178 CMAAO CORONA FACTS and MYTH RTPCR
Dr K Aggarwal
With inputs from Dr Monica Vasudev
1044: RTPCR can be redetected but not re-positive
1. A positive rRT-PCR antigen test is highly accurate, indicating presence of SARS-CoV-2 RNA.
2. There appears to be no significant cross-reactivity with other respiratory viruses or even other coronaviruses.
3. A small study conducted in Korea suggests that patients with persistent positive tests who are beyond 10 days from the initial positive test and are now symptom free are no longer infectious.
4. For patients with a high suspicion of COVID-19, a negative test should not rule out the infection.
5. The number of false-negative results is not well known, though the resultant risk is "substantial."
6. A number of factors affect the likelihood of a false-negative test, including when the sample was collected relative to the timing of illness and the type of specimen collected
7. nasopharyngeal swabs are more likely to be accurate vs nasal or throat specimens.
8. Repeat or serial testing increases the sensitivity but may not always be available.
9. Although rRT-PCR is the current criterion standard, more inclusive consensus-based criteria are likely to emerge because of the concern about these false-negative results.
10. Patients who are discharged from isolation after recovering from COVID-19 and who again test positive for SARS-CoV-2 are unlikely to be infectious, according to a report from the Korea Centers for Disease Control and Prevention (KCDC).
11. There's no relapse
12. Although the disease is now known to linger and that it affects more than one system of the body, other viral diseases, such as influenza and mononucleosis, also "work that way,"
13. As of May 15, Korean researchers had identified 447 patients who again tested positive on real-time reverse transcriptase–polymerase chain reaction (RT-PCR) testing for viral RNA. Of those, 285 (63.8%) patients had undergone epidemiologic investigation and contact investigation. Among the tested individuals, 59.6% were tested for screening purposes, and 37.5% underwent testing because of their having symptoms. Almost half (44.7%) of the 284 patients who underwent investigation were symptomatic.
14. Data from three groups of patients from different cities showed that between 25.9% and 48.9% of patients again tested positive after they had been discharged.
15. Among the 226 patients who were symptomatic when their case was initially confirmed, a repeat positive test result after discharge occurred an average of 44.9 days (range, 8 – 82 days) from the date symptoms initially developed. It took an average of 14.3 days (range, 1 – 37 days) from the time of discharge to the time of the second positive test.
16. More than half (59.6%) of patients who tested positive a second time were tested for screening purposes, without regard to symptoms. Of those who again tested positive, 44.7% had symptoms that included cough and sore throat.
17. To help determine whether a positive result on a second test is associated with infectivity, the Korean researchers investigated 790 contacts of the 285 patients who tested positive a second time. Of those patients, 351 were family members, and 439 were others. Among the contacts, the researchers identified three new cases; however, for all three patients, other sources of infection were possible. These sources included religious groups or family groups in which there were persons who were confirmed to have COVID-19.
18. The researchers tried to culture virus from 108 patients who tested positive a second time; all such cultures were negative.
19. Further, first and second serum samples were obtained from 23 patients who had tested positive a second time. Of those, 96% tested positive for neutralizing antibodies.
20. Based on active monitoring, epidemiological investigation, and laboratory testing of re-positive cases and their contacts, no evidence was found that indicated infectivity of re-positive cases
21. Patients who have been discharged from isolation need no further testing and are not likely to be infective, even if they again test positive on RT-PCR assay.
22. The patients will no longer be regarded as "re-positive cases" but as "PCR re-detected after discharge from isolation"