Testing for COVID-19

There are three main types of test for COVID-19, two are diagnostic (PCR and antigen tests) and one looks for antibodies from a previous infection (antibody or serologic tests). The test for antigens uses oral or nasal fluids as this is where it is easier to detect the virus, where as the antibodies will be found in the blood.

Diagnostic Tests

1. PCR Swab test 

This is the gold standard test as it is very accurate, but takes longer.

How does it work?

The Polymerase Chain Reaction (PCR) test works by looking for traces of the virus’s genetic sequences, the RNA.  The  sample is sent to a laboratory where the RNA is extracted and converted to DNA. The DNA is then amplified which means many copies of the viral DNA are made so that there is enough to get a result. 

 

By detecting the viral RNA which will be present in the body before antibodies form or even before any symptoms appear, the tests can tell if the person is infected very early on.

Test Tubes

How is someone tested?

A swab is pushed into the nose or the back of the throat, and the sample is sent off to a lab.  Reagents (chemicals) break open the viruses to expose the viral genome (RNA).  The test can take from a few minutes to a few days to get the results.

The test can only tell if the virus is currently there not if the person has already it before.

This tests is very important for helping governments to control the virus so they need to be able to perform as many tests as required. Restrictions arise from access to the the chemical reagents to perform the test, the number and location of testing centres across the country and sufficient laboratories who are able to conduct the tests.

2. Antigen tests

Antigen tests detect specific proteins on the surface of the coronavirus. The test looks for fragments of the virus without amplifying or replicating it in the lab. Swap samples can be used for antigen tests.

These tests can get results in as little 15 minutes and are often called rapid diagnostic tests, however although they are highly sensitive for a positive result, which means if you test positive you have the virus. However, there is a higher chance of a false negative.  If you have symptoms and get a negative result you may also require a PCR test to confirm the result.

Reliability of Tests

There are a number of factors which can affect the reliability fo the test including whether or not the sample was collected properly, the conditions it was kept in before and while it was shipped to the laboratory but also the timing of the test.

recent study by researchers at John Hopkins University found that the effectiveness of the test varies over the course of the the infection. The probability of a false negative (i.e a negative test for someone who actually does have the virus) ranged from 100% on Day 1 of the infection to 67% on Day 4. This decreased further to 20% on Day 8 but rose again every day after that. 

 

This timing difficulty together with the fact that some tests are not taken or handled properly, has resulted in a false negative result of 1 in 5 tests taken.