By Anne Ruth Dela Cruz
While the RT-PCR test is considered the gold standard in testing for the Covid-19 virus, it is important that the test is administered at the right time to ensure the accuracy of the diagnosis.
In a special episode of the webinar series organized by The Medical City (TMC) Monday titled “The New Norm Series: Coping in a Quarantine Community” which tackled “Covid-19 Tests: What are they and what are they not?” Dr. Karl Events Henson, Head of TMC’s Hospital Infection Control and Epidemiology Center, said the kind of test the patient will undergo will depend on where he is “in the cycle” of the virus.
“You have to know when you are infected,” Dr. Henson said. “You have to know when your symptoms started so that the doctors can determine the kind of test that will be administered to you.”
Onset of symptoms
He noted that the virus in the nose and in the respiratory tract start to grow around a week before the symptoms start to show and can already be detected two or three days before the onset of the symptoms. After the body encounters the virus, the body will start producing antibodies, usually beyond the first week, either on Day 7 or Day 8.
Currently, there are three tests, or more specifically, three molecules that are used to detect the Covid-19 virus—RNA, Antibody and Antigen. RNA is the genetic material that can be found at the center of the virus cell and this is what is duplicated to enable the virus to multiply.
The Antibody is produced by the body in response to a foreign body. The Antigens, on the other hand, are pieces of the virus that is detected by the human body in order to produce antibodies.
“When we consider detecting genetic materials, that usually means RT-PCR and this is known as the gold standard in detecting the Covid-19 virus,” Dr. Henson said. “The test sample starts to be positive two to three days before the onset of the symptoms and then it peaks at the start of the symptoms and then it tapers off.”
Test positivity
Hence, it is important that the RT-PCR test is administered at the onset of the symptoms. The positivity of the test, Dr. Henson said, also depends on the quality of the sample because if the sample is not taken properly, there will be a false negative result.
“Since genetic material can stay in the respiratory tract for a long time, RT-PCR should not be used to monitor recovery,” Dr. Henson said. “Getting two negative RT-PCR tests used to be the basis for the discharge of Covid-19 patients but this is no longer the practice.”
The Antibody test is the ECLIA test that is available at TMC and the ELISA test is the test being provided free of charge by the Manila City government. Antibodies start to show after seven days on the onset of the symptoms.
“Antibody tests should not be used to diagnose acute infection. If you just started to have symptoms then you don’t need an antibody test at that point in time. What you need is an RT-PCR,” Dr. Henson said. “An antibody test may be useful in several scenarios like a recovered or asymptomatic patient who wishes to confirm the presence of antibodies in preparation for plasma donation.”
Pieces of the virus
The third test are those that detect antigens. Antigens are pieces of the virus and “we know that when you detect the presence of the virus, we know that you are detecting acute infection.”
“So like RT-PCR, antigen tests can also be used to detect acute infection. The sample for Antigen tests can be obtained from the nose because it needs the sample of the virus,” Dr. Henson said. “Compared to RT-PCR tests, Antigen tests are much cheaper when cost is considered.”
At the end of his talk, Dr. Henson reminded that the best time to take the RT-PCR and antigen test is when the symptoms are visible. The antibody test should be used in the second, third and fourth week and should not be used to diagnose the acute infection.
“If you have moderate or severe symptoms, please get yourselves checked even through telemedicine,” Dr. Henson said. “If you feel that your symptoms are urgent enough, drive yourself to the nearest emergency department. Always consult your doctor so that they will know what test should be given to you.”
Stop rapid antibody tests
Meanwhile, Dr. Antonio Dans, spokesman for the Healthcare Professions Alliance Against Covid-19 reiterated their appeal to the government to halt the use of rapid antibody test kits for workers returning to work, stressing that it could not accurately detect the virus.
At the Philippine College of Physicians health forum titled “Solidarity in Combatting Covid-19,” Dr. Dans also warned that if half of the population in the workplace, for example, were undetected for Covid-19, they would become carriers.”
“Dapat ihinto na yang rapid antibody test nayan. Sa buong mundo, tayo na lang yata ang gumagamit niyan,” Dr. Dans said.
The call was echoed by Dr. Aileen Espina of the Philippine Society of Public Health Physicians who said the test would give a “false level of security.”
For their part, the Department of Health said they have been constantly reminding the public that rapid antibody testing should not be used as a stand alone test to definitively diagnose or rule out Covid-19 as stipulated in the Department Memorandum 2020-0258.
They added that they also stipulated in the DOH guidelines that consultation with a doctor with regard to rapid antibody testing is necessary to ensure proper timing of testing and correct interpretation of results.
“We are currently in the process of developing omnibus guidelines on the use of the different types of test kits so as to strictly specify what these different types of tests are for and the conditions under which they may be used,” the DOH concluded. —with a report from Claudeth Mocon-Ciriaco
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