DOH lays out pros, cons of virus tests: ‘No perfect testing for COVID-19’
MANILA, Philippines — It turns out there is no perfect test in the Philippines to detect coronavirus that causes COVID-19 as the reverse transcription polymerase chain reaction (RT-PCR), the “gold standard” of COVID-19 testing, has its own disadvantages.
This was what Health Undersecretary Rosario Vergeire said when she discussed the health department’s Omnibus guidelines for COVID-19 tests during a webinar with local government units on Thursday.
Vergeire cited the three existing methodologies for COVID-19 in the country which includes the RT-PCR test, antigen test and rapid antibody test. All these three tests have their own weaknesses, she also noted.
“Wala pong test methodology for COVID-19 na perfect. Lahat po there are disadvantages and there are weaknesses for each of these methods,” the health official said.
(There is no perfect test methodology for COVID-19 that is perfect. There are disadvantages and there are weaknesses for each of these methods.)
What tests detect
The RT/PCR is the most reliable test for screening of COVID-19 as it detects the viral genetic material of the disease.
“It is a widely used molecular biology technique to detect the genetic material of the virus,” Vergeire said.
The RT/PCR is used for confirmatory tests for suspects, probable cases and close contacts of COVID-19.
The antigen test, meanwhile, detects “viral protein or antigens, which is expressed only when the virus is replicating.”
For quick screening of the novel coronavirus, antigen tests must still be complementary with the RT/PCR test. It is also used for screening low-risk individuals and those high-risk for suspicion of COVID-19.
Meanwhile, Vergeire explained that rapid antibody tests detect presence or absence of antibodies against the virus present in the patient’s serum or blood sample.
The health official reiterated that the rapid antibody test is not a “stand alone” method to detect the presence or absence of SARS-CoV-2, causative agent of COVID-19.
“It is not confirmatory nor is it a diagnostic test but can determine whether the person was recently infected with SARS-CoV-2 and has developed the antibodies against the virus,” she said.
The health official said the rapid antibody test is not recommended for those returning to work and those entering the country or provinces.
“The antibody test is not recommended for screening individuals if they have COVID-19 or if they don’t have COVID-19,” Vergeire said.
How samples are collected, analyzed
For RT/PCR tests, health workers collect swab samples from patients. Specimens from RT/PCR are analyzed in biosafety level 2 laboratories, which are already 82 nationwide.
For antigen tests, samples are also collected through the nasopharyngeal/oropharyngeal swab.
Rapid antibody tests collect serum or the liquid part of blood after the clot is separated samples from patients.
The specimens from antibody tests and antigen tests are then sent to a laboratory or healthcare setting for evaluation.
When is the best time to conduct tests?
The Philippine health department also discussed the accurate use and timing of the three available methods for the coronavirus disease.
For RT/PCR, symptomatic patients must undertake the test shortly after the onset of their illness.
However, close contacts who are asymptomatic or presymptomatic must undertake an RT/PCR test at least five or seven days after exposure to a COVID-19-infected patient.
For antigen tests, the patient should undertake such a test after one to five days after onset of the symptoms.
For antibody tests, it is best to conduct the method 14 days after the onset of illness.
What positive result, negative result for tests mean
For positive RT/PCR results, it means the patient is infected with the coronavirus disease.
However, Vergeire said negative RT/PCR results do not mean that the patient has already recovered from the disease due to the timing of tests conducted.
“Kapag negative sa RT/PCR, depende sa timing ng pagkakakuha ng sample para masabi na talagang negative po kayo. (If you have negative results from the RT/PCR, it still depends on the timing of sample collection to say that they are negative for the virus.) There is an appropriate time and ideal time to get the samples for specific individuals,” she said.
For positive results in antigen tests, it means it is “presumptive that the patient has COVID-19.” If a patient yields negative results in antigen tests, it means infection for novel coronavirus is “unlikely.”
Both results, however, means the patient still needs to undergo confirmatory RT/PCR test for detection of the virus.
For antibody testing, its results show positive and negative results for antibodies namely, Immunoglobulin G (IgG) and the Immunoglobulin M (IgM).
If the patient is IgG positive, it means there is past infection while IgM positive suggests recent infection.
Similar to the antigen test, rapid antibody test still needs to be complemented with the swab test for RT/PCR.
To date, Philippines has a total of 178,022 cases of coronavirus disease. Of which, 114,114 are recoveries while 2,883 are deaths.
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