A Public Works Department (JKR) engineer from Cheras, Kuala Lumpur arrived in Kuching International Airport (KIA) on Monday 14 September. At the airport, she was randomly selected for an RT-PCR test.
She was allowed to proceed to her hotel and freely carry on with her official duties. The test result was confirmed positive on 16 September, by which time she was already on the return flight to Kuala Lumpur.
The Ministry of Health, on 17 September, classified her as a local Sarawak case, identified through screening at Sarawak’s international gates. The Sarawak state government, however, classified Patient 701 as an imported case, not a local transmission. Is this just petty semantics between states, each trying to keep their side “green” or an issue with real public health implications?
For the Sarawak Health Department, the contact tracing for Case 701 must start from the passengers and crew on the plane that she was on, for the 1.5-hour flight; through the KIA terminal, taxi, hotel, and all her contacts until she boarded her return flight.
If the federal health officials insist that she was infected in Kuching, their contact tracing work will only start from her fellow passengers on her return flight and presumably stop at the gates of Hospital Sungai Buloh, where she had been brought for isolation. Short and sweet. Case tutup?
But wait! How did she get infected in that short walk through the terminal in Kuching to test positive a few minutes later? Could she possibly have got infected in the KLIA2 terminal in Kuala Lumpur or on the plane?
Imagine this scenario: a passenger alights the plane at KIA and walks 150 metres through the terminal, and gets stopped by health inspectors at random. (Good job, guys!). A nasopharyngeal swab is taken for a free RT-PCR test. The collected swabs from KIA are sent to a laboratory in Kuching city.
The test results will be known 1.5 days later. Sarawak relies entirely on the RT-PCR for the screening of incoming travellers at its airports and international boundaries. The rapid antigen test is not used because of the number of false negatives among travellers who had tested negative on embarkation at KLIA, but were found to be positive by RT-PCR soon after their arrival.
How soon after exposure to a SARS-CoV-2 infectious case will a person test positive for RT-PCR?
While RT-PCR is reliable, the swabs can fail to pick up infected cells during the early stages of infection when virus levels are low. These early swabs will produce false negative results.
RT-PCR is most accurate when carried out between two and five days after the onset of symptoms, with the collection of material via oral/ nasal swab or sputum (Figure 1). The first Covid-19 symptoms will appear in most cases around four to five days after exposure. 97.5% of those infected who develop symptoms will do so within 11.5 days.
However, according to Dr Anthony Fauci, director of the US’ National Institute of Allergy and Infectious Diseases, in his statement on 11 September, up to 40-45% of infected cases may be asymptomatic, i.e. they will not have signs and symptoms of Covid-19 at all throughout the course of the infection. We will only know they are infected when their swabs are taken in the course of contact tracing, or randomly, or pre-operative.
Unfortunately, all cases are capable of spreading the virus to others two to three days before they start feeling sick or have symptoms. They remain infectious for seven to 10 days after symptom onset. This is why close contacts and people under investigation are required to quarantine for 14 days from day of known exposure. By Day 14, most infected persons would be either symptomatic or have become less infectious.
When is the best time to RT-PCR test for Covid-19?
Researchers from Johns Hopkins Medicine reviewed a pooled analysis of seven published studies on RT-PCR performance by time since symptom onset or SARS-CoV-2 exposure using upper respiratory swab samples from 1,330 patients (7 Aug 2020).
They warn that RT-PCR tests carried out too early in the course of infection increases the risk of false negatives. During the first four days of infection, the false negative rate may be as high as 67-100%.
Over the four days of infection before the typical time of symptom onset (Day 5), the probability of a false-negative result in an infected person decreases from 100% on Day 1 to 67% on Day 4. On the day of symptom onset, usually Day 5, the median false-negative rate was 38%. This decreased to 20% on Day 8 (three days after symptom onset), then began to increase again, from 21% on Day 9 to 66% on Day 21.
This review has some limitations so the exact percentages will need to be verified with future studies. However, the knowledge of this variation of false-negative rate by time since exposure can help optimise the timing for the testing of close contacts.
It will not be cost-effective to swab all the passengers on the same flights as Case 701 as they disembark. That, being Day One of exposure, will result in almost 100% negative results (unless some coincidentally are already infected prior to boarding).
These negative tests will give these contacts a false sense of security and make them less quarantine-compliant and potential super-spreaders. We have seen this in some, now infamous, cases. RT-PCR testing of contacts on Days 5-8 after their exposure to an index case will give more accurate results.
This is the rationale behind the two tests for close contacts – the first is taken as soon as the index case’s results is known. The contact tracer will ask you for the date that you last had contact with the positive case. If the first test is negative, a second test will be scheduled for seven to 10 days after the last known contact. You will need to restrict your movements even if the first test is negative, until the second test is also negative.
To save tests and resources, pooled RT-PCR testing can be used when large groups in a community under lockdown or detention centre need to be screened. Samples from five persons are pooled and tested with one test. If that test is negative, all the five in the group are presumed to be negative. If it tests positive, all five persons are recalled for repeat tests individually.
Using pooled testing, Wuhan, China was able to screen 9.9 million out of its 11 million population over a period of 14 days between 14 May and 1June 2020. Wuhan’s testing blitz cost the city government around 900 million yuan (U$127 million). The citywide Covid-19 test found only 300 positive cases who were all asymptomatic.
The timing of Covid-19 tests may be the key to its accuracy
A meta-analysis of 13 studies, published 28 August, confirmed that the sensitivity of the RT-PCR for coronavirus diagnosis was 86% (i.e. overall false negative rate of 14%). The estimate of specificity calculated for the studies was 96% (i.e. false positive rate of 4%).
The report identified the factors that can influence the results of the examination, thus producing false negative results, such as technique and place of collection, time of onset of symptoms, storage and transportation of the sample to the location of the examination.
When there is uncertainty regarding the diagnosis, a second sample collection can be indicated to confirm the diagnosis.
A fusion opinion from a Sarawakian public health specialist, paediatrician, ex-associate professor, disaster relief and medical volunteer, passionate about helping people learn.
- This is the personal opinion of the writer or publication and does not necessarily represent the views of CodeBlue.
