15th Jun 2020

COVID-19 FAQ: What is the difference between PCR and antibody testing?

15th Jun 2020

Curated by Endre Szvetnik

Currently, there are a number of tests that help us either detect the presence of COVID-19 in people or see if they have already experienced the disease and are immune to the virus (read more about immunity in our earlier COVID-19 FAQ post).

Covid-19 testing centre sign

Polymerase Chain Reaction (PCR) tests are the predominant way of finding disease-causing pathogens in the human body. To detect if someone is currently COVID-19-positive, nasal swabs, sputum or stool samples are taken from people. Stool samples can help detect the virus in 50% of patients and are important to capture patients who have developed gastrointestinal and not respiratory symptoms. The samples are analysed through a PCR method called Reverse Transcription Polymerase Chain Reaction (RT-PCR). The test identifies the genetic material of the virus in a human sample. As SARS-CoV-2 is a single strand RNA virus, it first needs to be converted to DNA – through ‘reverse transcription’ – hence the name. PCR tests are very sensitive and their benefit is that they multiply a tiny sample, so that enough material is produced for analysis. The tests help determine if someone is infected even if they are symptom-free at the time of testing (due to the variable, 2 to 14 days incubation period of COVID-19). The drawback of this technique is that even a small amount of contamination in the sample can lead to misleading results. These tests can produce a high proportion –  up to 20% – of false positives, so specialists are advising the retesting of suspect cases.

What are antibody tests? Also known as serologic diagnosis, this type of test is looking for IgG or IgM antibodies (proteins that emerge after a viral infection in people’s blood which can protect them from a repeat infection by the same pathogen, in this case, SARS-CoV-2). This test determines if someone has already had the disease, for example, a mild form of COVID-19, that they were not sure about, because they recovered at home and did not go to the hospital. Identifying such individuals is useful because they could donate their blood plasma to severely ill patients to help them recover from COVID-19 (there is an ongoing clinical trial in the UK to determine the efficacy of this potential treatment). Currently, there are several commercially available antibody tests and researchers are calling to establish standardised testing formats. A recent paper highlights that the clinical sensitivity of a particular antibody test varies depending on when it is performed in relation to the time of the infection. The test picked up antibodies in the samples only in 8.6% of the cases when performed less than six days after infection, but reached an 84% accuracy after 14 days.

Here is the current state of science on a Sparrho pinboard. NB: The pinboard contains research papers that have not been peer-reviewed yet, meaning that they have not gone through the standard scientific validation process yet.

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Curated by

Endre Szvetnik

Endre Szvetnik is Senior Editor at Sparrho. Endre works with Sparrho Heroes to curate, translate and disseminate scientific research to the wider public.

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