SN5 SHORT NOTE 5M ★ High-yield

Define chemoprophylaxis. Describe the principles of surgical and medical antimicrobial prophylaxis.

Pharmacology · 2nd Professional — Para-Clinical

Definition and Principles

Chemoprophylaxis is the use of an antimicrobial to prevent infection, rather than to treat an established one.

It is justified only when the risk of infection is high, the consequence is serious, the likely organism is predictable, an effective and safe drug exists, and the period of risk is defined and short
Prophylaxis directed at "any infection" fails — it selects resistant organisms and merely changes which infection occurs

Surgical Prophylaxis

Principle	Detail
Timing	A single dose within 60 minutes before incision, so that tissue levels are adequate when contamination occurs. Given after the incision it is much less effective; given hours before, levels have fallen
Duration	A single dose suffices for most procedures; continuing beyond 24 hours adds resistance, cost and C. Difficile risk without reducing infection — the commonest error in surgical practice
Choice	Cefazolin for most clean and clean-contaminated surgery; metronidazole added for colorectal and gynaecological procedures; vancomycin where MRSA is prevalent
Repeat dosing	If the operation is prolonged (beyond two half-lives) or blood loss is heavy
Not a substitute	Asepsis, skin preparation, normothermia, glycaemic control and surgical technique matter more

CLINICAL PEARL

Prophylaxis given after the incision is largely wasted, and prophylaxis continued after the operation is entirely wasted. The purpose is to have drug present in the tissues at the moment of contamination. Once the wound is closed, further doses cannot prevent an infection that has already been seeded — they only select resistance. The single most valuable improvement in most hospitals is stopping prophylaxis at closure.

Medical Chemoprophylaxis

Situation	Drug
Rheumatic fever — secondary prophylaxis	Benzathine penicillin every 3–4 weeks for years; the single most important prophylactic programme in India, preventing progressive valve damage
Meningococcal contacts	Ciprofloxacin (single dose), rifampicin or ceftriaxone
Tuberculosis contacts and latent infection	Isoniazid for 6 months, or rifampicin-isoniazid regimens; essential before anti-TNF therapy and in HIV
Infective endocarditis	Amoxicillin before dental procedures — now restricted to the highest-risk cardiac lesions only, as routine prophylaxis was found unjustified
HIV — opportunistic infection	CO-trimoxazole for Pneumocystis and toxoplasmosis when CD4 is low
Post-exposure prophylaxis	HIV (antiretrovirals within hours), hepatitis B (immunoglobulin and vaccine), rabies, tetanus
Malaria	Doxycycline, atovaquone-proguanil, mefloquine for travellers
Neonatal ophthalmia; recurrent urinary infection; asplenia	Topical antibiotic; low-dose nitrofurantoin; penicillin and vaccination in asplenia

WHY Prophylaxis Fails or Harms

Wrong timing — too early or too late relative to contamination
Excessive duration — the commonest fault, adding harm without benefit
Too broad an agent — selecting resistant flora
Used to compensate for poor technique or asepsis, which it cannot do
Superinfection — particularly C. Difficile and candidiasis
False reassurance — masking early infection and delaying diagnosis

Applied Aspects

Audit surgical prophylaxis in your own unit — timing and duration are the two measures that most reliably improve, and both are easy to record
Benzathine penicillin prophylaxis for rheumatic heart disease is transformative and under-delivered in India; the barriers are supply, injection pain and follow-up rather than efficacy
Do not give antibiotics "to cover" a procedure that does not need it, such as routine dental work in a low-risk patient
Written protocols reduce variation more effectively than exhortation, and a default stop order at 24 hours is among the most effective single stewardship measures
Prophylaxis is not a substitute for vaccination, which prevents disease without selecting resistance

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