During the peaks why was there more confirmed cases that PCR Positive results?

[djay]

At many points in the pandemic thailands PCR test positives was less than confirmed cases which doesn't make sense as MOPH claims to only confirm a case using PCR testing as recommended by WHO

From the situation reports you can also get a breakdown of cases that helps us understand what is going on

During the second wave positive results and cases were closer up until early February. At this time there was a government initiative to do large amounts of proactive testing in factories with migrant workers. This resulted in a big jump in confirmed cases however it didn't seem to result in a similar jump in positive test results. For some reason these tests seem to be excluded from the testing data. At the same time there isn't a large jump in numbers of tests either.

In the 3rd wave cases were again higher than positive tests. It appeared proactive and prison testing might not have been included. The initial large announcement of prison cases was on the 17th May. Curiously the 27th May was a large spike in positive test results, about ~7k greater than average for this time. There wasn't a spike in tests at the same time. If we look at cumulative figures since 1st April 2021 the positive tests is more than cases but only just.

We can also try to smooth out the data by using a 21 day moving average.

From the test data by area we can see which areas have more cases than public positive test results. A positive value means more cases than tests. NOTE the district testing data averaged over a week so comparing it on a day basis like this might not be accurate. In addition prison cases are separated out of the province data but the test data just has health districts and if prisons are included its not clear where which health districts.

[djay]

MOPH claimed in an email to OWID that the reason is there are two data entry systems that the PCR test labs use. One for confirmed cases (co-ward) and one for more detailed test result data (co-lab). Due to the huge demand for tests the some of the data doesn't get entered into co-ward.
The outcome is there is likely more PCR tests conducted than show in the data. This does mean that Confirmed Cases / Tests is perhaps a better estimate of postitive rate than positive results / tests.

There are few other possibilities that were looked into

• Maybe Combinational pooled Testing was sometimes used

  • e.g. Origami Assays can confirm/clear 600 patients with only 60 tests
  • But would labs report single assay as a "positive test" or the result of how many were confirmed by the combinational approach?
  • This would also drop the total number of tests performed also at times where this used used
  • News report from May 2020 says they use it for proactive tests
  • Thailand doesn't appear on the list of List of countries implementing pool testing strategy against COVID-19

• Maybe double entry failures

  • the system for reporting cases (co-ward) and reporting tests (co-lab) are two seperate system (TODO: reference)
  • Delays or omissions might happen if there isn't a need to always enter both at the same time, for example payment
    for some kinds of tests might rely on entries in co-lab but others might not?

• Maybe some Anit-body tests were used without PCR test confirmation afterwards

  • Reports seem to indicate some cases are "historical" which could mean antibody tests were used which would be unusual.
  • The MOPH testing data seems to only be for PCR tests
  • Bangkok Post article on 26th Dec 2020 where the governor of Samut Sakhon refers to using blood tests.

    Samut Sakhon governor Veerasak Vijitsaengsri said on Friday that he has ordered the testing method to be changed to ensure quick results and cover all at-risk groups of Thais and migrants in the province.
    The current method of inserting a swab into the nose to get a fluid sample will be replaced by taking blood samples from people, the governor said, adding that results from the new method come back in 30 minutes and costs are cheaper.

  • WHO report 26 Jan 2020

    Rapid tests that require a blood sample and that can provide results within 30 minutes will be used in high risk groups (both Thai and migrants) in Samut Sakhon to supplement the nasopharyngeal swab

  • WHO report on 31 Jan 2020 seems to indicate antibody tests were used to confirm cases in migrant workers

    The high daily case count of around 700 new cases per day for the last 6 days reflects the surveillance strategy of targeted active surveillance amongst the workers in the 4,000 plus factories located in Samut Sakhon. These large counts reflect the testing methods (see Sitrep 123 for an explanation) and represent both current and historical infections, and therefore the large daily case counts are not necessarily all new infections.

• Maybe some proactive tests are excluded from Test reports deliberately

  • Since positive rate is reported in these reports and proactive testing results in a misleading higher positive rate than a more random walk-in approach perhaps the decision was made to exclude some test results? OWID for example try to exclude proactive testing from positive rate calculation.

    OWID: "we don't think it is necessary to include tests or cases from proactive test finding in our calculation of the Positive Rate. This is because we use the Positive Rate as an estimator for testing capacity within a country, not the fraction of the population that is infected. Including tests from proactive case finding breaks the condition of random sampling, as they would obviously have a much higher probability of being positive, and so would lead to a biased estimator."

  • Both tests and positives don't seem to rise during times of proactive testing.

• Maybe Antigen tests were used without PCR test confirmation

  • No evidence rapid tests were used to confirm cases other than they are available and used by provinces for allowing travel in some circumstances.

• Maybe Simple Group Testing was used

  • This seems unlikely since a positive group result means further followup testing of each patient in the group.