Disinfectants and Disinfection Levels
On this page
Direct answer
Disinfectants reduce the microbial load on surfaces and instruments, and the level chosen must match the item's risk under the Spaulding classification. High-level agents such as 2% glutaraldehyde kill all organisms except high numbers of spores, while sodium hypochlorite and alcohols are intermediate-level agents. Disinfectant performance depends on concentration, contact time, organic load and in-use testing.
What you must remember
- Spaulding classification: critical items enter sterile tissue and need sterilisation, semi-critical items touch mucous membranes, and non-critical items touch only intact skin.
- High-level disinfection: kills all organisms except large numbers of spores, using agents such as glutaraldehyde, ortho-phthalaldehyde and peracetic acid.
- Intermediate level: kills mycobacteria, fungi and most viruses but not reliably spores, as with hypochlorite and alcohol.
- Low level: kills most vegetative bacteria and some viruses but not mycobacteria or spores, as with quaternary ammonium compounds.
- Hypochlorite: a cheap, fast agent that organic matter inactivates, so blood and pus must be cleared before disinfection.
- Alcohols: 70% ethanol or isopropyl alcohol inactivate vegetative bacteria and enveloped viruses, but they cannot sterilise.
- Glutaraldehyde: a high-level agent that irritates the airways, so it must be used in well-ventilated areas.
- In-use testing: test strips and titration confirm that a working solution still has the minimum effective concentration.
Disinfection levels and example agents
Each level is defined by the organisms it reliably kills, and the agent is then matched to the level required for the item. The table sets out the three levels and the agents most often named in examinations.
| Level | Organisms killed | Example agents |
|---|---|---|
| High | All except high numbers of spores | Glutaraldehyde 2%, ortho-phthalaldehyde |
| Intermediate | Mycobacteria, fungi, most viruses | Hypochlorite, 70% alcohol |
| Low | Most vegetative bacteria, some viruses | Quaternary ammonium compounds |
Efficacy depends on the agent's concentration, the contact time and the amount of organic material, such as blood or pus, that dilutes or inactivates it. Each product has a manufacturer-stated contact time that must be respected, and an exhausted solution gives false reassurance even when it looks unchanged.
In-use testing checks that a working solution still contains at least the minimum effective concentration. Glutaraldehyde strength is checked with test strips, and hypochlorite strength by titration, and the preparation date is recorded so that expired working solutions are discarded.
Where the exam tries to catch you
Disinfectant questions usually give an item or an organism and ask which level or agent is correct.
- Trap: saying alcohol can sterilise an instrument Correct: alcohol is an intermediate-level disinfectant that does not kill spores, so it cannot sterilise.
- Trap: choosing quaternary ammonium compounds for a tuberculosis spill Correct: they are low-level agents that do not kill mycobacteria, so hypochlorite is preferred.
- Trap: wiping a bronchoscope with a low-level disinfectant Correct: it is a semi-critical item, so a high-level agent such as glutaraldehyde is required.
Frequently asked questions
Which disinfection level is required for a bronchoscope?
A bronchoscope is a semi-critical item because it touches mucous membranes, so it requires high-level disinfection under the Spaulding classification. Agents such as glutaraldehyde or ortho-phthalaldehyde are used, and the scope is then rinsed with sterile water.
Why does organic matter reduce the activity of hypochlorite?
Organic matter such as blood or pus reduces the activity of hypochlorite because the chlorine reacts with proteins and other organic material before it reaches the microbes. Surfaces soiled with blood must be cleaned first, and a stronger solution is used for blood spills.
Which disinfectant is suitable for a Mycobacterium tuberculosis spill?
Sodium hypochlorite is suitable for a Mycobacterium tuberculosis spill because it is an intermediate-level agent that reliably kills mycobacteria. Quaternary ammonium compounds are unsuitable because they are low-level agents that do not kill mycobacteria.
What does in-use testing of a disinfectant check?
In-use testing of a disinfectant checks that the working solution still contains at least the minimum effective concentration. Test strips for glutaraldehyde or titration for hypochlorite confirm the strength, so that a weakened solution is not used on instruments or surfaces.
Why is 70% alcohol preferred over absolute alcohol as a disinfectant?
Seventy percent alcohol is preferred because the water in the mixture helps it penetrate and denature microbial proteins more effectively than absolute alcohol. Absolute alcohol coagulates the outer proteins and can leave inner microbes alive.