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Before the promotion of large-scale screening, we should first think about the purpose, target group, etc., in order to carry out meaningful screening.

Since the outbreak of COVID-19, Taiwan has experienced many debates over whether to implement “universal screening”, and the recent major outbreak in the country has inevitably been discussed (relevant news: free, mid-time). So, can Pu Siai work?

The outbreak in Taiwan was later than in other countries, so we can learn from the experience of other countries, and the United Kingdom is the first class in epidemic control. How do they deal with the general screening problem? [1]


What are the obstacles to the implementation of large-scale, unlimited general screening?

Ideally, with large-scale, almost unlimited screening, it is very attractive to wipe out the hidden infections at one time! But ideals are plump and reality is very skinny.To implement large-scale, almost unlimited general screening, the actual implementation will encounter many crises that need to be overcome. The following is divided into scientific, behavior, and resource aspects to explain [1]。

One,Scientific side

False positive:

Any screening method cannot avoid a certain percentage of “false positives.” Therefore, a certain percentage of healthy people will be put into quarantine after testing positive. People’s work, business, interpersonal, nursery and care for the elderly, etc., may be affected by the difficulty of recovery.

The excessive number of false positive cases will even occupy wards, ambulances and other equipment, and consume manpower such as nursing, epidemic investigation, and information. It even further squeezes out conventional medical treatment (such as surgery, cancer screening, etc.), affecting Taiwan’s overall medical resources.

In small-scale screening, the number of false positives is small and has little impact; but if the screening volume is scaled up to tens of thousands or hundreds of thousands, false positives will become a very serious problem.

Personnel proficiency will jeopardize the accuracy of screening:

If mobilized to implement unlimited general screening, novices will be recruited to assist in testing. Past studies have found that the accuracy of screening is related to the technical proficiency of the tester. Statistics show that testers who lack training or experience will test more false negative cases. If there is no plan to force general screening rashly, the incidence of false negatives will increase and the screening results will be more unreliable.

The excessive number of false positive cases may be further crowded out to routine medical resources. Photo/ENVATO ELEMENTS

two,Behavioral aspect

False confidence:

The sense of security after the screening may bring “false self-confidence” to the public and weaken the defense.

For example, people who have received a negative judgment may neglect to wear masks, wash their hands frequently, or even arrange trips to visit relatives and friends because they feel safe.According to a poll in the United Kingdom, after the people are tested negative, 20% (17%) will go shopping and 10% (9%) will visit relatives and friends. [2] (Extended reading: What should I pay attention to for large-scale screening?).

However, there must be false-negative (infected, but tested negative and returned to the community) infected persons. After screening, there will be a greater chance of infecting the virus to relatives and friends. We should not be cautious as a lesson from the UK.

“Visit screening” waste of resources

Under unrestricted screening, some people may regard the screening as a means of praying for peace of mind, and at the request of themselves and even their bosses, they will carry out the screening repeatedly and daily, thus wasting medical treatment.

People who are screened negative may have false self-confidence and slack in epidemic prevention. Photo/ENVATO ELEMENTS


Screening is not only as simple as buying reagents, but also includes procurement, personnel training, data transmission, and cross-unit / cross-county / cross-central information integration… and other tasks.

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After the test, the “positive, what should I do?” Transportation, ward scheduling, as well as epidemic investigation, social welfare, care resource intervention, and information integration also need to be planned in advance. On the other hand, it is equally important to establish the correct concept of “female, what should I do?” (even if they are female, they still need to face masks, wash their hands, and stay away from the community).

Therefore, considering the above-mentioned situation, almost no country has “extensive screening” based on the “universal screening”, but mainly “extensive screening” with a planned and clear purpose.

In most countries, “extensive screening” is planned with a clear purpose. Photo/ENVATO ELEMENTS

How to do a planned “extensive screening”?

Since a high proportion of COVID-19 infected people are asymptomatic, large-scale screening is still necessary. After exploring, Europe and the United States each launched a planned and purposeful extensive screening.

The following references and simulates the situation that may be applicable in Taiwan [1]。

1. Key areas of infection

  • Be applicable:Hot spots in the epidemic area, such as Wanhua, institutions with outbreaks of nosocomial infections
  • practice:For the population with high estimated prevalence and potentially many infected people, testing is provided by anyone; even knocking on doors, door-to-door, and repeated (the disease has an incubation period, and the date of onset of each person may vary).
  • Matters needing attention and effects:Extensive screening of hot spots can detect infected people in a short time and in the largest number, preventing them from spreading the virus across counties, cities, and agencies. And screening for ethnic groups with high prevalence can reduce the impact of false positive cases. However, a short period of time and a large number of screening tests will also impact the medical system and put them under great pressure. After a large number of screening tests, there may still be false-negative infections returning to the community.

2. Protect vulnerable ethnic groups or life-sustaining facilities (Test-to-Protect)

  • Be applicable: Places where vulnerable ethnic groups gather, such as hospitals, nursing homes, etc.; or important life-sustaining facilities, such as supermarkets, police stations, etc.
  • practice:Regularly (such as twice a week) conduct a comprehensive screening of the personnel of the institution, taking the hospital as an example, including at least medical care, logistics, hospital admissions, discharges, etc.
  • effect:In hospitals and nursing homes, regular screening can reduce the possibility of disease outbreaks in fragile environments. In important social operation facilities such as supermarkets and police stations, the collapse of life-sustaining facilities can be avoided, or the interaction between supermarkets, agency personnel, and the people can instead infect the people.
Extensive screening of important social operating facilities such as supermarkets and police stations can prevent the collapse of life-sustaining facilities. Photo/ENVATO ELEMENTS

Three, evaluation of release isolation (Test-to-Release)

  • Be applicable:Areas that have been closed for a long time and have suffered too much economic damage
  • practice:After it has been closed for a period of time and continues to be closed, there is a risk that there will be economic damage areas that are difficult to recover. Periodically and publicly screen data, explain the meaning of each number to the public, and set a target value for reopening.
  • Things to note:It is necessary to set a meaningful value for unblocking (such as: positive rate + number of screening tests) to encourage people to abide by the quarantine policy, and at the same time strike a balance between economy and epidemic prevention.

Four, evaluation of open activities (Test-to-Enable)

  • Be applicable:Assess whether it can reopen and close activities that have been closed for a long time, but are important for returning to normal social operations, such as restaurants.
  • practice:Regular screening of event personnel and participating vulnerable groups in order to gradually open up group activities. Take a meal for the elderly as an example, at least include the staff of the meal system and the participating elders.
  • Effects and matters needing attention:It can promote the increase of social and economic activities, and at the same time give the people the confidence that “the society is gradually recovering”, and enhance the people’s willingness to abide by the isolation policy.
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Gradually opening up group activities can increase people’s confidence and increase their willingness to comply with policies. Photo/ENVATO ELEMENTS

Use 5W1H to plan extensive screening

From the extensive experience of screening in Europe and the United States with clear goals and planning, we can know that before planning to expand screening, we can learn the 5W1H of management and think about its purpose and strategies:

  • Why: The purpose of mass screening? What do you want to achieve?
  • Who: In order to achieve the goal, which ethnic groups and institutions should be screened? How to let them know and willing to come?
  • Where: In order to improve the fluency of testing, how should screening, logistics, etc. be arranged?
  • What: In order to match the goals and existing resources, what testing, information system, and notification process should be used?
  • How: After knowing the positive and negative, how to plan the follow-up?
  • When: In order to achieve the goal, when should we start the screening, review and explain the results?
The 5W1H of management can be used to plan a wide range of screening strategies. Photo/ENVATO ELEMENTS

Pursuing peace of mind is human nature, and it can be expected that discussions on screening will become more and more heated in the future. Each county and city government should explain to the public the pros and cons of screening in detail, and should even discuss with the public in the form of popular science, so that the public can better understand “Why not do it?” and “Why do it?” Compliance. After all, the ultimate key to this epidemic prevention battle is the people.

Remarks:Taipei City, Taichung CityscreeningPositive rate, decision-making after false positives

Positive rate

Refers to the ratio of positives in the test. It can be used to infer the prevalence (or prevalence) of this group of diseases, which is extremely important for public health policy making.

Taking the recent data from Beishi Quick Screening Station as an example, it can be inferred that about 5-10% of people with a history of Wanhua activities have been infected. With such a high positive rate, further expansion of screening can be considered to catch potential infections as much as possible.

However, it should be noted thatThe positive rate is affected by the will of the people

If people with a history of activities/contacts dare not be screened due to attacks and stigmatization caused by netizens, the positive rate will be reduced as a result, and a false sense of security will be created instead. Or low-risk people with the intention of “screening and having peace of mind” occupy the quota of quick screening, and the positive rate will also be reduced, causing the government and the public to misjudge the situation and affect subsequent epidemic prevention.

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Recently, the positive rate of rapid screening in Beijing has changed.Photo/Taipei City Health Bureau

False positive

It means that the quick screening result is positive, but when it is retested by RT-PCR, it is found to be a negative result. This data can infer the prevalence (or prevalence) of this group of diseases, which is extremely important for the setting of public health policies and quick screening goals.

According to Taichung City’s 05/21 press release, 1791 people were screened quickly, 19 were positive, and only one was confirmed by RT-PCR. The false positive rate was about 94%. The possible reasons are inferred: 1) The true prevalence rate in Taichung City is extremely low; 2) Most of the screened persons are not high-risk groups, and the data cannot reverse the real epidemic situation.

Regardless of the real reason, the high false positives indicate that the vast majority of people currently tested are healthy people. Taichung City may need to adjust the strategy of fast screening stations to allow the truly high-risk groups to be tested.

False positive and false negative

If I am tested negative/positive, how likely is it that I am actually a false negative/false positive?

Assume that you are in a hot spot in an epidemic area such as Wanhua, its prevalence is assumed to be 10%, the sensitivity of the rapid screening used is 90%, and the specificity is 98%. Then the ratio of false yang to false yin is approximately as follows:

  • The false positive rate is about 19% → 100 quick screens are positive, about 19 people need to be detained for a few days, waiting for RT-PCR re-screening
  • The false negative rate is about 2% → 100 quick negatives, 2 of them are actually infected, returning to the workplace and the community
Given the sensitivity and specificity of the screening test, the false negative and false positive rates of the tested population are estimated after the prevalence of diseases. The Chinese information is annotated by the author, and the source of the calculation is: “BMJ

Assuming that 1,500 cases are screened quickly, the results are as follows:

  • 30 false positive healthy people were quarantined and their lives were greatly disrupted.Such as: receiving the epidemic investigation, recalling the contact history, asking for leave from the company, asking someone to bring a child, etc.
  • Fifteen false-negative infected persons returned to the community to gain a sense of security, eat with relatives and friends, take public transportation to work, discuss work with customers, etc.

So you can know that even after a quick screening

  • If the result is negative, please wear a mask, wash your hands, and keep it at home. As my condition is still in the incubation period.
  • If the result is positive, please don’t be nervous, call contacts, companies, pet hotels, and use them as a drill. Wait a few days for the Health Bureau to give PCR data.

Note in particular that the prevalence, sensitivity, and specificity of the above assumptions are all high estimates. In the real world, each value may be lower.

Table 1: Under the assumption that the prevalence rate is 10%, the sensitivity is 90%, and the specificity is 98%, it is estimated that the number of false positive and false negative cases after 1,500 people are tested


1. Alex Crozier, Selina Rajan, Iain Buchan, Martin McKee (2021) Put to the test: use of rapid testing technologies for covid-19. BMJ. DOI: Liverpool covid-19 community testing pilot—interim evaluation report. 2020.

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