A Facebook post with a picture of a positive control for a COVID-19 test misleadingly describes it as a positive test kit and suggests it may be “preloaded with the virus”.

The post (here) shows a  photo of two packages. One has a biohazard symbol and says “COVID-19 Ag RAPID TEST DEVICE Positive Control”. Text above the photo says: “Can anyone explain this??? Why are there positive and negative test kits? And why does the positive test show that it’s a biohazard? Is the test preloaded with the virus?”

As part of quality control measures for COVID-19 tests, “control” samples are included in batches to help to detect any faults.

To make sure the test is not detecting the disease in people who have not been infected (called a false positive), a negative control is used. This consists of generic cells that have not developed antibodies or been infected with the disease that can be run through the test. Medical practitioners know that these cells are not infected, so if the test comes back positive, it shows that something went wrong with the test and indicates they should not be used.

Conversely, if the tests don’t detect viruses such as SARS-CoV-2 in people who have been infected, there is little point using them. To test for this, a positive control is used. This is a swab that has been treated in such a way that it should give a positive result when run through the test, so that health workers know they are performing the test correctly.

While these control swabs are widely used for other tests, including PCR tests (here), the particular product shown in the post is the  Panbio™ Covid-19 Ag Rapid Test Device, an antigen test.

The product literature for this device (here) says “The controls are specifically formulated and manufactured to ensure performance of the Panbio™ COVID-19 Ag Rapid Test Device and are used to verify the user’s ability to properly perform the test and interpret the results.” It also says: “Good laboratory practice suggests the use of positive and negative controls to ensure that: Test reagents are working, and The test is correctly performed.”

The literature also says that these controls should be used when a new operator begins testing specimens, with each test shipment and at “periodic intervals” dictated by applicable laws and guidelines.

The tests are not “preloaded” to give a result. The swabs are used to make sure that a sample test is working correctly.

Missing context. A Facebook post shows the positive and negative controls for an antigen test.

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