Swab testing is the preferred method of sampling because it comes in direct contact with the sampling surface, allowing for the detection of substances that are not easily rinsed off or soluble.

It’s also preferred because it can be used to test the cleanliness of difficult areas and should pick up hard to remove contamination.

These swabs can be used to determine both microbial and chemical contaminants.

Entire Equipment Surface

Swab sampling does not cover the entire equipment surface area so sites to be swabbed must be chosen with care.

It is important that, at a minimum, the swab sites represent worst-case locations on the equipment as well as representative samples of the area.

Sample site selection should be based on areas that are deemed hardest to clean.

Criteria include:

• Equipment complexities (areas of different geometry that are likely to be difficult to clean)
• Areas of different materials of construction
• Ability to access and reproducibility of the sample

Riboflavin Clearance Test

One common method of determining the hardest to clean areas is to perform a Riboflavin Clearance Test.

This test is performed by spraying the internal surfaces with an aqueous solution of Riboflavin and allowing it to dry.

Visual examination using ultraviolet light illumination is used to confirm that all internal surfaces are coated (the riboflavin fluoresces under Ultraviolet light).

Internal Surfaces

Internal surfaces are given a (usually brief) clean and then are again visually examined under ultra-violet light. The “hardest to clean” areas can then be identified by those areas that still show the presence of Riboflavin.

Those areas, along with other chosen representative areas, should be designated as areas for swab sampling.

The result from the swab tests are then extrapolated to account for the total product contact surface area. This calculation makes it possible to make a worst-case determination of potential carryover into subsequent product.

The number of sites to sample will be based on the above considerations as well as on the overall dimensions of the equipment.

The Advantages and Disadvantages of Swab Sampling

Advantages:

• They dissolve and physically remove samples. It is possible to sample insoluble residues due to the physical action associated it
• Economical
• Allow sampling of the defined area
• Usable on a variety of surfaces
• And Applicable to active ingredients, microbial and cleaning agents

Limitations

• They may introduce fibres and material to the sampling area
• Sometimes the design of the swab may inhibit the recovery and specificity of the method
• They are difficult to use in crevices, pipes, or large vessels
• It is necessary to ensure that extractables of the swab do not interfere with the sampling method
• Due to the nature of this method, which employs physical forces as well as chemical forces, it may be necessary to perform sampling technique evaluations
• Swabbing efficiency (% recovery) for the swabbing method must be determined
• And The swab may be further contaminated during transit