UKMLA Infectious Diseases: Sepsis to Stewardship

Infectious diseases run through every UKMLA specialty. A respiratory stem might open with CAP, a neurology stem with bacterial meningitis, a haematology stem with neutropenic sepsis, a surgery stem with necrotising fasciitis. Candidates who silo ID as "microbiology" miss the thread. The UKMLA tests it as the connective tissue of acute medicine: recognise infection, recognise severity, give the right antibiotic in the right time window.

This pillar consolidates the examinable ID surface into one reference — the syndromes, the UK empirical antibiotic choices, the stewardship rules, and the traps. Most candidates lose marks in ID not from obscure microbes but from hesitating on sepsis six timing or confusing penicillin allergy with intolerance. Those are the reflexes this pillar builds.

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1. Why ID integrates with every specialty

The GMC content map embeds infection across 32 of its 212 presentations. Expect ID to appear wherever:

• Fever is part of the stem — paediatrics (traffic-light), oncology (neutropenic sepsis), O&G (chorioamnionitis, PID), surgery (collection/peritonitis).
• New confusion appears in an older adult — UTI, pneumonia, encephalitis, meningitis.
• Antibiotic prescribing is asked — always a good prompt to check allergies and renal function first.
• Returning traveller is mentioned — malaria, typhoid, dengue, Zika.
• Immunocompromise is hinted — HIV, post-transplant, steroids, chemotherapy.

The AKT rewards candidates who move from "symptoms → likely organism → empirical antibiotic → source control" in a single mental sweep. Pair this pillar with the UKMLA Content Map 2026 to see where ID sits in the syllabus, and with the NICE Guidelines pillar for antibiotic-prescribing ladders.

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2. Sepsis (NICE definition, Sepsis Six, BUFALO)

NICE NG51 definition (2016 update): life-threatening organ dysfunction caused by a dysregulated host response to infection.

NEWS2 + red flag criteria drive ED recognition — a NEWS2 ≥5 with suspected infection → screen for sepsis. Red flag criteria include:

• Responds only to voice/pain or unresponsive.
• Acute confusion.
• Systolic BP ≤90 mmHg (or drop >40 from baseline).
• Heart rate >130.
• Respiratory rate ≥25.
• SpO₂ <92% (≥88% in chronic respiratory disease).
• Non-blanching rash, mottled/ashen/cyanotic.
• Not passed urine in 18 h.
• Lactate ≥2 mmol/L.

Sepsis Six (within 1 hour of recognition — UK Sepsis Trust "BUFALO"):

Take 3:

1. Blood cultures before antibiotics (if no delay).
2. Urine output — catheterise and measure.
3. Lactate and FBC/U&E/CRP.

Give 3: 4. IV fluids — 500 mL crystalloid bolus over 15 min, repeat up to 30 mL/kg. 5. IV broad-spectrum antibiotics — local empirical protocol. 6. Oxygen — aim SpO₂ 94–98% (88–92% in chronic respiratory disease).

Septic shock = sepsis + vasopressor required to maintain MAP ≥65 + lactate ≥2 despite adequate fluid resuscitation. Transfer to ICU.

Neutropenic sepsis (NICE CG151):

• Temperature >38°C in a patient with neutrophil count <0.5 (or <1 and falling).
• IV piperacillin-tazobactam empirically within 1 hour (local guidance may differ).
• Do not wait for culture results.
• Add vancomycin if line infection suspected.

For integration with ABCDE and the 10 must-master emergencies, see our UKMLA Acute & Emergency Presentations pillar.

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3. Antimicrobial stewardship principles (UK)

UK hospitals and GPs work to the Start Smart, Then Focus framework (UKHSA). The AKT repeatedly tests the principles rather than specific choices.

Start Smart:

• Send cultures before antibiotics if safe to do so.
• Document indication, dose, route, and duration on the drug chart.
• Review at 48–72 hours and make an active decision: stop, switch IV→oral, change antibiotic, extend, or refer to OPAT.

Then Focus — five options at the 48 h review:

1. Stop.
2. Switch IV to oral.
3. Change antibiotic (ideally narrower spectrum, to target identified organism).
4. Continue.
5. Outpatient parenteral antimicrobial therapy (OPAT).

Key stewardship rules:

• Avoid the 4Cs where alternatives exist — ciprofloxacin, clindamycin, co-amoxiclav, cephalosporins — because of C. difficile risk.
• Minimum effective course. Many infections (cystitis 3 days, CAP 5 days, cellulitis 5 days) are now shorter than historic regimens.
• De-escalate to narrowest effective agent once sensitivities known.
• Check allergy — true anaphylaxis to penicillin precludes all β-lactams.

For prescribing-safety depth and PSA overlap, see our upcoming UKMLA Prescribing Safety pillar.

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4. Bacterial vs viral meningitis — LP findings, empirical treatment

Adult presentation: fever + headache + neck stiffness + photophobia + altered consciousness. Kernig's and Brudzinski's signs have low sensitivity — their absence does not exclude.

Investigations:

• Bloods — FBC, U&E, CRP, glucose, clotting, blood culture, meningococcal/pneumococcal PCR.
• CT head before LP if: focal neurology, new seizures, GCS <13, immunocompromise, papilloedema, age >60 (relative).
• LP — opening pressure, protein, glucose (+ paired serum), cell count, Gram stain, culture, PCR (meningococcal, pneumococcal, HSV, VZV, enterovirus).

CSF interpretation:

Empirical antibiotics (don't delay for LP if unsafe):

• Community-onset, age 3 months – 50 yrs: ceftriaxone 2 g IV.
• Age >50 or immunocompromised: add amoxicillin 2 g IV (Listeria cover).
• Penicillin anaphylaxis: chloramphenicol + vancomycin (local policy).
• Dexamethasone 10 mg IV QDS 4 days reduces neurological sequelae in pneumococcal meningitis — give with first antibiotic dose.
• Community GP with suspected meningococcal disease (non-blanching rash) — IM benzylpenicillin 1.2 g before transfer.

Public health: notify within 24 h. Prophylaxis with ciprofloxacin 500 mg stat for close contacts (household, kissing within 7 days before onset).

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5. Encephalitis (HSV) — CT/LP sequencing

HSV encephalitis — most common sporadic viral encephalitis; temporal lobe predilection.

Presentation: fever + altered behaviour/personality + focal seizures + dysphasia + focal neurology. Gradual onset over hours–days.

Workflow:

1. Empirical IV aciclovir 10 mg/kg TDS — start on suspicion, do NOT wait for imaging/LP.
2. CT head first (look for temporal lobe oedema) then LP (lymphocytosis, raised protein, normal/mildly low glucose, HSV PCR).
3. MRI brain — temporal lobe hyperintensity on T2/FLAIR.
4. EEG — lateralised periodic discharges.
5. Continue aciclovir for 14–21 days if PCR positive.

Do NOT delay aciclovir for test results — every hour delay worsens outcome. This is a common AKT trap.

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6. UTI: community vs catheter-associated, asymptomatic bacteriuria

Uncomplicated lower UTI (community):

• Women: nitrofurantoin 100 mg MR BD for 3 days OR trimethoprim 200 mg BD for 3 days (local resistance guides).
• Men: 7 days of the same (all male UTIs considered complicated).
• Recurrent (≥2 in 6 months or ≥3 in 1 year): consider daily low-dose prophylaxis, vaginal oestrogen in post-menopausal women.

Upper UTI / pyelonephritis:

• Oral cefalexin or co-amoxiclav 7–10 days (or local empirical).
• IV if septic — gentamicin + amoxicillin, OR co-amoxiclav, OR piperacillin-tazobactam per local protocol.
• Imaging — USS within 24 h if no response, suspected obstruction, or recurrent.

Catheter-associated UTI (CAUTI):

• Urinary symptoms + positive culture + indwelling catheter >48 h.
• Change the catheter before sending culture.
• Antibiotics 7 days (oral if tolerating, IV if septic).
• Do NOT treat asymptomatic bacteriuria in catheterised patients — selects for resistance.

Asymptomatic bacteriuria — treat only in:

• Pregnancy — reduces pyelonephritis and preterm delivery risk.
• Before urological procedures that breach mucosa.

Everyone else → do not treat. This is a heavily tested stewardship point.

Pregnancy-specific: avoid trimethoprim in first trimester (folate antagonist); avoid nitrofurantoin in third trimester (neonatal haemolysis). Amoxicillin or cefalexin preferred.

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7. Community- vs hospital-acquired pneumonia (CURB-65, NICE)

CURB-65:

• Confusion (AMT ≤8).
• Urea >7 mmol/L.
• Respiratory rate ≥30.
• BP <90 systolic or ≤60 diastolic.
• Age ≥65.

Stratification (NICE NG138):

• 0–1 — low risk, home treatment possible.
• 2 — hospital consideration.
• ≥3 — severe, admit; consider ICU.

CAP empirical (UK):

• Mild (CURB-65 0–1): amoxicillin 500 mg TDS 5 days PO.
• Moderate (CURB-65 2): amoxicillin + clarithromycin (dual cover) 5–7 days.
• Severe (CURB-65 ≥3): IV co-amoxiclav + clarithromycin 7–10 days; consider levofloxacin if Legionella suspected.

HAP (onset >48 h into admission) — piperacillin-tazobactam OR ceftriaxone ± amikacin; add vancomycin if MRSA risk. De-escalate with culture.

Atypical pneumonias: Mycoplasma (dry cough, erythema multiforme, cold agglutinins), Legionella (hyponatraemia, diarrhoea, hepatitis), Chlamydia psittaci (bird exposure). Cover with macrolide.

Immunocompromised: Pneumocystis jirovecii (PCP) — presents with dry cough, exertional desaturation. Treat with high-dose co-trimoxazole + steroids if PaO₂ <9.3. Confirm with BAL.

For breathlessness differential and ABG integration, see our UKMLA Respiratory Masterclass.

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8. Aspiration pneumonia

Risk factors: reduced GCS (alcohol, post-ictal, stroke, sedation), swallowing dysfunction (neurological, post-radiotherapy, elderly), NG feeding, dental disease, oesophageal disorders.

Features: right lower lobe (right bronchus more vertical), delayed presentation with necrotising infiltrate, anaerobic organisms common.

Management:

• Co-amoxiclav OR piperacillin-tazobactam OR metronidazole added to ceftriaxone (local protocol).
• Prevent recurrence — SALT review before re-introducing oral intake, head-up feeding, oral care, consider NG/PEG if recurrent.

Lung abscess (subacute after aspiration) — fever, productive foul-smelling sputum, cavitating lesion on CXR. Needs prolonged antibiotics (weeks) ± drainage.

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9. TB — active vs latent, contact tracing, RIPE regimen + side effects

Active TB:

• Cough >3 weeks, haemoptysis, night sweats, weight loss, fever, apical cavitation on CXR.
• 3 sputum samples (one early morning) for AFB smear + culture + NAAT.
• Induced sputum or bronchoscopy if unable to expectorate.

Latent TB:

• No symptoms, normal CXR.
• Mantoux (tuberculin skin test) OR interferon-γ release assay (IGRA — Quantiferon, T-SPOT.TB).
• IGRA preferred in BCG-vaccinated.

RIPE regimen (active TB):

Adverse effects:

• Rifampicin — orange secretions, hepatitis, induces CYP450 (reduces OCP, warfarin, methadone efficacy).
• Isoniazid — peripheral neuropathy (give pyridoxine B6), hepatitis, lupus-like syndrome.
• Pyrazinamide — hepatitis, hyperuricaemia/gout.
• Ethambutol — optic neuritis (colour vision screen before start).

CNS TB — 12-month regimen + adjunctive steroids. MDR-TB (rifampicin ± isoniazid resistance) — specialist regimens, often 18+ months.

Contact tracing: close contacts (household, workplace ≥8 h/day) offered IGRA + CXR. Latent TB → 3 months rifampicin-isoniazid OR 6 months isoniazid.

Notification — statutorily notifiable. Directly observed therapy (DOT) for non-adherent patients.

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10. HIV — screening, ART basics, opportunistic infections

Screening — UK opt-out testing in high-prevalence areas; universal screen in new antenatal bookings, STI clinics, TB patients, new registrants from endemic regions.

Seroconversion illness — 2–6 weeks post-infection: fever, rash, pharyngitis, lymphadenopathy (mononucleosis-like).

Diagnosis: 4th-generation combined HIV antigen-antibody assay (window 4 weeks). Confirm with second assay.

Staging:

• Viral load — untreated typically >10,000 copies/mL.
• CD4 count — <200 = AIDS-defining vulnerability.
• CD4 <200 → prophylactic co-trimoxazole (PCP).
• CD4 <50 → azithromycin prophylaxis for MAC.

ART (start at diagnosis regardless of CD4, BHIVA):

• Standard first-line: two NRTIs + integrase strand transfer inhibitor (e.g., tenofovir + emtricitabine + dolutegravir).
• Single-tablet regimens dominate UK prescribing.
• Undetectable = untransmissible (U=U).

Opportunistic infections — pattern recognition:

PEP (post-exposure prophylaxis):

• Within 72 hours of exposure (preferably <1 h).
• Tenofovir + emtricitabine + raltegravir for 28 days.
• Baseline + 8-week + 12-week HIV test.

PrEP available through sexual health services for high-risk populations.

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11. Malaria — species, Plasmodium falciparum severity

Presentation: fever + chills + headache + myalgia + returning traveller (up to a year for P. vivax/ovale). Always ask about travel in any fever stem.

Five human species: P. falciparum (most dangerous), P. vivax, P. ovale, P. malariae, P. knowlesi.

Diagnosis: three thick-and-thin blood films over 24 h (one negative does not exclude) + rapid diagnostic test (RDT).

Severe falciparum malaria criteria (any of):

• Parasitaemia >2%.
• Schizonts on blood film.
• Impaired consciousness, seizures.
• Haemoglobin <80 g/L.
• Hypoglycaemia <2.2 mmol/L.
• Acidosis (pH <7.3, lactate >5).
• AKI (creatinine >265 μmol/L).
• Pulmonary oedema / ARDS.
• Spontaneous bleeding / DIC.
• Shock (BP <80 systolic).

Management:

• Severe falciparum → IV artesunate 2.4 mg/kg at 0, 12, 24 h then daily. Switch to oral after 24 h improvement. Transfer to ICU if criteria met.
• Uncomplicated falciparum → oral artemether-lumefantrine (Riamet) OR atovaquone-proguanil.
• P. vivax / ovale → chloroquine + primaquine for hypnozoite eradication (check G6PD status first — primaquine causes haemolysis in G6PD deficiency).

Notifiable disease. All falciparum admitted; vivax/ovale can be treated outpatient if well.

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12. C. difficile — risk factors, management, recurrence

Risk factors: recent antibiotics (especially 4Cs — ciprofloxacin, clindamycin, co-amoxiclav, cephalosporins), PPI use, age >65, hospitalisation, immunocompromise.

Diagnosis: diarrhoea (≥3 loose stools/day) + risk factors + positive stool glutamate dehydrogenase (GDH) screen confirmed by toxin A/B EIA or PCR. GDH+ / toxin− = carriage, do not treat.

Severity (UKHSA):

• Mild — <3 loose stools/day, no WCC rise.
• Moderate — 3–5 stools, WCC <15.
• Severe — WCC >15, temperature >38.5, creatinine rise >50% baseline, abdominal pain.
• Life-threatening — hypotension, ileus, toxic megacolon.

Management (NICE NG199):

• Stop causative antibiotic where possible; isolate patient; enteric precautions (soap-and-water hand hygiene, not alcohol gel).
• First episode: oral vancomycin 125 mg QDS 10 days.
• Recurrent (within 12 weeks): fidaxomicin 200 mg BD 10 days.
• Life-threatening: oral vancomycin 500 mg QDS + IV metronidazole 500 mg TDS; early surgical review for subtotal colectomy if deteriorating.
• Refractory/multiply recurrent: faecal microbiota transplant (FMT).

Do NOT use loperamide — risks toxic megacolon.

For deeper GI context including diagnostic ladders for bloody diarrhoea and IBD, see our UKMLA Gastroenterology & Hepatology pillar.

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13. Cellulitis and necrotising fasciitis (LRINEC)

Cellulitis:

• Diffuse erythema, warmth, swelling, pain of skin + subcutaneous tissue.
• Most commonly Strep pyogenes or Staph aureus.
• Eron classification stratifies severity (I–IV). Class III/IV → admit IV.

Treatment:

• Oral flucloxacillin 500 mg QDS 5–7 days first-line.
• Clarithromycin or doxycycline if penicillin allergic.
• IV if systemic features or rapid progression.
• Mark margin with pen; review daily for progression.

Necrotising fasciitis — red flags:

• Pain out of proportion to visible signs.
• Rapid progression (hours).
• Systemic toxicity disproportionate to skin findings.
• Crepitus, skin necrosis, bullae.
• Anaesthesia over affected skin (nerve destruction).

LRINEC score:

• CRP ≥150.
• WCC 15–25 = 1 point, >25 = 2.
• Haemoglobin 11.0–13.5 = 1, <11 = 2.
• Sodium <135.
• Creatinine >141.
• Glucose >10.
• Score ≥6 highly suggestive; ≥8 strongly predictive.

Management:

• Surgical emergency — urgent debridement within 12 h is the definitive treatment.
• Broad-spectrum IV antibiotics — piperacillin-tazobactam + clindamycin (anti-toxin effect) ± vancomycin for MRSA cover.
• ICU/ITU support.
• Do not wait for imaging — operate on suspicion in unstable patients.

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14. Common antibiotic classes + notable adverse effects

Gentamicin monitoring:

• Once-daily dosing (5–7 mg/kg) — check trough before next dose; target <1 mg/L.
• Twice-daily dosing — peak 5–10 mg/L, trough <2 mg/L.
• Stop if AKI develops; check audiology if ototoxicity symptoms.

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15. Allergies vs intolerances vs true adverse reactions

True allergy (immediate, IgE-mediated):

• Anaphylaxis (see adrenaline IM 500 μg in adult), urticaria, angioedema, bronchospasm, hypotension.
• Onset minutes to 1 hour.
• Penicillin anaphylaxis precludes all β-lactams including cephalosporins (though cross-reactivity is lower than historically taught — ~1–2% with later-generation cephalosporins).

Delayed hypersensitivity (T-cell mediated):

• SJS/TEN, DRESS, serum sickness, interstitial nephritis, haemolysis.
• Onset days to weeks.
• Usually contraindicates all β-lactams.

Intolerance (not allergy):

• GI upset, headache, mild rash without systemic features.
• Does NOT preclude re-use of the drug class.

Adverse effects:

• Cholestatic jaundice with flucloxacillin/co-amoxiclav — not allergy but avoid re-prescribing.
• C. difficile — not allergy, but stewardship consideration.

Documentation matters — vague "penicillin allergy" flagged without specifics leads to sub-optimal antibiotic choice and worse outcomes. Always ask: what happened, when, and how quickly?

Allergy testing / challenge — outpatient skin testing and graded oral challenge available for de-labelling. Useful for patients with lifelong "penicillin allergy" that may not be real.

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Exam technique — ID SBA archetypes

Five frequently-tested ID stem patterns:

1. "Patient with fever + neutrophil count 0.3 post-chemotherapy" → neutropenic sepsis → piperacillin-tazobactam within 1 hour, blood cultures, sepsis six.
2. "Young adult + fever + neck stiffness + non-blanching rash" → meningococcal sepsis → IM benzylpenicillin in community + ceftriaxone on arrival + dexamethasone; notify public health.
3. "Returning traveller from sub-Saharan Africa with fever" → malaria until proven otherwise → three blood films + RDT; IV artesunate if falciparum with severity criteria.
4. "Elderly care-home resident + confusion + low-grade fever" → UTI (commonest) but screen for pneumonia, cellulitis, sepsis; dipstick unreliable — send MSU.
5. "Post-surgical patient + fever + pain out of proportion to redness" → necrotising fasciitis → urgent surgery + piperacillin-tazobactam + clindamycin.

Common ID traps:

• Don't give aciclovir after LP results in suspected HSV encephalitis — give empirically first.
• Don't treat asymptomatic bacteriuria outside pregnancy or pre-urological procedure.
• Don't give loperamide in C. difficile.
• Don't forget dexamethasone in pneumococcal meningitis.
• Don't prescribe trimethoprim in first trimester or nitrofurantoin at term.

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Summary — five reflexes that win ID SBAs

1. Sepsis recognition + sepsis six within 1 hour. NEWS2 ≥5 + infection = screen.
2. Empirical antibiotics on suspicion, before cultures if cultures delay treatment. Meningitis, encephalitis, neutropenic sepsis, necrotising fasciitis.
3. Know your empirical choices by syndrome. CAP (amoxicillin ± clarithromycin); severe meningitis (ceftriaxone ± amoxicillin for Listeria); neutropenic sepsis (piperacillin-tazobactam).
4. De-escalate at 48 h. Start smart, then focus. Shortest effective course.
5. Differentiate allergy from intolerance. Anaphylactic penicillin allergy precludes β-lactams; GI upset does not.

Pair this pillar with the Cardiology Masterclass for endocarditis, Respiratory Masterclass for pneumonia and TB depth, and Emergency Presentations pillar for sepsis six in context. ID marks are won by recognition speed and the first-dose empirical choice — rehearse those until they're instinctive.