How many tests do we need to say we are testing enough?
It’s a question that’s been going around since the start of the pandemic. Of course, while some would profess that the more we test the better, testing everyone makes no sense.
The right utilization of resources is fundamental in the delivery of health care in countries with limited funds. Remember, even if one tested negative now, it is not a guarantee that you don’t have the virus (tested too early) or that you will never have the virus – ever!
What is a good parameter to say that we’re probably testing ‘enough’?
In communities where RT-PCR testing is done, and a high number of those tested are coming back positive, it’s a sign that we’re not testing enough. In short, there are many more cases that have not been found.
Because we’re currently prioritizing who’s getting tested, more positive result means that the tests are being used to confirm the obvious (clinically symptomatic) cases.
In areas where close contacts and suspected cases are being tested as well, the negative tests coming back are high. Michael Ryan, executive director of the WHO Health Emergencies Program points out that among countries with extensive testing, <12% of their tests are positive.
The benchmark of a system that’s doing enough testing to pick up all the cases is 10-30 negative cases per confirmed positive case (or an overall positivity rate of 3-10%).
The scatter chart above shows another way of seeing the extent of testing relative to the scale of outbreak in various countries in Asia. The one encircled in black is the Philippines.
On a logarithmic scale, the X axis represents the daily confirmed cases per million people. The vertical or Y axis shows the number of tests done (as of the date in the graph).
Based on latest data, the total number of tests the Philippines has done is 378,396 or 3456/M population (with our population of almost 110M). The scatter chart shows that countries that have done more tests have actually lower case fatality rates. For example, Thailand, South Korea, Malaysia, Saudi Arabia, Singapore have had more tests done per million population. And considerably a lesser case fatality rate.
Then there are other countries who have done less tests than us (based on tests per million population) and have had case fatality rates lower than ours.
Except for Iran that had seen an unprecedented number of deaths (8,012) and one of the highest number of cases (160,696), countries like Bangladesh and Taiwan have done less testing compared to us and yet have had very low fatality rates. Indonesia and Japan on the other hand, have done less testing than the Philippines and have seen higher fatality rates than us.
As of June 2, 2020, we have tested 378,296 people with a positive rate of 7.3%. This positive rate of 7.3% is not exclusive to new confirmed cases as the health agency does not discern whether some of these reported tests are retests done among previously positive patients or not. This is one gap that should be addressed as the public feel that the almost 20,000 total cases yesterday still contains a backlog of more than 5000 patients because the positive individuals is more than 25,000. Because of this unknown variable, there may be a further decline in positive rate if we now exclude patients whose positive results are retests.
More importantly is the judicious use of the RT-PCR as a testing tool for declaring patients as recovered. The graph below shows the usefulness of varying testing methods. While PCR is most useful within the first three weeks of symptom onset, its value declines significantly among most patients after the second week of illness.
Patients who have clinically recovered after 14-21 days, may be tested ONCE and when negative end testing there, instead of having to test negative two consecutive times (just to fulfill a diagnostic end point that is not clinically justified). Now, we know that there are quite a number of patients that continue to test positive even after 30 days of recovery and attribute that to genetic viral remnants in the oropharyngeal tract. It is therefore recommended that after a patient has tested ONCE to be negative after 14 days of isolation/quarantine AND is clinically well, no additional testing should be done. If patients continue to test positive after this period, they can be observed for 7 more days and if they are clinically well after an extended week of observation, should be considered a recovery with no additional tests done.
Last April 30, 2020, LiveScience published a report on the phenomenon of patients still testing positive even when they clinically recovered. And this phenomenon may be shortcomings in the current testing kits available.
More than 260 COVID-19 patients in South Korea tested positive for the coronavirus after having recovered, raising alarm that the virus might be capable of “reactivating” or infection people more than once. But infectious disease experts now say both are unlikely.https://www.livescience.com/coronavirus-reinfections-were-false-positives.html
It is a fact that the PCR is unable to distinguish genetic material from an infectious virus and the “dead” viral fragments may linger in the body even after some people have recovered. And that can be explained by the fact that the body has its way of cleaning up broken down cells. This “garbage” is referred to a s cellular corpses destroyed by the virus and within this “garbage” are fragmented remains of non-infectious viral particles.
As a result of the findings in South Korea, under the new protocols, no additional tests are required for cases that have been discharged from isolation and will refer to “re-positive” cases as “PCR re-detected after discharge from isolation” and people should no longer be required to test negative for the virus before returning to work or school after recovering from their illness and completing their isolation period.
On May 30, Singaporean Daviest Ong tested negative for the virus after 68 days in isolation and 22 uncomfortable swab tests in Gleneagles Hospital. On May 28, Health Minister Gan Kim Yong announced that COVID-19 patients in Singapore who are assessed to be clinically well by day 21 of the onset of illness can be discharged without further tests. This new policy took into consideration clinical and scientific evidence on the limitations of the RT-PCR test after a window of recovery.
This comes a few days after the World Health Organization also published its recommendation to shift from transmission-based discharge to time-based discharge for patients who are clinically well. The WHO now recommends that patients who are clinically well can now be discharged after 10 days from the onset of illness, plus at least 3 days without respiratory symptoms or fever.
As we know more about the clinical course of the disease, and see more recoveries as well as the actual picture of SARS-CoV-2 and its disease COVID-19, we are learning how to manage this infectious agent more rationally today than when it began its story in Wuhan, China 6 months ago. Without a vaccine or treatment on the horizon yet, we will need to reinvest on our clinical acumen to reboot the economy…and our lives.