Antimicrobial Prophylaxis & Stewardship
A Step 2 CK–focused tour of antimicrobial prophylaxis across three arenas — surgical, post-exposure, and HIV opportunistic-infection — paired with stewardship decisions, including the high-yield traps of when NOT to give antibiotics (asymptomatic bacteriuria, viral URIs) and how to avoid C. difficile.
The big picture: prevent, don't just treat
Antimicrobial prophylaxis means giving antibiotics before infection is established, in patients whose risk is predictable. Boards test three arenas: (1) surgical prophylaxis to prevent surgical-site infection (SSI), (2) post-exposure prophylaxis after a defined contact (meningococcus, HIV, rabies, pertussis, tetanus), and (3) opportunistic-infection prophylaxis in the immunocompromised, keyed to CD4 count or transplant status. The paired discipline is antimicrobial stewardship — the narrowest effective drug, correct dose, and shortest duration, with de-escalation once cultures return.
The recurring Step 2 CK decision is whether to give an antibiotic at all, and if so, which one and for how long. Overtreatment is frequently the wrong answer: it breeds resistance and C. difficile colitis. Learn to spot the traps — treating asymptomatic bacteriuria or a viral URI — where the correct next step is often no antibiotic.
- Cefazolin is first-line for most clean/clean-contaminated cases — covers skin flora (S. aureus, streptococci).
- Timing: infuse within 60 min before incision; vancomycin and fluoroquinolones within 120 min (longer infusion time).
- Colorectal / appendiceal / GI cases: add anaerobic coverage — cefazolin + metronidazole, or cefoxitin — plus oral antibiotic bowel prep.
- MRSA colonization or severe β-lactam allergy: use vancomycin (add gram-negative coverage if needed).
- Redose intraoperatively for long cases (cefazolin ~every 4 h) or blood loss >1.5 L.
- Weight-based dose: cefazolin 2 g (3 g if ≥120 kg).
- Duration: a single preoperative dose is usually enough; stop within 24 h. Continuing antibiotics until drains/catheters are removed is a classic wrong answer and does not lower SSI.
Give prophylaxis only for a highest-risk cardiac condition undergoing a qualifying procedure.
Highest-risk cardiac conditions:
- Prosthetic valve or prosthetic valve-repair material (includes TAVR).
- Prior infective endocarditis.
- Unrepaired cyanotic CHD; repaired CHD with prosthetic material within 6 months, or with a residual defect adjacent to the patch/device.
- Cardiac transplant with valvulopathy.
Qualifying procedures:
- Dental work manipulating gingiva/periapical tissue or perforating oral mucosa.
- Respiratory-mucosa incision (e.g., tonsillectomy); surgery on infected skin/musculoskeletal tissue.
- NOT routine GI/GU procedures.
Regimen: Amoxicillin 2 g PO 30–60 min before. Penicillin allergy → cephalexin (only if the reaction was not anaphylaxis/angioedema/urticaria), azithromycin, or doxycycline. Clindamycin is no longer recommended (2021 update) due to C. difficile risk. MVP, bicuspid valve, HCM, rheumatic disease, prior CABG → no prophylaxis.
A 58-year-old man with a bioprosthetic aortic valve (placed 3 years ago) is scheduled for a tooth extraction. No drug allergies. Best next step?
Decision: A prosthetic valve is a highest-risk cardiac condition, and extraction manipulates gingival tissue = qualifying dental procedure → prophylaxis is indicated.
Next step: Amoxicillin 2 g PO 30–60 minutes before the procedure (single dose).
Contrasts to lock in:
- Same patient for a colonoscopy → no prophylaxis (GI procedure).
- Patient with only mitral valve prolapse or a bicuspid aortic valve → no prophylaxis, regardless of procedure.
- Penicillin-allergic → cephalexin (if non-anaphylactic), azithromycin, or doxycycline — not clindamycin.
Post-exposure prophylaxis at a glance
| Exposure | Organism | First-line prophylaxis |
|---|---|---|
| Close contact, meningococcal disease | N. meningitidis | Ciprofloxacin 500 mg once (or rifampin; ceftriaxone IM in pregnancy) |
| Household w/ vulnerable child, invasive Hib | H. influenzae type b | Rifampin |
| Close contact, pertussis | B. pertussis | Azithromycin (macrolide) |
| Needlestick / high-risk sexual, HIV | HIV | Tenofovir/emtricitabine + dolutegravir ×28 d, start <72 h |
| Animal bite, previously unvaccinated | Rabies virus | Wound wash + RIG + vaccine series |
| Tetanus-prone wound, <3 or unknown doses | C. tetani | Tdap + TIG |
| Sexual contact, early syphilis | T. pallidum | Benzathine penicillin G IM |
| Newborn of HBsAg+ mother / exposure | HBV | HBIG + vaccine |
A 78-year-old nursing-home woman has a screening urinalysis with pyuria and a culture growing >10⁵ CFU/mL *E. coli*. She is afebrile with no dysuria, frequency, urgency, or suprapubic pain, and is at her baseline mental status. Next best step?
Decision: This is asymptomatic bacteriuria, not a UTI. Do not give antibiotics — treatment doesn't help and drives resistance and C. difficile.
Treat asymptomatic bacteriuria ONLY in:
- Pregnancy — screen and treat (prevents pyelonephritis/preterm complications).
- Before a urologic procedure with expected mucosal bleeding (e.g., TURP).
Traps that do NOT warrant treatment: elderly, diabetic, or chronically catheterized patients without symptoms; cloudy or malodorous urine alone.
HIV opportunistic-infection prophylaxis — "200, 100, 50":
- CD4 <200 → PJP prophylaxis: TMP-SMX (also covers Toxoplasma). Alt: dapsone (check G6PD), atovaquone.
- CD4 <100 + positive Toxoplasma IgG → TMP-SMX (one drug covers both).
- CD4 <50 → MAC: azithromycin weekly (may be omitted if effective ART is started promptly, per current guidance).
Discontinue each once ART raises CD4 above the threshold for ~3 months.
Antibiotics most likely to cause C. difficile — the classic "4 C's":
- Clindamycin (classic, highest relative risk)
- Cephalosporins (esp. 3rd/4th generation)
- Ciprofloxacin / fluoroquinolones
- Co-amoxiclav (amoxicillin-clavulanate) / broad-spectrum penicillins
- The "D's": right Drug, Dose, Duration, De-escalation — after establishing the correct Diagnosis.
- Culture before you treat (blood, urine, wound), then narrow to the organism's susceptibilities.
- IV → PO switch once the patient is stable and tolerating oral: highly bioavailable agents include fluoroquinolones, metronidazole, linezolid, fluconazole, TMP-SMX, doxycycline, clindamycin.
- Shortest effective course: CAP ~5 days, uncomplicated cystitis 3–5 days, cellulitis ~5 days, pyelonephritis 5–7 days (fluoroquinolone).
- No antibiotics for: viral URIs, acute bronchitis, or asymptomatic bacteriuria (except pregnancy / urologic procedures).
- Procalcitonin can support stopping antibiotics in respiratory infection/sepsis.
- Penicillin-allergy delabeling restores narrow options and avoids broad-spectrum alternatives.
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