Disinfection
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Direct answer
High-level disinfection — 2 per cent glutaraldehyde for about 20 minutes of contact, or peracetic acid — destroys mycobacteria, fungi, enveloped and non-enveloped viruses, but not necessarily spores, and is the standard demanded by endoscopes that touch mucous membranes. Intermediate-level agents (70 per cent alcohol, iodophors, phenolics) kill mycobacteria and most viruses but spare spores; low-level agents (quaternary ammonium compounds) do not reliably kill mycobacteria or non-enveloped viruses. For a blood spill carrying hepatitis B or HIV risk, the classroom-to-clinic answer is 1 per cent sodium hypochlorite left in contact for half an hour before wiping. An antiseptic is simply a disinfectant diluted enough to be safe on living tissue — chlorhexidine and povidone-iodine hold that dual registration.
What you must remember
- Glutaraldehyde 2 per cent: about 20 minutes for high-level disinfection, up to several hours as a sterilant; irritant to eyes and airways, so ventilation and a closed container matter.
- Alcohols (60–80 per cent) act fastest by denaturing protein, have no sporicidal activity, and are inactivated by organic matter — clean the wound before you paint it.
- Chlorhexidine shows persistence (residual activity for hours) and is odourless, but is ototoxic and neurotoxic in the middle ear and central nervous system — never near the meninges or tympanic membrane.
- Povidone-iodine is broad-spectrum but partly inactivated by blood and sputum; iodophors release iodine slowly.
- Sodium hypochlorite: 1 per cent for blood spills; bleaching powder and chlorination disinfect water supplies; corrodes metals and loses activity with organic load.
- Quaternary ammonium compounds (benzalkonium, cetrimide) are low-level, better against gram-positive organisms — and Pseudomonas can multiply in stock solutions of them, a viva favourite.
- Formaldehyde: 10 per cent formalin fixes and decontaminates specimens; formaldehyde vapour with potassium permanganate has fumigated operating theatres, though safer alternatives are now preferred.
- Laboratory evaluation: the Rideal–Walker and Chick–Martin tests give a phenol coefficient; the Kelsey–Sykes in-use test checks whether a working-strength solution in an actual ward bucket is still effective.
Choosing the agent for five real surfaces
Reason from contact time, organic load and toxicity. The bronchoscope used on a tuberculosis suspect needs glutaraldehyde 2 per cent for at least 20 minutes — alcohol cannot be trusted with mycobacteria on a semi-critical device, and autoclaving would melt it. A blood spill in the dialysis unit gets gloves, mechanical removal of the bulk, then 1 per cent hypochlorite for 30 minutes; glutaraldehyde would be wasted expense. Pre-operative skin painting pairs 7.5 per cent povidone-iodine scrub with paint, or 2 per cent chlorhexidine in 70 per cent alcohol — the alcohol accelerates killing while chlorhexidine persists through the surgery. For mucosa, dilute the iodine and abandon chlorhexidine (neurotoxicity). The ward floor, a non-critical surface, takes a phenolic or a quat; here the question that matters is the in-use test — is the bucket solution in the corridor still working, or has it been topped up for a fortnight?
The exam's favourite trap
Read the word "spores" in the stem before answering. A question asking which agent kills spores is fishing for glutaraldehyde as a sterilant (hours of exposure) or peracetic acid — alcohol, chlorhexidine, iodophors and quats all fail. The second trap is level-matching: "bronchoscope after use on a sputum-positive tuberculosis patient" is the high-level disinfection stem, while "stethoscope and blood-pressure cuff" is the low-level stem, and answering the same agent for both loses the mark. Third is the phenol coefficient question — it compares bactericidal efficiency against phenol, not a measure of toxicity or tissue safety. And remember the classical Indian-scenario twist: multidose vials and reused quat containers have grown Pseudomonas in hospital outbreaks, which is exactly why the in-use test exists.
Frequently asked questions
Which disinfectant is preferred for bronchoscopes and endoscopes?
Two per cent glutaraldehyde for about 20 minutes for high-level disinfection, or peracetic acid in automated reprocessors; total immersion and full contact time are mandatory, followed by rinsing and drying.
How is a blood spill containing hepatitis B virus disinfected?
Wearing gloves, absorb the spill, then apply 1 per cent sodium hypochlorite for 30 minutes before cleaning; alcohols are inactivated by the organic load of blood itself.
What does the phenol coefficient measure?
The dilution at which a test disinfectant kills a standard organism, compared with phenol under identical conditions — a Rideal–Walker or Chick–Martin laboratory exercise, higher values meaning greater potency.
Why can quaternary ammonium solutions themselves cause infection?
Gram-negative bacilli, classically Pseudomonas, can survive and multiply in stock quat solutions, turning the disinfectant into a reservoir; the Kelsey–Sykes in-use test detects such contamination.
Which skin antiseptic is avoided before middle-ear surgery?
Chlorhexidine — it is ototoxic and neurotoxic on contact with the middle ear and meninges; povidone-iodine is the safer choice for procedures near the tympanic membrane.