15/16 February 2024
Two sampling techniques are generally used in cleaning validation: the rinse test and the swab test. The following are notes on potential issues with the swab test.
In general, the use of swabs for the detection of chemical or microbiological residues is possible. Usually, the recovery of microbiology swabs is lower compared to agar plates for microbial surface sampling, only the swab for the detection of chemical residues (product or detergent residues) is addressed in the following.
The swab sampling process itself, in addition to the analysis, which will not be discussed here, has two critical steps that must be optimized considering surface materials and residues:
In addition to a reproducible swab technique, the material of the swab itself is crucial. Using cotton made swabs is no longer "state of the art". These can themselves release particulates on the product contact surface, disintegrate during the swab process depending on the swab technique, or fail to release the residues into the extraction solution. Modern swabs have an abrasive material which, in addition to dissolving the residues by using a moistened swab head, also removes them mechanically from the surface. In this case, the correct pressure, which can be checked if the swab shows a slight bending, is important. In general, rectangular sampling surfaces sampled in a check pattern are preferable. To ensure a correct sampling area, it is advisable to swab in overlapping lanes and sample at least the specified surface area. More surface area swabbed means a "worst-case" and ensures that a falsely low result is generated due to a smaller surface area swabed. After swabbing, as little extraction solution, which is used for pre-moistioning the swab head, as possible should remain on the surface. In special cases it may be useful to swab the surface again with a second dry swab. In addition, it should be noted that after cleaning processes with warm or hot media or a drying step, the equipment surface needs to cool down to room temperature.
Depending on the swab technique, the transfer of the residues is carried out sequentially in step 2. Here, the swab is swirled in the extraction solution between the individual swab steps. After sampling, the swab is typically transferred into the extraction solution and left there until analysis. Prior to this, additional enhanced extraction can be achieved using a vibratory shaker or an ultrasonic bath. Sampling recovery can be improved by optimizing the extraction solution. This mainly depends on the type and condition of the residue. However, the extraction solution itself should also be easily cleanable and furthermore not interfere with the analysis.
Author: Dipl.-Ing. (FH) Robert G. Schwarz
PS. During the ECA Live Online Training Cleaning Validation with Pre-Conference Workshop: Impact of Annex 1 Revision on Cleaning Validation on March 21-23, the author will go into detail about sampling techniques for cleaning validation.