling, discharge, spreading redness, fever or metabolic deterioration.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

NICE first-choice narrow therapy for likely streptococci and methicillin-susceptible staphylococci in superficial disease.

Flucloxacillin for mild infection

Give 500 mg to 1 g orally four times daily for seven days for mild diabetic foot infection, with further duration only after clinical review.

Clarify immediate penicillin allergy, review liver and renal function and culture history, and escalate anatomy rather than merely prolonging treatment when deep disease is suspected.

Provides a NICE-listed alternative for selected mild diabetic foot infection without deep or systemic features.

Clarithromycin allergy alternative

Give 500 mg orally twice daily for seven days when penicillin allergy makes flucloxacillin unsuitable and local guidance supports macrolide activity.

Review QT prolongation, hepatic dysfunction and interactions including statins, warfarin and colchicine; local resistance or previous cultures may require another agent.

Offers an alternative oral option in penicillin allergy where organism probability and susceptibility make tetracycline activity appropriate.

Doxycycline allergy alternative

Give 200 mg orally on the first day, then 100 mg once daily for a total seven-day course when locally selected for mild infection.

Avoid in pregnancy, separate from iron and antacids, counsel about oesophagitis and photosensitivity and do not use as underpowered monotherapy for severe polymicrobial disease.

Provides setting-specific broad cover while urgent drainage, debridement and perfusion restoration are arranged.

Severe diabetic-foot empirical regimen

Use the current local weight-, allergy- and renal-adjusted intravenous combination for severe diabetic foot infection immediately after deep and blood cultures when feasible.

Regimen choice depends on previous isolates, recent antibiotics, MRSA status, local resistance, renal function and tissue penetration; microbiology review and early narrowing are mandatory.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Osteomyelitis

Contiguous spread into bone causes persistent drainage, structural destruction and relapse unless infected bone and antimicrobial duration are addressed.

Deep abscess and necrosis

Tracking pus and devitalised tissue can cross foot compartments, requiring urgent drainage, fasciotomy or excisional debridement.

Sepsis and metabolic crisis

Invasive infection triggers systemic organ dysfunction and can precipitate life-threatening diabetic ketoacidosis or hyperosmolar hyperglycaemic state.

Amputation

Uncontrolled infection, non-reconstructable ischaemia or extensive necrosis may require minor or major amputation to preserve life or function.

Recurrent ulceration

Persistent neuropathy, deformity and pressure create new skin failure even after infection resolves, requiring lifelong prevention and footwear care.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record infection grade, ulcer dimensions, depth, drainage, erythema, tissue viability, pain and photographs according to consent and local practice.
Trend observations, glucose, ketones, renal function and inflammatory markers in admitted or systemically unwell patients.
Review wound and deep-culture results with clinical response and remove broad empirical cover when organisms and source control permit.
Reassess perfusion after debridement and during healing; a clean wound that does not granulate may remain critically ischaemic.
Monitor offloading adherence, pressure redistribution, footwear, nutrition and smoking because each affects healing independently of antibiotics.
Confirm a named multidisciplinary follow-up service, dressing plan, antimicrobial stop date and emergency re-entry route before discharge.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Culture follows diagnosis

A chronic ulcer will grow organisms, so examination establishes infection first and a deep sample then refines treatment.

Pain is an unreliable safeguard

Sensory neuropathy may make osteomyelitis, abscess and ischaemia painless; visual depth and systemic or metabolic change deserve priority.

Perfusion governs delivery

Antibiotic susceptibility in a laboratory cannot compensate for critical ischaemia preventing oxygen, immune cells and drug from reaching tissue.

Probe-to-bone changes probability

The bedside finding is most useful when interpreted with ulcer depth, prevalence, radiography and inflammatory evidence rather than as an absolute verdict.

Charcot can mimic infection

Warmth, swelling and bony change may reflect acute neuropathic arthropathy; ulcer communication, MRI pattern and multidisciplinary review help discriminate.

Offloading is active treatment

Continued pressure repeatedly injures granulating tissue and permits deeper extension, so a workable offloading plan is as important as dressings.

11Common pitfallsFrequent interpretation and management errors.

01
Do not prescribe antibiotics for an uninfected diabetic foot ulcer or use culture positivity as the definition of infection.
02
Do not submit a superficial swab when cleaned deep tissue or bone can be obtained safely.
03
Do not assume absence of pain excludes abscess, osteomyelitis or ischaemia in neuropathy.
04
Do not let MRI delay urgent drainage of a clinically evident deep collection or necrotising process.
05
Do not manage infection without assessing pulses, Doppler signals and need for revascularisation.
06
Do not focus on glucose and antibiotics while leaving pressure, necrotic tissue or inadequate perfusion untreated.