Control swabs aren’t meant to be used for patient testing

An Instagram post shares an image of what appears to be two SARS-CoV-2 antigen (lateral flow) tests produced by ACON Laboratories. One of the tests is labelled as a “positive control swab”, and another is labelled as a “negative control swab”. The picture also has a caption which says that the picture was taken “secretly” by an on-shift nurse, whose name has been concealed for “obvious reasons”. 

It’s not clear what is meant by comparing the images in this way, but it could be taken to imply that the results of lateral flow tests are already set, and that the tests’ results are therefore unreliable or untrustworthy.

As described by Reuters fact check, control tests are used to make sure that tests are being conducted correctly and the tests themselves are working properly. They are not used to swab patients or people being tested. 

For example, a negative control swab shouldn’t test positive, so if it does something has gone wrong with how the tests were carried out, or with the tests themselves, and therefore they should not be used without checking what the error is. Similarly, a positive control test should return a positive test result. If it does not, there is a problem with the tests, or the testing method.

Control tests can be used to verify “proper performance of the procedure and reagents (a substance used in the chemical reaction in the test)”. If a negative control swab does not produce a negative result, or a positive control swab does not produce a positive result, it indicates that either the test was not carried out properly or that the test reagents were not “functioning properly”.

ACON Laboratories confirmed to Full Fact that control swabs are not provided in the kit for at-home lateral flow testing. However, they are included in testing kits for professional use, such as at testing centres. 

The leaflet included in the package says that positive and negative control swabs are “supplied with each kit”, and should be used:

1.  When a new lot of tests are used and/or when a new operator performs the test.
2.  At periodic intervals as dictated by local requirements, and/or by the user’s quality control procedures. 

Talking to French fact checkers AFP, Liliane Grangeot-Keros, a virologist and permanent secretary of the French National Pharmacy Academy, said the control swabs in the Covid-19 test kits are used to "check whether the technology has worked". 

Images of similar control swabs in test kits from the manufacturer Abbott have also been shared on social media. The Abbott kit instruction leaflet says that control swabs should not be used for patient testing. 

A spokesperson for Abbott added that “these control swabs are not used as part of patient testing” and explained that this is “standard” for “most rapid diagnostic tests throughout the world”.