A paper produced for the Scientific Advisory Group for Emergencies (SAGE) in June said that both the false negative and false positive rate were unknown, and noted that the accuracy in an idealized lab setting might not apply in real life conditions as “there may be inefficient sampling, lab contamination, sample degradation or other sources of error.”īut in general, these tests have “very high specificity”, which means they don’t return many false positives. It’s hard to say how many false negatives and positives PCR tests produce. They can also indicate that someone has the virus when they don’t (false positive). PCR tests can sometimes indicate that someone does not have the virus when they do (false negative). That’s not to say that PCR tests are perfect. The US Food & Drug Administration (FDA) says: “This test is typically highly accurate and usually does not need to be repeated.” Antigen tests are more likely to miss a Covid-19 infection than PCR tests according to the FDA. PCR tests are generally seen as the gold standard for Covid-19 testing. We’re going to focus on the accuracy of PCR tests, as the Department of Health and Social Care confirmed to us that all diagnostic tests for Covid-19 in the UK are PCR tests. “So when people talk of “antigen” tests and others talk of tests for viral RNA or “PCR tests” they are actually talking about the same thing.” However, the test being used for COVID-19 is actually looking for viral RNA (which is technically not a viral antigen). Professor of Immunology and Infectious Disease at the University of Edinburgh, Eleanor Riley has said: “Tests for the virus (current infection) are often called “antigen” tests – where antigen refers to some component of the virus, typically the external (coat) protein of the virus. These terms are sometimes (confusingly) used interchangeably. Antigen tests are “ not widely used for coronavirus testing currently”. The presence of antigens normally triggers an immune response by the body. These primers are designed to target unique segments of the virus’s genome.Īntigen tests, by contrast, do not detect the viral genetic material, but rather usually proteins in the virus. This testing involves specially designed “primers” and “probes” which attach themselves to specific sequences of the virus’s genetic code, and send out a signal that indicates the genetic material has been found. This is so enough copies are present to be picked up when that sample is then tested.
These substances, enzymes known as “reverse transcriptase” and “DNA polymerase”, work to make many copies of any viral RNA present. First, various substances are added to the sample ( usually a nose or throat swab) taken from the person tested. PCR (polymerase chain reaction) tests detect the virus’s RNA (genetic material) in a sample. The two diagnostic tests are PCR tests and antigen tests. Diagnostic tests tell you whether a person has the virus now, and they’re what we’re normally talking about with programmes like the UK’s contact tracing systems.Īdditionally there are antibody tests, also called serology tests, which can detect if someone has had the virus in the past, but not whether they still do. There are two main types of diagnostic test which can detect the presence of viruses like SARS-CoV-2 (the virus which causes Covid-19). This is based on the fact that the common cold can be caused by a number of human coronaviruses, which are related to (but not the same as) the new coronavirus which causes Covid-19.īut it’s a misunderstanding of how the main type of test for Covid-19 works. We’ve been asked by readers whether a Covid-19 test can return a positive result if the person tested only has the common cold rather than Covid-19.