This scanning electron microscope image shows SARS-CoV-2 (round blue objects) emerging from the surface of cells cultured in the lab. Photo and caption: niaid/Flickr, CC BY 2.0.
Specificity is a measure of a test kit’s “true-negative” rate. A 100% specificity means that if 100 blood samples that lack antibodies to SARS-CoV-2 are tested with the kit, the kit will return a “negative” result all hundred times. This feature is important because antibody kits rely on an “antigen” – a protein on the surface of the SARS-CoV-2 virus – to detect the antibodies. When antibodies in a blood sample bind with the antigen, the kit gives a positive signal.
However, many other viruses, including the Middle Eastern Respiratory Syndrome (MERS) virus or the human coronavirus 229 E, could have similar antigens. So, the SARS-CoV-2 kit antigen must be unique enough that it doesn’t confuse antibodies to other viruses with the ones it is seeking. If the test does mix-up antibodies – or cross-reacts with them – it will give an incorrect positive result. Scientists call this a “false positive”.
Sensitivity, on the other hand, is a kit’s “true positive” rate. So, a 100% sensitivity means that if the test kit is run on 100 samples that have SARS-CoV-2 antibodies, the kit detects the antibodies in all of them. This feature is crucial too, because some kits don’t catch low levels of antibodies in blood samples, leading to what scientists call “false negatives”.
Note: This article was updated at 6:33 pm on May 29, 2020.