Cambridge University Hospitals - NHS
Professional training, consultancy and specialist services for decontamination testing and validation

Terms Index

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z



A dispersion of solid or liquid particles in a gas.


The integration of moist heat time and temperature to produce a defined lethal effect. For medical instruments disinfection requires 600 Ao units i.e. 80°C for 600 seconds (10 minutes) or 90°C for 60 seconds. The lowest usable temperature for disinfection is 65°C.

Air Detector

Device designed to detect the presence of non-condensable gases in a stream of steam and condensate or in the sterilizer chamber.

Aseptic technique

Procedures used to prevent microbial contamination of living tissue or sterile materials.

Automatic controller

Device that, in response to pre-determined cycle variables, operates the WD sequentially through the required stages of the cycle(s)/process.


Bacterial endotoxins

The shell of dead Gram-negative bacteria, the material is very poisonous and will give rise to toxic shock in high levels.  Also called pyrogens. 


The level of organic material adherent to a surface.  High bio-burden is normally associated with a high bacterial count. 


An agent that destroys microorganisms.


A matrix of microorganisms and extracellular material that attaches to surfaces that are bathed with liquids.



The set of operations that establish, under specified conditions, the relationship between values of a quantity indicated by measuring instrument or measurement system, or values represented by a material measure or a reference material, and the corresponding values realised by standards. 


This mark on equipment shows a declaration by the manufacturer that the product meets all appropriate provisions of the relevant legislations implementing certain European Directive(s). 


Choice Framework for local Policy and Procedures 01-01 – Parts A to E Management and decontamination of surgical instruments (medical devices) used in acute care.


The part of the washer disinfector in which the load is processed. 

Chemical additive

One or more chemicals added to the chamber and load of a washer disinfector during one or more stages of the process.

Chemical disinfection

Disinfection achieved by the action of one or more chemicals, the primary purpose of which is to be microbicidal. 


Obtaining and documenting evidence that equipment has been provided and installed in accordance with its specifications and that its functions within pre-determined limits when operated in accordance with operational instructions. 


Measurement of electrical energy between two points i.e Reverse Osmosis (RO) = >30µScm-1 @ 25°C.


An unwanted microorganism and/or foreign matter.

COSHH chemical exposure levels

MSDS – material safety data sheet, should be supplied with all potentially dangerous chemicals.

OES – operator exposure standard, now replaced by MEL.

MEL – maximum exposure level for glutaraldehyde 0.2mg/m-3 or 0.05pppm.

Critical site

Sterile tissue, cavity or bloodstream.

Cycle variables

The physical and chemical properties (e.g. times, temperatures, disinfectant concentration, pressures and flows) that influence the efficacy of the washing and processes. 


D value

For a micobiological process the extent of exposure under defined conditions which cause a 90% decrease in the viable population of a specified micro-organism. 


The combination of processes (including cleaning, disinfection and sterilization) that will remove or destroy contamination, preventing infectious agents reaching susceptible sites.


A system that will remove ionic chemicals from water e.g. salts. Activated granules are held in a container through which water is passed.  As the water flows through the granules chemicals are attracted to the granules from the water. The resulting water output will be free of contaminating chemicals. The granules have a limited life, but can be reactivated. Measurement of the outlet water conductivity will enable the granules to be changed when expired. Bacteria will grow in a deioniser; therefore a post-bacterial filter will be required to remove contaminating organisms.

Direct contact

Close or intimate contact that results in the spread of infectious agents.


A process that reduces or completely eliminates all pathogenic microorganisms except spores. See also high-level disinfection.

Divalent cations

Ions with a double positive charge.


Device provided as a means of closing and sealing the chamber.

Droplet transmission

Transmission of infectious agents in droplets from respiratory secretions.

Duty of care

An obligation of all participants in the workplace to ensure the health and safety of all persons in the workplace.


Endogenous infection

An infection caused by organisms from a person's own body.

Endovascular surface

The surface of the endothelial cells that lines blood vessels.

Environmental mycobacteria 

Non-pathogenic organisms found widely in the environment, particularly in wet conditions, e.g. in the spout of taps.  These organisms frequently contaminate washer disinfector final rinse water and may find their way into biopsy specimens.  When these specimens are examined environmental mycobacteria cannot be distinguished from Mycobacteria tuberculosis and misdiagnoses may occur.


Organic compounds that have the property of breaking down other organic materials into their constituent parts that is digesting proteins, fats and sugars to a soluble form.  The main enzymes used for cleaning are:

  • Proteases – break down proteins
  • Lipases – break down fats and grease
  • Amylases – breakdown sugars and carbohydrates

Enzymes are not broken down during the digestion process, but continue to work over long periods, but they do need suitable conditions including time and warmth.

Exogenous infection

An infection caused by organisms from a source external to the person.



A quantity, measured in minutes, used to determine the efficacy of an operating cycle and equivalent to a continuous period at a temperature of 121°C.

Faecal-oral transmission

Mode of transmission of pathogens that enter the body via the mouth and are excreted in faeces.

Fail safe

Attribute of washer disinfector design, component or its associated services that minimises a possible safety hazard.


Recognition by the automatic controller that the pre-set cycle variables for the washer disinfector cycle have not been attained. 


An inanimate object that can be the source of an infection.



Inflammation of the gastrointestinal tract.

Gram negative bacteria

A bacterium that stains red with the Gram stain.

Gram positive bacteria

A bacterium that retains the blue colour of the Gram stain.

Gram stain

A procedure for staining bacteria that is the first step in classifying and identifying them.



Something with the potential to cause harm.

Hazardous substance

A substance designated as hazardous because it is listed as such or it meets the classification criteria laid down by the National Occupational Health and Safety Commission (NOHSC).

Health care associated infection

An infection that is acquired in a hospital or other health care establishment.

High-level disinfection

Is the minimum treatment recommended for reprocessing a device or item of equipment for use in a semi critical site, if it cannot be sterilised. It involves killing all microorganisms, with the exception of high numbers of bacterial spores.

Horizontal transmission

Transmission of an infectious agent from one person to another.



Infections that arise as a result of health care interventions.


The exposure of a person to material that is antigenic but not pathogenic, to make them immune to a particular microorganism.

Indirect contact

Contact with contaminated surfaces or objects that results in spread of infectious agents.

Installation test

Series of checks and tests performed after installation of the washer disinfector in the place of use.

Intermediate-level disinfection

Kills tubercle bacilli, vegetative bacteria, most viruses and most fungi but not necessarily bacterial spores.


Originating from or on the inside of an organ or part.

In-use life

The maximum number of days a biocide may be reused provided it is still above the minimum effective concentration (MEC).

Ionic contamination

Water containing salts and other ionic chemicals that will give a high conductivity level.  Ionic contamination needs to be removed to generate pure water by the use of reverse osmosis or deionisation. 



A collective term used to describe all the goods equipment and materials that are put into a washer disinfector at any one time for the purpose of processing it by an operating cycle. 

Low-level disinfection

May kill most vegetative bacteria, some fungi and some viruses.


Medical Device

See EN 46001:1997

Mesophilic bacteria

Bacteria normally found in the environment and will grow at temperatures from 15°C to 40°C.  This group of organisms are detected during cultivation for Total Viable Count of the final rinse water. 

Minimum effective concentration (MEC)

The lowest concentration of active ingredient necessary to meet the label claim of a reusable high level disinfectant/sterilant.


The measurement of physical variables, such as the function of the automatic controller to check the attainment, or otherwise, of the pre-set cycle variables essential to the efficacy of the operating cycle. 


Non-critical site

Intact skin.

Normal flora

The microorganisms that inhabit the human body without causing disease.


Operating cycle

The complete set of stages of the process that is carried out in the sequence as regulated by the automatic controller. 



Capable of causing disease.

Peak (Peak Limitation)

A maximum or peak concentration of a hazardous substance to which workers may be exposed.

Performance qualification

Obtaining and documenting evidence that the equipment as commissioned will produce acceptable product when operated in accordance with the process specification.

Permit to Work

This should ensure the formal removal of equipment from, and return to, service and will provide certification of acceptance by the user.

PLC processor

Programmable logic controller used to control a washer disinfector.

Portal of entry

The point of entry of an infectious agent to the body.

Potable water

Water of a standard to drink and is subject to meeting the requirements of “Water Supply (Water Quality) (England) Regulations 2000”, Statutory Instrument 2000 No 3184, Incorporate 98/83/EC.  Bacterial count not above 0/100ml. 


The chemical strength or efficacy.

Process residue

Traces of chemicals left on processed items carried over from the disinfection or cleaning stages. 

Product compatibility

Ability of the washer disinfector operational cycle to achieve the intended results without detrimental effect on the product or its intended use. 

Prophylactic antimicrobials

An antibiotic administered in order to prevent the occurrence of an infection.


The sudden rise in the incidence of a disease due to pseudoinfection.


The presence of microorganisms in a stain or culture of a body fluid or tissue that does not correlate clinically with the signs or symptoms of infection characteristic of that microorganism.


Reference load

Specified load made up to represent the most difficult combination of items to be processed in a particular washer disinfector operational cycle.

Reservoir of infection

A site where organisms persist and which acts as a continual source of infectious agents.

Resident flora

The microorganisms that normally inhabit that particular site of the human body without causing disease.

Reverse Osmosis

A method of producing water by ultra filtration of water under high pressure to remove large molecules.  The system operated in the reverse to osmosis, when water molecules move from the low solute to the high solute.  The water recovered from RO systems will be in the order of 5% to 15%, they are therefore expensive to operate.


The likelihood that harm might result because of a hazard.

Risk assessment

A process to assess and prioritise any risks to health and safety associated with identified hazards.

Risk management

A process of systematically identifying hazards, assessing and controlling risks, and monitoring and reviewing activities to make sure that risks are effectively managed.

Routine test

Series of tests intended to be performed by the user, or their representative, at various pre-determined intervals to demonstrate that the performance of the washer disinfector remains within the limits established during type/works/installation.


Semicritical site

Intact mucous membranes or non-intact skin.


A clinical syndrome resulting from the presence of organisms in the blood stream.


The change of a serological test from negative to positive, indicating the development of antibodies in response to infection or immunisation.


Different antigenic strains of a microorganism.

Short term exposure limit

Airborne concentrations averaged over a period of 15 minutes which should not be exceeded and should not be repeated within 60 minutes or more than four times per day.

Source of infection

The person or object from which an infection is acquired.


A specialised type of resting Gram positive bacterial cell, with a thick coat. Highly resistant to heat and chemicals.


Ability to kill bacterial spores.

Standard precautions

Work practices which require everyone to assume that all blood and body substances are potential sources of infection, independent of perceived risk.

Steam generator

Vessel designed to contain water and a heating system (e.g. a steam coil or a fully immersed electric element) which is used to heat water to its vapour state.

Steam under pressure sterilisation

The most efficient and reliable form of sterilisation of instruments and equipment. The latent heat of condensation is transferred to the load causing it to heat rapidly resulting in coagulation of protein structures, thus inactivating infectious agents.


See EN 556, HTM 01-01 Pt’s A-E


The use of physical and/or chemical procedures to completely eliminate or destroy all forms of microbial life (including resistant bacterial spores).


The equipment (sealed chamber) used to achieve steam under pressure sterilisation.

Sterilising agent

The medium used for the sterilising process (physical/chemical).

Sterility assurance level

The probability of a viable organism being present on an item after sterilisation.

Surrogate device

An item made up to have similar characteristics for cleaning purposes as the actual equipment being cleaned.  For instance, a collection of PTFE tubes of varying diameters to simulate an endoscope. 

Susceptible (as in susceptible sites)

Vulnerable or lack resistance to an infection.



A process vessel, integral to the washer disinfector, designed to hold solutions during processing. 

Test soil

Substance used to test the washing efficacy of the washer disinfectors. 

Thermal disinfection

Disinfection achieved by the action of moist or dry heat. 

Time weighted average

An average concentration over an 8-hour day for 5 working days.

Total viable count (TVC)

A method of enumerating the number of live bacteria present in the final rinse water measured per 100ml.  A sample of at least 200ml is required. 

Type tests

Tests performed by the manufacturer to establish the working data of a particular machine design.



A frequency above the human ear's audibility limit of about 20,000 hertz.


Vegetative bacteria

Bacteria that are in the growth and reproductive phase, ie. not spores.

Vertical transmission

Transmission of an infectious agent from mother to foetus.

Viable micro-organism

Micro organisms, including viruses, which are capable of multiplication under specified culture conditions. 


Washer disinfector

Machine intended to clean and disinfect medical devices and other articles used in the context of medical, dental, pharmaceutical and veterinary practices.  When used for flexible endoscopes it is known as an EWD (Endoscope Washer Disinfector).

Window period

The period immediately after a person is infected with an agent, during which the infection is not detectable by laboratory tests, although the person may be infectious.

Works test

A list of tests for Washer Disinfectors and Sterilizers performed by the manufacturer at their premises to demonstrate compliance with a pre-determined specification. (BS EN ISO 17665,BS EN ISO 285, HTM Pt A,B & C for sterilizers) & (BS EN ISO 15883 Pt 1-5, HTM Pt A,B & D for WD’s & EWD’s)  


Z value

For a thermal microbicidal process the change in temperature required to cause a tenfold change in D value.