Facts on Antigen Antibody and PCR Tests

Real-Time Reverse Transcriptase Polymerase Chain Reaction (rRT-PCR) Testing for COVID-19

What is an rRT-PCR assay?

RT-PCR is a molecular diagnostic technique to detect viral genetic material (viral RNA) in a biological sample after having amplified it to allow for its detection. This testing methodology represents the current gold standard for detecting presence of the virus in the respiratory tract, i.e. for identifying active infections. This technique has very good sensitivity and specificity, meaning that it is very reliable.

Why are the rRT-PCR assays so much better for detecting SARS-CoV-2 in persons?

The inherent sensitivity of real-time PCR allows for detection of minute amounts (e.g., <100 copies/ml) of target nucleic acid in clinical samples. Additionally, most of the currently available, extensively validated and approved tests have sensitivities of more than >95% and specificities of >99%. 

Put plainly, the characteristics of this testing methodology make it extremely useful in identifying pre-symptomatic and early symptomatic COVID-19 persons, as well as those persons that are being assessed for COVID-19 5 to 7 days after their initial infection and presentation with symptoms. 

What are some of the pitfalls of rRT-PCR testing for COVID-19?

While rRT-PCR tests are highly sensitive and specific, negative test results must be interpreted in the context of timing of sample collection (early post-onset versus late post-onset of symptoms), the type of specimen tested (e.g., nasopharyngeal [NP] swab versus throat swab), the quality of specimen collected, and the performance characteristics of the assay. Additionally, RT-PCR tests can yield false negative results if the level of viral RNA in a particular sample is too low for detection.

To compound these challenges, rRT-PCR assays are costly to perform and require trained and qualified staff that are capable of analysing the test results.

Turn Around Time 

48 hours

 

Antigen Tests for COVID-19

What are Antigen tests?

Rapid antigen tests or rapid diagnostic tests (RDTs) or antigen point-of-care testing (POCT) for SARS-CoV-2 detect the presence of viral antigen (protein) in a patient’s sample without amplification. 

Can antigen tests be used to determine if a person has coronavirus disease 2019 (COVID-19)?

Yes. COVID-19 Antigen tests offer multiple benefits in comparison to RT-PCR tests for the detection of SARS-CoV-2. They have been developed as both laboratory-based tests and for near-patient use (point-of-care), and results are normally generated in 10 to 30 minutes after the start of the analysis, and at low cost. Additionally, the tests are sensitive enough to detect cases with a high viral load, i.e. pre-symptomatic and early symptomatic cases (up to five days from symptom onset; or low RT-PCR cycle threshold (Ct) value, ≤25). 

What are the pitfalls of using COVID-19 antigen tests?

The sensitivity [i.e., ability to detect a person who truly has the virus] of these antigen tests decline by about 15% to 20% seven or more days after symptom onset. 

Most currently available rapid antigen tests show a lower sensitivity compared to the standard RT-PCR test, while their specificity [i.e., ability to identify a person as truly negative] is generally reported to be high. 

However, there is concern that, given the variability of viral loads in COVID-19 patients, antigen detection may miss cases due to low infectious burden or sampling variability. 

For this reason, the U.S. FDA and the World Health Organization have recommended that a negative COVID-19 antigen test be followed up with a PCR test, especially for patients with the classical symptoms.

 

Antibody Tests for COVID-19

What are Antibody tests?

A COVID-19 antibody test is a blood test that can, to some extent, determine if you’ve had COVID-19 before. 

Are antibody tests used to determine if a person has coronavirus disease 2019, that is, diagnose COVID-19?

No. These test cannot be used diagnose COVID-19 infection. These are prone to producing false negative results particularly in the early course of infection, when antibodies directed towards the virus have not yet developed, or they may generate false positive results (for example, if antibodies to another coronavirus type are present in the patient).

Therefore, a positive antibody test in the absence of symptoms associated with COVID-19 may have little clinical meaning. For these and other reasons, the results of antibody tests may not be useful in identifying persons with COVID-19, and the results of these tests should be interpreted with extreme caution.

When should I consider doing an antibody test?

The most clinically significant antibody produced after SARS-CoV-2 infection is immunoglobulin G (IgG). This antibody becomes detectable in the patient’s blood between 10-14 days after an infection with SARS-CoV-2. 

Ideally, persons seeking to do a COVID-19 antibody test should be tested at least 14 days after their infection and/or if they meet the following criteria:

  1. They were previously diagnosed with COVID-19
  2. They suspect they might have had COVID-19
  3. You suspect that you might have had a past exposure to a person who had COVID-19