empiric antibiotic quick reference
One-page empiric choices for common adult inpatient infections, with the triggers for adding Pseudomonas, MRSA, anaerobic, atypical, Enterococcus, Listeria, ESBL, or toxin-suppression cover. Default is the narrowest agent that covers the likely organisms; broaden only for a named trigger. Doses assume normal renal function. Most penicillin-allergic patients can still receive ceftriaxone or cefazolin — see penicillin allergy. Detail and evidence live in each syndrome note.
Septic shock, bacterial meningitis, necrotising soft tissue infection, febrile neutropaenia — cultures if they cause no delay, then antibiotics; source control in parallel.
Defaults below follow Canadian stewardship guidance. Switch the default when local resistance crosses the threshold: E. coli TMP-SMX resistance >20% (empiric cystitis/pyelo), fluoroquinolone resistance >10% (give a parenteral ceftriaxone/aminoglycoside dose before oral FQ for pyelo; FQ-based IAI regimens need ≥90% susceptibility), cefazolin susceptibility low (use ceftriaxone for cholecystitis), pneumococcal macrolide resistance >25% (macrolide monotherapy), MRSA >20% of S. aureus (empiric MRSA cover in HAP; >10–20% in VAP), >10% Gram-negative resistance to the planned monotherapy (double Gram-negative cover in HAP/VAP).
empiric choice by syndrome
“β-lactam avoided” = SCAR, serum sickness, AIN, haemolysis, or near-fatal anaphylaxis within 10 years — not simple penicillin allergy.
respiratory
| syndrome | first-line | β-lactam avoided | add / escalate if | duration |
|---|---|---|---|---|
| CAP, ward | ceftriaxone 1–2 g IV q24h + azithromycin (ATS/IDSA); ceftriaxone alone if non-severe, no Legionella risk | levofloxacin 750 mg | azithromycin mandatory: CURB-65 ≥3, step-down/ICU, Legionella risk, worsening | 5 days (3 if stable day 3) |
| CAP, ICU | ceftriaxone 2 g + azithromycin 500 mg IV | levofloxacin + vancomycin (if MRSA risk) | Pseudomonas/MRSA triggers below | 7 days |
| aspiration | usually pneumonitis — observe; ceftriaxone if pneumonia | levofloxacin | metronidazole only for abscess/empyema | 5 days |
| AECOPD (purulent sputum or admitted) | amoxicillin-clavulanate 875 mg PO BID or cefuroxime 500 mg PO BID | levofloxacin | different class if antibiotics within 3 months; CAP regimen if infiltrate | 5 days |
| HAP, no Pseudomonas risk | ceftriaxone 1–2 g IV q24h | levofloxacin | — | 7 days |
| HAP with Pseudomonas risk | piperacillin-tazobactam 4.5 g IV q6h | meropenem | meropenem if ESBL colonised or pip-tazo-resistant Pseudomonas; vancomycin if MRSA colonised | 7 days |
| VAP | piperacillin-tazobactam | meropenem | septic shock → + ciprofloxacin 400 mg IV q8h (double Gram-negative); vancomycin if MRSA colonised | 7 days; stop at 1–3 days if minimal, stable vent settings |
urinary
| syndrome | first-line | β-lactam avoided | add / escalate if | duration |
|---|---|---|---|---|
| cystitis | nitrofurantoin 100 mg PO BID | TMP-SMX DS BID (3 days) | fosfomycin/pivmecillinam if resistant | 5 days (men 7) |
| pyelonephritis / complicated UTI | ceftriaxone 1–2 g IV q24h | gentamicin 7 mg/kg IV q24h or ciprofloxacin | + ampicillin 2 g q6h: men, catheter, instability (Enterococcus); ertapenem: ESBL within 6 months; meropenem: ESBL + shock | 7 days (5–7 if FQ) |
| catheter-associated | as pyelonephritis + change catheter | as above | as above | 7 days |
skin, bone, and joint
| syndrome | first-line | β-lactam avoided | add / escalate if | duration |
|---|---|---|---|---|
| cellulitis (non-purulent) | cefazolin 2 g IV q8h → cephalexin 500 mg QID | vancomycin; clindamycin PO | no MRSA cover | 5–7 days |
| abscess / purulent | I&D ± TMP-SMX or doxycycline PO | same | vancomycin if systemically unwell | 5–7 days |
| necrotising infection | vancomycin + piperacillin-tazobactam + clindamycin 900 mg IV q8h + surgery | vancomycin + ciprofloxacin + clindamycin (ID) | stop vancomycin once MRSA excluded; IVIG if streptococcal TSS | until debridement complete |
| diabetic foot, severe | ceftriaxone + metronidazole, or pip-tazo | vancomycin + ciprofloxacin + metronidazole | debridement ± bone biopsy | 6 weeks if osteomyelitis |
| septic arthritis, native | cefazolin 2 g IV q8h + drainage; vancomycin + ceftriaxone if Gram stain negative and high risk | vancomycin | vancomycin if MRSA risk; ceftriaxone if gonococcal risk or Gram-negative rods | 2–4 weeks after drainage |
abdominal
| syndrome | first-line | β-lactam avoided | add / escalate if | duration |
|---|---|---|---|---|
| IAI, community, mild–moderate | ceftriaxone 1–2 g + metronidazole 500 mg q12h | ciprofloxacin + metronidazole | pip-tazo if shock/ICU; meropenem if prior ESBL | 4 days after source control |
| IAI, healthcare-associated severe | piperacillin-tazobactam | meropenem | vancomycin for E. faecium risk (linezolid/daptomycin if VRE colonised); echinocandin for tertiary peritonitis | 4 days after source control |
| cholecystitis / cholangitis | cefazolin (cholecystitis) if local E. coli cefazolin susceptibility is high, else ceftriaxone; ceftriaxone (cholangitis) | ciprofloxacin | pip-tazo if shock; ampicillin if post-ERCP/stent (Enterococcus) | 4 days after source control |
| SBP | ceftriaxone 1–2 g IV q24h + albumin | ciprofloxacin | pip-tazo/meropenem if nosocomial or on FQ prophylaxis with resistance | 5 days |
| C. difficile | vancomycin 125 mg PO QID | — | fulminant: vancomycin 500 mg QID + IV metronidazole | 10 days |
CNS and bloodstream
| syndrome | first-line | β-lactam avoided | add / escalate if | duration |
|---|---|---|---|---|
| meningitis, community | ceftriaxone 2 g IV q12h + vancomycin + dexamethasone | meropenem + vancomycin (ceftriaxone allergy) | ampicillin: >50, pregnant, immunocompromised; acyclovir: encephalitic features | organism-specific |
| undifferentiated sepsis | piperacillin-tazobactam 4.5 g IV q6h | meropenem, or ceftriaxone + metronidazole | vancomycin: shock, line, MRSA risk; meropenem: prior ESBL | source-specific |
| febrile neutropaenia | cefepime 2 g IV q8h or pip-tazo | vancomycin + aztreonam or ciprofloxacin | see febrile neutropaenia | until ANC recovery |
| suspected line infection | vancomycin | — | + pip-tazo/cefepime if septic, neutropaenic, or femoral line; remove line for S. aureus, Pseudomonas, Candida | organism-specific |
| S. aureus bacteraemia | vancomycin → cefazolin 2 g q8h or cloxacillin 2 g q4h if MSSA | vancomycin | ID consult, TTE | ≥14 days from first negative culture |
when to add anti-pseudomonal cover
Triggers:
- Pseudomonas isolated from the relevant site previously — respiratory isolate within the past year
- CAP: hospitalisation + IV antibiotics within 90 days (ATS/IDSA)
- HAP with any of: severe illness (hypotension, intubation, ICU transfer), ICU stay within 2 weeks, antibiotics during this admission, hospitalised ≥2 weeks
- VAP; neutropaenic fever; bronchiectasis/CF with prior colonisation
- healthcare-associated IAI with shock
Not triggers on their own: nursing home residence, dialysis, COPD, diabetes, “HCAP”.
Agents: piperacillin-tazobactam, cefepime, ceftazidime (no Gram-positive or anaerobic cover), meropenem; ciprofloxacin/levofloxacin are the only oral options. Ceftriaxone, cefazolin, ertapenem, amoxicillin-clavulanate, and moxifloxacin do not cover Pseudomonas.
Double Gram-negative cover (β-lactam + ciprofloxacin or aminoglycoside): septic shock with high MDR risk, VAP with shock, or local resistance >10% to the chosen β-lactam — stop the second agent once susceptibilities return.
when to add MRSA cover
Triggers:
- known MRSA colonisation or prior MRSA infection (especially within 1 year)
- purulent SSTI (oral TMP-SMX/doxycycline suffice if mild)
- septic shock without a source, or suspected line infection
- necrotising or cavitary pneumonia, empyema, post-influenza severe pneumonia
- people who inject drugs with SSTI or bacteraemia
- HAP/VAP with MRSA colonisation, or (IDSA) IV antibiotics within 90 days / ventilated HAP / unit MRSA >20% (HAP) or >10–20% (VAP)
- post-neurosurgical meningitis, prosthetic device infection
Not triggers: non-purulent cellulitis, CAP without risk factors, UTI.
Agents: vancomycin (bacteraemia, most sites), linezolid (pneumonia, SSTI; oral), daptomycin (bacteraemia/endocarditis; not pneumonia), TMP-SMX/doxycycline (SSTI PO).
Stop at 48–72 h if MRSA nasal PCR negative (pneumonia) and cultures negative.
redundant add-ons
- metronidazole on top of pip-tazo, a carbapenem, or amoxicillin-clavulanate — already anaerobic
- ampicillin on top of pip-tazo — already covers E. faecalis
- clindamycin for cellulitis already on a β-lactam — adds only C. difficile risk
- echinocandin for Candida in sputum or urine alone — colonisation
escalation when not improving at 48–72 h
Before broadening, check:
- diagnosis — PE, heart failure, drug fever, vasculitis, malignancy, gout
- source control — undrained collection, retained line/catheter, obstruction
- new hospital complication — C. difficile, line infection, VAP, DVT
- wrong organism — review cultures for resistance or unexpected pathogen
- dose/penetration — CNS, abscess, obesity, augmented renal clearance
Escalate only for a plausible uncovered organism or clinical deterioration. Fever alone at 48 h in an improving patient is not failure. Typical ladder: ceftriaxone → piperacillin-tazobactam → meropenem; add vancomycin only for an MRSA trigger.
coverage gaps and targeted therapy
What each agent misses, and the preferred agent once an organism is identified (ESBL, AmpC, Pseudomonas, VRE, Candida): targeted antibiotic therapy by pathogen.
IV → PO
Switch when afebrile, haemodynamically stable, improving, and absorbing. Near-complete oral bioavailability: fluoroquinolones, TMP-SMX, linezolid, metronidazole, doxycycline, clindamycin. Oral β-lactams achieve lower levels — fine for step-down in CAP, UTI, and SSTI; not first choice for bacteraemia step-down without ID input.