VALIDATION OF THE SURFACE ACTIVITY OF DISINFECTANTS
It is generally accepted that although the Time-Contact Test provides good evidence for the activity of a particular agent, the method does not take into account the physical factors that may effect the performance of the agent when applied to actual surface materials such as Stainless Steel, Vinyl, Ceramics, PVC, Perspex, Glass. There are a number of factors that may affect the performance of the disinfectant when in normal use. A number of them such as dirt, temperature and pH and other factors that may need to be taken into account are as follows.
a) The chemical nature of the materials.
The material itself may react with the disinfectant causing unwanted by products. The activity of the disinfectant may be adversely effected by such materials. The activity of the disinfectant may also be affected by other cleaning agents. Advice should be obtained from the supplier regarding chemical compatibility with certain agents e.g. other disinfectant products that may be used on the same surface. Agents that are alcohol based may also cause the surface material to dry out. This may cause the material to become brittle and cracks may develop. Care must be taken to avoid the use of alcohols on certain synthetics and organic materials.
b) The porosity and durability of the materials.
Materials that are porous may enable the organisms to embed themselves into the material. The act of wiping a surface even in the absence of a disinfectant can often help to remove microorganisms. When porous materials are present it may become difficult to remove organisms by this physical action. In this situation, additional care must be taken when cleaning the surface to ensure that the materials remain adequately wetted with the disinfectant for the recommended contact time. Additional information regarding the compatibility of the disinfectant agents with certain materials can often be obtained from the supplier. The following guidelines may be used to develop local SOP is giving a detailed description of the requirements for validation of the method.
Test Method For Validation Of Surface Activity.
Test Surface Preparation The purpose of this procedure is to evaluate the ability of a disinfectant to clean a surface material. The test is designed to show that a known bioburden will be eliminated from a surface when wiped with the disinfectant. Additional information can be gained regarding the variation in cleaning between different materials. Each material to be tested should be prepared as a square or coupon measuring anywhere between 45 mm2 ñ95 mm2 . It should be sterile for the purpose of the test. This can be achieved by autoclaving where practicable using an appropriate decontamination cycle. Alternatively the material can be cleaned using a 70% alcohol solution and allowed to dry. Test organisms should be selected as appropriate. The list of organisms detailed previously can be used as a guideline. Representative environmental isolates must also be employed in the test.
Inoculum Preparation
The test organisms should be prepared in a suitable diluent such as 0.1% peptone solution. The viable count should be >108 cfu/drop. (>107 cfu/drop for Fungi and >106 cfu/drop for bacterial spores). The count should be verified by preparing log dilutions of the test suspension. The Miles and Misra technique or membrane filtration of 1 ml aliquots of the dilutions can be used to determine the viable count.
Test Detail:
To perform the test add 30-100µl of the test organism onto the surface of 3 separate pieces of the material. Spread the inoculum over the surface using a disposable loop or spreader and allow to dry at room temperature (approximately 20°C). The appropriate test disinfectant is then applied to the surface by spraying or by pouring the agent directly onto the surface. NB. The material must remain wetted for the recommended contact time.
Recovery Procedure:
There are basic methods that can be used for recovery of the organisms,
Method A- Bioburden Washing Method
The squares are placed into separate irradiated polythene bags. A 250ml aliquot of a neutralizing solution such as 0.1% Peptone 2% Tween 80/0.3% Lecithin is then poured over the material. The contents are then agitated gently for up to 3 minutes. A set of serial dilutions should be prepared using a suitable diluent such as 0.1% Peptone solution. A 1ml aliquot of the 10-2, 10-3 and 10-4 dilutions should be filtered through a 0.45µm membrane and rinsed with 250ml of a suitable diluent such as 0.1% Peptone solution. Place the membrane onto a suitable neutralizing media such as Eugon Neutralizing Agar (ENA) or TSA containing 2% Tween 80/0.3% Lecithin. The plates are incubated at 30 ñ 35°C with daily inspection. Fungi should be incubated at 20 ñ 25°C.
Method B- Surface Swab Technique.
A sterile swab should be used to recover the organisms from the surface of the material. The swab should then be transferred to 10mls of a suitable neutralizing solution such as 0.1% Peptone/ 2% Tween 80/0.3% Lecithin. The solution is vortex mixed to free the cells. A set of serial dilutions should be prepared using a suitable diluent such as 0.1% Peptone solution. A 1ml aliquot of the 10-2 , 10-3 and 10-4 dilutions should be filtered through a 0.45µm membrane and rinsed with 250ml of a suitable diluent such as 0.1% Peptone solution. Place the membrane onto a suitable neutralizing media such as Eugon Neutralizing Agar (ENA) or TSA containing 2% Tween 80/0.3% Lecithin. The plates are incubated at 30 ñ 35°C with daily inspection. Fungi should be incubated at 20 ñ 25°C. The number of viable cells (B) remaining after the recommended contact time with the disinfectant can be determined.
Test Drying Control
The procedure should be repeated with the omission of the disinfectant. The dried inoculated plate is placed directly into the irradiated bag and processed as detailed previously. NB. The level of recovery from the surface will determine the actual number of viable cells present (A) at time zero. A proportion of the inoculum will be lost by the process of natural decay during the drying stage. The Log reduction can be determined for the challenge organism. Log Red n = A (number of cells at T= 0) ñ B (number of cells at T= t) A result of no recovery of the test organism from the surface material will provide evidence for the effectiveness of the agent on the test surface.
VALIDATION OF THE SHELF LIFE OF DISINFECTANTS
The methods detailed previously may be used to determine the shelf life of the in use disinfectant solutions. The efficacy of the disinfectant should be evaluated after manufacture and the tests repeated after the solution has been stored for its recommended shelf life. Solutions of the disinfectant should be prepared to simulate the manufacture procedures that are employed when in use. If the disinfectants are stored in Polycarbonate containers during routine use , the same type of container must be employed for any shelf life evaluation studies. A comparison of the efficacy of the disinfectant after manufacture and following its nominal storage period will provide an indication of the stability of the disinfectant solution.