antimicrobial reference
empiric antibiotics
sepsis & septic shock
first hour — do now
septic shock = vasopressor for MAP ≥65 + lactate >2 despite fluids
within 1 hour: cultures before antibiotics · antibiotics within the hour if shock · lactate · at least 30 mL/kg crystalloid within 3 h, then reassess and individualize (conditional, low certainty — not a fixed bolus for every patient) · noradrenaline (then vasopressin) to MAP ≥65 (60–65 if aged ≥65 with septic shock) · source control.
adjuncts
refractory shock hydrocortisone 200 mg/day IV.
glucose target 7.8–10 mmol/L.
unknown source — empiric
broad-spectrum gram-negative cover for shock of unknown source; add MRSA or double gram-negative cover only if risk factors are present, not routinely (SSC 2026)
obtain: blood cultures ×2 · urine culture · CXR · relevant imaging
empiric — first line
| Pip–tazo | or cefepime · meropenem · imipenem |
adjust to renal
add MRSA cover ifprior MRSA infection or colonization · recent IV antibiotics · recurrent skin infection / chronic wound · invasive device · hemodialysis · recent hospital admission → vancomycin 25 mg/kg load → 15–20 mg/kg q8–12h (local trough 15–20)
add / double gram-negative cover ifknown MDR colonization or infection in the past year · high local resistance prevalence · healthcare-associated infection · broad-spectrum antibiotics in the past 90 days · recent travel or hospitalization abroad
watch out: daptomycin does not treat pneumonia (inactivated by surfactant) — don’t use it for a respiratory source.
by organism
MRSA vancomycin 25 mg/kg load → 15–20 mg/kg q8–12h.
ESBL ertapenem · meropenem · imipenem.
Pseudomonas ceftazidime · meropenem · pip–tazo · ceftolozane–tazobactam.
rickettsial doxycycline 100 mg IV/PO q12h.
febrile neutropenia — high-riskafebrile 72h + recovered
fever ≥38.3 + ANC <500 · aerobic GNB (incl Pseudomonas) · GPC · fungi
refer haematology / oncology co-manage — they own the chemo context and antifungal escalation.
empiric — antipseudomonal monotherapy
| Cefepime | 2 g IV q8h |
| or Piperacillin–tazobactam | 4.5 g IV q6h |
| or Meropenem | 1–2 g IV q8h |
| or Imipenem | 500 mg IV q6h |
adjust to renal / local antimicrobial protocol
add vancomycin ifline infection · severe mucositis · SSTI · pneumonia · instability
add / escalate
line / mucositis / SSTI / pneumonia / instability add vancomycin (local trough 15–20).
persistent fever day 4–7 add antifungal: echinocandin, or liposomal amphotericin B.
post-splenectomy / asplenic
encapsulated — S. pneumoniae · N. meningitidis · H. influenzae · Capnocytophaga (dog bite)
emergency: a febrile asplenic patient can crash within hours — antibiotics now.
empiric — first line
| Ceftriaxone | 2 g IV q24h (2 g q12h if meningitis) |
after a dog bite
pip–tazo · meropenem · severe allergy → clindamycin 900 mg IV q8h.
bacterial meningitis
meningitis — age <50 y
S. pneumoniae · meningococcus · (Listeria if immunocompromised/pregnant)
don’t delay: LP + antibiotics + dexamethasone, with or before the first antibiotic dose.
empiric — first line
| Ceftriaxone | 2 g IV q12h |
| + Vancomycin | 15–20 mg/kg IV q8–12h (local trough 15–20) |
| + Dexamethasone | 0.15 mg/kg IV q6h ×2–4d |
adjust to renalpen-allergy → chloramphenicol + TMP-SMX + vancomycin (check local availability of chloramphenicol)
Listeria (immunocompromised, pregnant) → add ampicillin 2 g IV q4h; stop dexamethasone if CSF grows Listeria.
meningitis — age >50 y or comorbidity
N. meningitidis · S. pneumoniae · Listeria → ampicillin always
don’t delay LP + antibiotics + dexamethasone, with or before the first dose.
empiric — first line
| Ampicillin | 2 g IV q4h |
| + Ceftriaxone | 2 g IV q12h |
| + Vancomycin | 15–20 mg/kg IV q8–12h (local trough 15–20) |
| + Dexamethasone | 0.15 mg/kg IV q6h |
adjust to renalpen-allergy → vancomycin + aztreonam + TMP-SMX
stop dexamethasone if the pathogen is not S. pneumoniae.
community-acquired
CAP — admitted adult5–7 days
S. pneumoniae · atypicals (Legionella · Mycoplasma · Chlamydophila)
empiric — first line
| Ceftriaxone or Cefotaxime | 2 g IV q24h · 2 g q8h |
| + Azithromycin | 500 mg IV q24h |
| or Levofloxacin (mono, non-severe) | 750 mg IV/PO q24h |
adjust to renal
broaden ifICU / severe · structural lung disease · prior Pseudomonas · recent admission
watch out: fluoroquinolones (levofloxacin, moxifloxacin) prolong QTc; avoid with other QT-prolonging drugs.
conditionals & alternatives
severe CAP use the combination, not monotherapy. Corticosteroids: suggested in severe CAP only (ATS 2025, conditional / low certainty; IDSA did not endorse) — hydrocortisone 200 mg/day IV. Not recommended in non-severe CAP.
MRSA risk (prior MRSA, post-influenza, IVDU) add vancomycin or linezolid 600 mg q12h; nasal PCR to de-escalate.
Pseudomonas risk swap the cephalosporin for pip–tazo, meropenem, cefepime, or ceftazidime.
alternatives doxycycline (for azithro) · moxifloxacin (for levo) · ampicillin–sulbactam + macrolide · lefamulin / omadacycline.
duration minimum 5 days, and only stop once clinically stable; typically 5–7 days.
hospital-acquired · HAP / VAP
HAP / VAP — low MRSA risk7–8 days
early onset · no MDR risk · MRSA prevalence <10–20%
empiric — choose one
| Cefepime | 2 g IV q8h |
| or Piperacillin–tazobactam | 4.5 g IV (over 4h) q8h |
| or Meropenem | 1 g IV q8h |
adjust to renal
add vancomycin if MDR risk, prior MRSA, or positive nasal NAAT (local trough 15–20).
notes
also an option: levofloxacin 750 mg q24h. VAP: avoid pip–tazo / levofloxacin if ESBL suspected.
HAP / VAP — severe / MDR risk7–8 days
late onset · sepsis / hypotension · MRSA prevalence >10–20% · MDR GNB (ESKAPE)
empiric
| Vancomycin | 15–20 mg/kg IV q8–12h (local trough 15–20) |
| + one β-lactam | cefepime · pip–tazo · meropenem |
adjust to renalresistant GNB / CRE → ID consult
MRSA nasal screen negative → safe to stop empiric MRSA cover (NPV ~96%).
add for specific organisms
Legionella levofloxacin 750 mg q24h or azithromycin 500 mg IV q24h.
Pseudomonas / MDR GNB double cover: add ciprofloxacin 400 mg IV q8h · tobramycin · amikacin.
other lower respiratory
lung abscess4–6 weeks
oral anaerobes · oral strep (S. milleri) · S. aureus · enteric GNB
empiric — first line
| Ampicillin–sulbactam | 3 g IV q6h |
| step-down: co-amoxiclav | 875/125 mg PO BID |
pen-allergy → levofloxacin + metronidazole
alternatives
levofloxacin + metronidazole (750 mg daily + 500 mg q8h) · moxifloxacin 400 mg daily · ceftriaxone + metronidazole.
no response → bronchoscopy, fungal/Nocardia serology. severe / immunocompromised → treat as admitted CAP.
infective endocarditis
IE — native valve or late prosthetic (≥12 months), empiric
S. aureus · Streptococcus · Enterococcus · HACEK · culture-negative
refer cardiology + cardiac surgery early — surgery if heart failure, large vegetation, or uncontrolled infection.
cultures first: take three sets 30 min apart before antibiotics. If the patient is stable and not in heart failure, wait for them. If septic, start promptly.
empiric — first line
| Ampicillin | 12 g/day IV in 4–6 divided doses |
| + Ceftriaxone | 4 g/day IV in 2 divided doses |
adjust to renalpenicillin allergy → cefazolin 6 g/day IV in 3 doses or vancomycin (local trough 15–20), each + gentamicin 3 mg/kg IV once daily
alternative
Ampicillin 12 g/day IV in 4–6 divided doses + flucloxacillin (or cloxacillin) 12 g/day IV in 4–6 divided doses + gentamicin 3 mg/kg IV once daily.
gentamicin: max 240 mg/day. Check renal function and levels weekly, trough <1 mg/L and peak 10–12 mg/L.
no proven preferred empiric regimen — narrow to the organism as cultures return, usually within 24 to 48 h.
IE — early prosthetic (<12 months) or healthcare-associated, empiric
methicillin-resistant staphylococci · coagulase-negative staphylococci · Enterococcus · non-HACEK Gram-negatives
refer cardiac surgery early if S. aureus, heart failure, diabetes/renal failure, or valve-ring abscess.
empiric — first line
| Vancomycin | (local trough 15–20) or daptomycin 10 mg/kg IV once daily |
| + Gentamicin | 3 mg/kg IV once daily |
| + Rifampicin | 900–1200 mg/day IV or PO in 2–3 divided doses |
adjust to renal
alternative
gentamicin: max 240 mg/day. Check renal function and levels weekly, trough <1 mg/L and peak 10–12 mg/L.
no proven preferred empiric regimen — narrow to the organism as cultures return, usually within 24 to 48 h.
secondary peritonitis · intra-abdominal
secondary peritonitis4d after source control
perforation · ruptured appendix/diverticula · abscess · Enterobacteriaceae · Bacteroides · enterococci
surgical general surgery for source control in virtually all; the regimen doesn’t work without drainage or repair.
empiric — first line
| mild/moderate: pip–tazo | 4.5 g IV (over 4h) q8h |
| or ertapenem / moxifloxacin | 1 g q24h · 400 mg q24h |
| severe / shock: meropenem | 1 g IV q8h |
adjust to renal
β-lactam-free options
(cipro or levo) + metronidazole · cefepime + metronidazole · eravacycline. any cephalosporin / aztreonam regimen needs metronidazole added for anaerobes.
spontaneous bacterial peritonitis · SBP
SBP — community-acquired~5 days
cirrhosis · ascites · ascitic PMN ≥250/µL = SBP · GNB (E. coli) · GP cocci
empiric — first line
| Cefotaxime | 2 g IV q8h |
| or Ceftriaxone | 2 g IV q24h |
| or Piperacillin–tazobactam | 4.5 g IV (over 4h) q8h |
pen-allergy → ciprofloxacin 400 mg IV q12h
always add IV albumin 1.5 g/kg at diagnosis, 1 g/kg on day 3 (reduces renal impairment).
nosocomial & prophylaxis
nosocomial / high MDR meropenem + daptomycin.
prophylaxis cirrhosis + UGI bleed → ceftriaxone. chronic ascites → norfloxacin or ciprofloxacin PO daily.
C. difficile · CDI
CDI — initial episode10 days
≥3 unformed stools/day · stop the inciting antibiotic · avoid antiperistaltics
empiric — first line
| mild/moderate: fidaxomicin (preferred) | 200 mg PO BID ×10d |
| or vancomycin PO | 125 mg PO QID ×10d |
| fulminant/severe: vancomycin PO | 500 mg PO q6h ± IV metronidazole |
fidaxomicin is guideline-preferred over oral vancomycin, but cost and availability often decide locally — oral vancomycin remains an appropriate choice where fidaxomicin is not available. avoid metronidazole alone in severe disease (higher mortality). ileus → vancomycin 500 mg PR retention enema q6h. toxic megacolon → surgery.
recurrent CDI
first recurrence fidaxomicin, or vancomycin taper (QID → TID → BID → daily → q48h → q72h, 1 wk each).
multiple recurrences FMT (treatment of choice), or vancomycin → rifaximin. infection control: soap & water (gel doesn’t kill spores).
urinary tract
UTI — uncomplicated cystitis
adult female · E. coli · Klebsiella · S. saprophyticus
empiric — first line
| Nitrofurantoin | 100 mg PO q12h ×5d |
| or TMP/SMX DS (if resistance <20%) | 1 tab PO q12h ×3d |
asymptomatic bacteriuria → don’t treat (exceptions: pregnancy, before urologic procedure).
watch out: nitrofurantoin is ineffective if CrCl <30 mL/min and does not cover pyelonephritis.
alternatives & pregnancy
fosfomycin 3 g PO ×1 · ciprofloxacin 250 mg q12h ×3d · co-amoxiclav · cephalexin · pivmecillinam.
pregnant 5–7 days; nitrofurantoin / TMP-SMX first-line in 2nd–3rd trimester; avoid fluoroquinolones.
ESBL oral: fosfomycin, pivmecillinam, gepotidacin.
UTI — complicated / catheter (CA-UTI)7 days if improving
obstruction · reflux · transplant · Enterobacterales · P. aeruginosa · Enterococcus
empiric — first line
| Ceftriaxone (IV / pyelonephritis) | 1 g IV q24h |
| or Ciprofloxacin | 500 mg PO BID · 400 mg IV q12h |
| or Levofloxacin | 750 mg PO/IV q24h |
adjust to renal
systemic illness / high MDR risk
broaden, choose one ertapenem 1 g IV q24h · pip–tazo 4.5 g (over 4h) q8h · cefepime 2 g IV q12h · ceftazidime–avibactam · plazomicin.
cephalosporins & ertapenem are not reliable vs Enterococcus or Pseudomonas.
duration 7 days for most patients who are improving. Most do not need 10–14 days. Longer only for a specific reason: undrained obstruction or abscess, inadequate source control, slow clinical response, prostatitis, or persistent bacteremia. IV→PO once improving, absorbing, and an active oral agent is available on susceptibility — no arbitrary afebrile window.
cellulitis & erysipelas
cellulitis / erysipelas — extremities5 days if improving
non-diabetic · S. pyogenes & β-haemolytic strep · S. aureus (non-purulent)
empiric — hospitalised
| Cefazolin | 1 g IV q8h |
| or Flucloxacillin | 1–2 g IV q6h |
add MRSA cover ifprior MRSA · IDU · purulent · β-lactam allergy → vancomycin or linezolid
MRSA swab negative → β-lactam is fine (NPV 97.5%). bilateral & afebrile? stasis dermatitis, not cellulitis — no antibiotics.
outpatient, MRSA & alternatives
outpatient cephalexin 500 mg QID ×5d (extend only for inadequate response) · amoxicillin / penicillin VK. purulent → TMP/SMX DS.
MRSA risk (prior MRSA, IDU, purulent, allergy) vancomycin or linezolid 600 mg BID.
alternatives: ceftriaxone · dalbavancin · clindamycin · doxycycline (MRSA). portal of entry is often tinea pedis — treat it.
purulent · abscess
skin abscess · boils · furuncles5–7 days
S. aureus (MSSA / MRSA)
surgical incision & drainage is the treatment. antibiotics are adjunct; refer surgery for large, deep, or loculated collections.
empiric — I&D plus
| low MRSA: dicloxacillin / cephalexin | 500 mg PO QID |
| mod/high MRSA: TMP/SMX DS | 1 tab PO BID |
| or Clindamycin | 300–450 mg PO TID |
inpatient & alternatives
inpatient empiric I&D + cultures, vancomycin (local trough 15–20); de-escalate (MSSA → cefazolin / flucloxacillin).
no response in 2–3 days → look for an undrained abscess. alternatives: dalbavancin · linezolid · daptomycin · ceftaroline.
necrotising · surgical emergency
necrotizing fasciitis
GAS or mixed aerobic/anaerobic · S. aureus/MRSA · Clostridia · Vibrio
surgical emergency immediate surgical takeover for debridement. antibiotics are adjunct; do not wait on imaging. Tdap if >10 y.
empiric — broad
| gram-positive cover | vancomycin · linezolid · daptomycin |
| + broad β-lactam | pip–tazo · carbapenem · ceftriaxone + metronidazole |
adjust to renal
streptococcal toxic shock → add clindamycin 900 mg IV q8h for toxin suppression.
native joint · septic arthritis
septic arthritis — acute
synovial WBC >50k, PMN predominant · cultures + Gram stain before starting
orthopaedics urgent washout / drainage is essential; antibiotics alone do not clear a septic joint.
empiric — Gram-stain led
| Gram-positive cocci | vancomycin (local trough 15–20) |
| Gram-neg diplococci (GC) | ceftriaxone 1 g IV q24h |
| Gram stain negative | vancomycin + ceftriaxone |
adjust to renal
disseminated gonococcal (polyarthralgia + tenosynovitis ± pustular lesions) → ceftriaxone 1 g IV q24h; culture all mucosal sites.
allergy & chronic
serious β-lactam allergy vancomycin + ciprofloxacin · linezolid / daptomycin.
chronic monoarticular → treat the cause (Brucella, mycobacteria, fungi). exclude gout/pseudogout — look for crystals.
spine · vertebral osteomyelitis
vertebral osteomyelitis≥6 weeks
S. aureus ~50% · strep · aerobic GNB · cultures before empiric therapy in the stable patient
empiric — cover GP + GN
| Vancomycin | 15–20 mg/kg IV q8–12h (local trough 15–20) |
| + Ceftriaxone | 2 g IV q24h · or cefepime 2 g q8h |
adjust to renal
refer spinal surgery / neurosurgery for epidural abscess with neurological signs or cord compression.
epidural abscess red flags: severe back pain + radicular pain, new weakness, sensory loss, or incontinence → urgent MRI.
alternatives & duration
linezolid / daptomycin (MRSA) · pip–tazo (for the cephalosporin).
duration 6 weeks; ≥8 weeks if MRSA, ESRD, undrained abscess, or hardware.
MDR risk factors — when to suspect resistant organisms
consider resistant organisms if
- previous MDR organism
- antibiotics within 90 days
- recent hospitalisation
- long-term care facility
- immunocompromised state
- indwelling devices
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
local antibiogram takes precedence. Empiric therapy must be adapted to local resistance patterns, prior cultures, recent antibiotic exposure, organ function and individual patient risk. These tables are a starting point, not a substitute for your local microbiology policy.
vancomycin monitoring should follow local pharmacy protocol. AUC/MIC 400–600 is the 2020 guideline standard for serious MRSA (less nephrotoxicity than trough-guided dosing); where AUC monitoring is not available, trough-based targets apply (10–15 mg/L standard; 15–20 mg/L selectively for deep-seated infection, with renal monitoring). Trough ranges in this table are local practical targets, not AUC surrogates — see vancomycin trough.
once cultures return — de-escalate
- narrow coverage when possible
- stop unnecessary agents
- switch IV → PO when appropriate
- reassess duration
MOC+ library · volume 1
Acute Medicine Guide (Infectious Diseases)
These empiric regimens in full clinical context: workup, complications, and de-escalation for the common acute infections.
primary guidelines: IDSA (A–Z) · Surviving Sepsis 2026 · Sanford Guide
Last clinically reviewed · August 2026 · Last updated · August 2026