PCR & Antibody Testing for COVID19: What’s the Difference?

As the global community comes to grips with the scale and effects of the COVID19 virus, one of the tools we’re using to better understand the pandemic is diagnostic testing.  Currently, there are two primary methods used when it comes to testing for the COVID19 coronavirus: RT-PCR or Serological (antibody). 

These testing methods have two very different goals and consequences, yet, they are often conflated. Furthermore, they are not equal in their accuracy, the Center for Disease Control (CDC) estimates that Serological or ‘antibody’ tests are inaccurate 50% of the time. 

RT-PCR Test

In order to see if a person is currently infected with the coronavirus, doctor’s will employ the PCR test. According to a news article from Heavy.com, the most commonly used diagnostic test is the RT-PCR which stands for “Reverse-Transcription Polymerase Chain Reaction”. The sample needed is typically collected from the patient’s nose by way of a nasal swab and diagnostics are run to establish whether or not viral RNA is detected in the person’s cells. 

This PCR testing method is being employed by most large diagnostic labs.

ANTIBODY Test

If a person previously exhibited symptoms of COVID19 but currently shows no sign of the virus, then an antibody (otherwise known as “Serological”) test can be used to identify signs of a previous infection. The antibody test is utilized by taking a blood sample from the patient and identifying  whether antibodies to the corona virus have been produced. If antibodies are found, this means that the person was infected and their blood created antibodies in response to the virus. 

How Do They Compare?

There are positives and negatives to all things in life and this truancy does not escape the various testing methods utilized to detect past or present infections of COVID19. Let’s look at some points of testing criteria.

TIMING

Timing is crucial when it comes to testing. Those exposed to COVID19 can experience symptoms in a window ranging from two to fourteen days, though some may be infected and never experience any symptoms at all. If researchers perform the PCR test too early, there may not be enough of the virus in the patient’s cells to yield an accurate result. The same can be true if testing is done too late because the viral RNA will breakdown given enough time; this becomes problematic for samples sent off to a lab which can take a few days or, in some cases weeks to produce results. 

It’s important to note that a PCR test does not detect antibodies. Sample provided by a patient that has ‘beaten’ COVID19 may not yield a positive result from a PCR test, even though they have COVID19 antibodies present.

ACCURACY

PCR tests: When one nonprofit research center in Geneva called, “Researchers at the Foundation for Innovative New Diagnostics” tested five COVID19 RT-PCR tests they found that all five achieved 100% sensitivity on positive samples, and at least 96% specificity on negative samples. That being said, these tests were conducted within a laboratory setting. 

In the real world, testing conditions and processes are far from idyllic, and accuracy suffers when faced with various abnormalities encountered in the field. When gathering samples, PCR testing can produce false negatives if the health care provider has failed to adhere to effective guidelines. In some cases, the HCP did not follow proper procedure in collection, such as swabbing deep enough in the nasopharyngeal passage way to obtain a proper sample. In other cases where the correct sample size was taken, the researcher may have tested too early or waited too long. 

If sample quality, size and the time of testing are not done just right the result can be a false positive or negative. The danger here is that some people will think they have been infected when they have not and this may lead them to believe they have developed antibodies when they have not.

Researchers still don’t know what the real-world false positive rate is, but clinical sensitivity of RT-PCR tests currently ranges from 66% to 80%. That means that if proper procedures are not followed during collection, it’s possible that nearly one in three infected people who are tested will receive false negative results (Heavy.com). 

The PCR testing speed depends mostly on workflow of the laboratory. It is possible to complete a test cycle within a couple of hours, the logistics of collection, delivery, and testing at a clinical laboratory are often the cause of delays for PCR testing.

Antibody tests: When it comes to speed, antibody tests have PCR’s beat. Instead of a swab of the throat, which require a sealed sample being delivered to the diagnostic testing location, the antibody test uses blood by pricking a patient’s finger. This blood sample can yield results in minutes as opposed to the several hour minimum of a PCR. 

These tests vary in their results. Some of these ‘test kits’ simply identify whether antibodies are present (referred to as lgG positive). Some COVID19 antibody test kits are able to indicate a recent or active infection (lgM positive) as well. 

Once more, the challenge becomes timing. It can take one to two weeks for an infected person’s body to build up antibodies to fight the virus. So, if the serological test is performed too early, there may not be enough antibodies to be detected which would result in a false negative. 

While Antibody tests may win out for speed, the accuracy of the PCR test has yet to be matched.

The CDC as well as the Food and Drug Administration (FDA) have each expressed caution in regards to the accuracy of antibody tests versus PCR tests. The FDA has taken measures to ban sales of some Antibody test kits due to their inaccuracy and promises. The consistent inaccuracy of antibody kits can be attributed to how common a certain virus is within the population being tested. The CDC has said, “For example, in a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%. In other words, less than half of those testing positive will truly have antibodies”. 

Which Testing Method is Better?

Antibody and PCR tests have different functions. While blood-drop antibody tests deliver quick results, they are often inaccurate, and depending on the quality of the test, limited in their reporting (potentially only reporting antibodies once an a strong immune response to COVID19 has begun). Diagnostic testing using a Nasopharyngeal or Orophrayngeal samples can take as few as a couple hours and deliver much more accurate results indicating a live virus in the sample.