Why are rapid antibody tests not recommended for COVID-19?

MANILA, Philippines — Despite previous pronouncements from the Department of Health (DOH) as well as health experts warning against the use of rapid antibody test kit to check if a person is infected with the coronavirus disease (COVID-19) or not, the test kits continue to be in use. 

But why?

No less than former health secretary Manuel Dayrit, the person who led the country’s successful battle against the severe acute respiratory disease (SARS) back in 2003, said that results given by rapid tests are “inconclusive” mainly because the test detects the presence of antibodies that our immune system produces and not the virus that causes COVID-19.

Rapid testing vs swab testing

In an interview with INQUIRER.net, Dayrit pointed out the “huge” difference between rapid testing and real-time reverse transcription-polymerase chain reaction (RT-PCR) test or more commonly known as swab testing.

Dayrit explained that rapid test measures the presence of antibodies that the body produces against the virus while RT-PCR detects the presence of the virus in one’s body.

“So malaki na ang deperensya noon. Yung RT-PCR you detect the actual virus, nandun talaga. Yung rapid antibody test, ang dinedetect mo yung antibodies against the virus. And yan, kinukuha mo sa blood, hindi mo nakukuha sa swab,” Dayrit said.

(There is a huge difference. RT-PCR tests can detect the actual virus, that they are still there. The rapid antibody test only detects the antibodies against the virus and you get that through blood and not swab.)

To easily explain the difference between the two tests, Dayrit likened the coronavirus to a thief breaking into a house, representing the human body.

Dayrit said RT-PCR tests can catch the thief while still inside the house while rapid tests can only see the  evidence left by the thief who entered the house—and in some cases, there is even no evidence.

“‘Yung RT-PCR yun ang test para mahuli mo yung magnanakaw habang nandun pa siya kasi yun ang papasa, ipapasa, at makakahawa sa iba. Kaya kung malaman natin na yung taong yun, dala dala pala niya ‘yung magnanakaw sa kanya, ‘yan kailangan mong i-isolate,” Dayrit said.

(RT-PCR test seeks to catch the thief while they are still there because that is what can be passed onto others. That’s why if we find out that that person is still carrying the ‘thief’, then we have to isolate that person.)

“Ngayon kung gumawa kayo ng rapid antibody test, anong makukuha mo dun? Mga ebidensiya lang na nandun siya, dumaan siya. Minsan nga wala pang ebidensya,” he added.

(Now, if you do a rapid antibody test, what will you get from it? You will just get evidence that the ‘thief’ has been there. But sometimes there is even no evidence.)

This means that rapid tests if used to check whether a person is currently afflicted with COVID-19, could produce false positive and false negative results, said Dayrit.

“Kasi may mga taong nagkaroon ng impeksyon, pero walang ebidensya. Hindi siya nag-develop ng antibodies. Kaya yung rapid antibody test hindi ganun ka-useful,” Dayrit said.

(There are people who have had infection, but there is no evidence because they did not develop antibodies. That’s why rapid antibody tests are not that useful.)

“Kaya sinasabi ng DOH, wag na masyadong gamitin yan at wag mong gagamitin yan para sabihin kung ang tao ay okay o hindi kasi wala kang ebidensya para sabihin ‘yun kasi wala na, wala na yung magnanakaw. Ang alam mo lang dyan nagkaroon ng ebidensya na dumaan pero hindi mo alam kung makaka-impeksyon pa siya,” he added.

(That’s why the DOH has been saying, do not use that to determine if a person is okay or not because you do not have evidence that the ‘thief’ is already gone. What you only know is that there is evidence that the ‘thief’ has been there but you do not know if that person can still infect.)

If that is the case, then, are rapid tests of any use in this pandemic?

Dayrit said that rapid tests can be used to measure how many people in a population have had exposure to the virus.

“Ibig sabihin nyan, sa ating analogy, ilan sa mga bahay dito sa ating syudad ang napasukan na ng magnanakaw. Yun ang usefulness noon. Pero hindi natin masasabi na yung mga bahay na yun kung mayroon pang virus doon,” Dayrit said.

(In our analogy, it means that you can find out how many homes in our city have been visited by the thief. That’s it’s usefulness. But we cannot say that those homes still have the virus.)

How it is like in other countries

Dr. Supachai A. Basit, Dean of School of Medical Technology in Emilio Aguinaldo College in Dasmariñas, Cavite, cited some of the best practices being implemented by countries that are deemed to be winning their fight against COVID-19.

According to Basit, some first-world countries are even offering their tests for free.

As per those near the Philippines, Basit pointed to Taiwan, Singapore, and South Korea as some of the countries with the best practices in fighting the pandemic as these are the countries where testings are prevalent.

“[In South Korea], they have really this low fatality count and one of the reasons for low fatality count is that because of the wide-scale mass testing. They even have this drive-thru mass testing center for COVID-19,” Basit told INQUIRER.net.

“Because of that particular widespread massive mass testing, they were able to conduct contact tracing much easier, and are able to quarantine those people who had been exposed to COVID-19,” he added.

Meanwhile, Basit said Taiwan has a “very smooth, systematic healthcare system.”

“So of course, taxpayers have definitely contributed for this particular system work, but they’re able to efficiently direct the funding to concentrate on COVID-19 testing,” Basit said.

“So, I’m not saying that our government is not actually doing it, but I think, if we’re going to compare the outcome of the efficiency of a particular country as compared to ours, relatively compared and it comes to fighting COVID-19, they’re doing much better than us primarily because they were able to appropriately use the funding for a particular problem such as COVID-19. ,” he added.

As per the cost of the tests, Basit there is a “big disparity” between rapid testing and RT-PCR tests.

He said that RT-PCR testing kits could cost from P3,000 to P8,000 while rapid tests only cost around P400 to P800.

Despite this difference in the cost, Basit said that having a rapid antibody test may end up being more expensive for the patient since the results still have to be confirmed with RT-PCR results.

Further, Basit said that results from rapid tests have to be interpreted with caution.

Testing isn’t the end

Dayrit said that countries must not only test but also isolate those who will be found positive.

As per the Philippines, Dayrit believes that the country should further ramp up its testing capabilities while targeting the right population to test.

“Ang ibig sabihin nyan, you have to have a strategy to test who needs to be tested. ‘Yan ang problema ngayon, malaki na ang capacity for testing but sino ang ite-test at kailangan mong kumuha ng mga swab mula sa mga taong yun. Dun medyo nagla-lag behind tayo,” Dayrit said.

(It means there has to be a strategy to test who needs to be tested. That is the problem now. We have a huge capacity for testing but the question now is who should be tested? That’s where we are lagging behind.)

“We’ve been able to build capacity but we are not really getting the people who should be tested  and really you want to test regularly. Because you can be tested today but be infected tomorrow. There has to be a program for regular testing particularly for people that are in high-risk situation,” he added.

Another point of concern that has to be addressed, said Dayrit, is government support towards the private sector.

Meanwhile, Basit echoes Dayrit’s proposal, saying that an algorithm that is applicable to the country has to be implemented so that match testing would be efficient.

“In other countries, they have developed an effective algorithm for mass testing. So let’s say for example, in some countries, they are actually, they have this data if the local transmission is high on that particular area, then mass testing has to be there,” Basit said.

“And then the result of the contact tracing, the data that you’ll get from contact tracing will be also very important in order for you to conduct mass testing… So when it comes to mass testing, we do not prioritize officials, we do not prioritize people who are not an at-risk group. So, at the end of It all, it all boils down to proper identification of the target population in a cohesive algorithm,” he added.