The hows and whys of COVID testing

Published June 25, 2020, 12:00 AM

by manilabulletin_admin



There is lot of misinformation regarding COVID testing, so let’s try to unravel the facts from the smoke. The main problem with this issue is that the real experts in laboratory testing, the pathologists, were not consulted or involved in the planning and decision making in COVID testing at the start. In mid-May, the Philippine Society of Pathologists (PSP) issued its statement on the matter, which finally made the health authorities sit up and take notice.

By then, a lot had been said about rapid antibody testing (RAT) and for lack of anything better and with the shortage of RT-PCR testing, almost everyone, including major medical associations, jumped on the RAT bandwagon. Millions of said tests were purchased, often for a lot of money, and are still being used by anyone with access to the kits.

Unfortunately, since these kits were hurriedly developed and marketed, and the FDA was under pressure to provide authorization for COVID testing, they were allowed to be used without validation. It now turns out that these kits have poor accuracy, such that out of 100 results, only 40-60% were correct. Thus, diagnosis using these kits is more like tossing a coin, where you’ll be right less than half the time. So, why shell out P750 when you can guess it yourself?

Even now, news stories about people testing positive for COVID-19 feature RAT. There are several versions of RAT. Some test for IgM only, other IgG only and some test for both IgM and IgG. These are often interpreted by doctorsand/or nurses, who have not trained in the basics of laboratory testing. For instance, assuming the RAT is accurate, what was tested and what was positive? Were they truly infected? Should you rejoice if you tested negative for both? NO. It only means you can still be infected.

But are the persons interpreting the tests doing it properly? So many questions, so few clear answers.

Let’s back up for a bit and learn the basics of antibodies. Antibodies (immunoglobulins or Ig for short) are produced by the body to fight an infection, in this case, the SARS-COV-2  which causes COVID-19, the disease. The body produces Immunoglobulin M (IgM) first starting the first week of infection and can be detected soon after that.  Antibody production then shifts to Immunoglobulin G (IgG) soon after. IgG is the main antibody to fight an infection and its appearance in the patient’s blood signals that the tide has turned in favor of the host (patient) against the invader (virus).

The PSP has stated that rapid antibody tests are not reliable. Instead, it recommends laboratory-based automated testing employing more robust methods such as ELISA (Enzyme-Linked ImmunoAssay) and CLIA (Chemi-Luminescence ImmunoAssay). These tests are now available in the country and can be set up quickly since the machines needed are commonly available in many laboratories locally. And results are very reliable compared to RAT. Similarly, some test for IgM only, some IgG only, other test for IgM and IgG while a few test for total antibody (IgG, IgA and IgM). The beauty of these tests, aside from their accuracy, is that they are amenable to mass testing with results available within a few minutes from each other. Cost is similar to or competitive with RAT.

With the combined use of RT-PCR (Reverse Transcriptase Polymerase Chain Reaction) that detects the virus RNA (not to be confused with the virus antigen which is the protein structure of the virus; antigen testing for COVID-19 is not currently available locally) and laboratory-based antibody testing, we can now decipher the COVID status of a person with certainty. A positive RT-PCR result means that person is infected. A positive total antibody or IgM-IgG result may mean two things, either the person is currently battling the infection, thus still infected, or that he /she is already recovered. When that person who tests positive for total antibody is then tested for IgG and is positive, it means he/she has recovered. If IgG is negative, we can presume current infection and proceed to RT-PCR for confirmation. If all tests are negative, that person has not had exposure to the virus and may still be infected.

Why test for antibody at all? Even if RT-PCR is the gold standard for diagnosis, it can still be negative (false negative) in 20-30% of infections due to many factors like low viral load, or being tested in the early phase (window period) when viral load is too low to be detected or late phase when the virus has been cleared by the patient’s immune system. Another factor is improper collection, storage or transport of the sample for RT-PCR. Testing for COVID antibodies can pick up early and late phase infections.

Thus, a combined testing approach will give us a clearer picture of a person’s infection status. Even more, if employed in a mass testing strategy, we can now see the extent of the pandemic in the population tested.  This gives a clear guide to further public health measures to control the spread of infection (test, trace and treat) until a vaccine can be given and avoid repeated lockdowns that ruin the economy.

It can also tell us the herd immunity status of the population where 60% of the population should have recovered based on the presence of IgG antibodies and presumed immune to the disease, thereby protecting the remaining vulnerable population.

The antibody status will also be useful in determining who should get the vaccine (negative for IgG) and who should not (IgG positive), thus providing guidance for vaccination priorities.

So let’s hear it from the true laboratory experts- the pathologists.


The author is a US and Philippine Board Certified Pathologist, subspecialization in immunopathology with a total of 9 years of pathology training in both countries and a practice spanning more than 30 years. He pioneered the use of molecular diagnostics in the field of paternity testing and transplantation in 1992. He is affiliated with St. Luke’s Medical Center, Quezon City, Philippine Childrens Medical Center (head of COVID laboratory), Cardinal Santos Medical Center, and Diliman Doctors Hospital.