antimicrobial reference
empiric antibiotics
CrCl calculator, for renal dosing
first hour
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
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 neutropeniaafebrile 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.
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.
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 ×4d, with or just before the first dose; stop if not pneumococcal |
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.
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.
CAP, admitted5–7 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.
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 q6h |
| 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 or 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.
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.
native valve or late prosthetic (≥12 months)
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)
alternative
Ampicillin 12 g/day IV in 4–6 divided doses + flucloxacillin (or cloxacillin) 12 g/day IV in 4–6 divided doses.
gentamicin and rifampicin are not added empirically on a native valve (AHA 2015); they belong to the prosthetic-valve row below.
no proven preferred empiric regimen — narrow to the organism as cultures return, usually within 24 to 48 h.
healthcare-associated, native valve (dialysis, lines, devices, recent admission)
S. aureus incl. MRSA · coagulase-negative staphylococci · Enterococcus · non-HACEK Gram-negatives
refer cardiology + cardiac surgery early if S. aureus, heart failure, diabetes/renal failure, or valve-ring abscess; remove the line or catheter.
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 or embolising, start promptly.
empiric — first line
| Vancomycin | (local trough 15–20) or daptomycin 10 mg/kg IV once daily |
adjust to renal
alternative
vancomycin is the backbone; no empiric gentamicin or rifampicin on a native valve (AHA 2015). Gram-negative cover is an ID decision on the local antibiogram when septic from a line source.
no proven preferred empiric regimen — narrow to the organism as cultures return, usually within 24 to 48 h.
early prosthetic (<12 months) or prosthetic-valve staphylococci
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 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.
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.
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).
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.
complicated or catheter (CA-UTI)7 days from first effective dose 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 from the first effective dose for most patients who are improving (IDSA 2025; 5–7 days with a fluoroquinolone; the same with Gram-negative bacteremia). 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 / erysipelas5 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.
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.
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 |
| + Clindamycin | 900 mg IV q8h from the start, for toxin suppression |
adjust to renal
clindamycin is empiric, not conditional (23 Oct 2026): the organism cannot be known before the surgeon looks; continue it in streptococcal toxic shock.
septic arthritis
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.
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
consider resistant organisms if
- previous MDR organism
- antibiotics within 90 days
- recent hospitalisation
- long-term care facility
- immunocompromised state
- indwelling devices
once cultures return — de-escalate
- narrow coverage when possible
- stop unnecessary agents
- switch IV → PO when appropriate
- reassess duration
MOC+ library · volume 1
notes & sources
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 follows local pharmacy protocol; trough ranges in these tables are practical local targets. AUC-guided dosing and full targets: vancomycin trough.
primary guidelines: IDSA (A–Z) · Surviving Sepsis 2026 · Sanford Guide
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
Last clinically reviewed · August 2026 · Last updated · October 2026