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SINGAPORE – Royal Caribbean International’s cruise ship Quantum of the Seas returned to Singapore on the third day of a four-day cruise after an 83-year-old man on board tested positive for Covid-19.
The man was taken to hospital on Wednesday afternoon (December 9), where further tests were conducted. That same night, the Health Ministry said the man’s original sample was retested at a national laboratory and returned negative for Covid-19 infection.
A second new sample analyzed by the lab also tested negative on Wednesday, while a final confirmatory test on Thursday showed the man did not have the coronavirus, indicating that his test result on board the ship was a false positive.
The Straits Times looks at what false positives are, how likely they are to occur, and how the testing capabilities on board cruise ships differ from those of national labs.
Q: What is a false positive or false negative test result and why do they occur?
A: A false positive test result means that a person without the infection is misdiagnosed as infected, while a false negative means that a true infection has not been detected. False positives and negatives occur as there is a margin of error in all Covid-19 tests and none can accurately diagnose Covid-19 infections all of the time.
Q: What are the chances of false positives or negatives?
A: Two measures are used when talking about the accuracy of Covid-19 tests: sensitivity and specificity.
Sensitivity refers to the ability of a test to identify those infected as positive. A test with lower sensitivity will have more false negatives, which means that more true infections are missed.
On the other hand, specificity refers to the ability of a test to correctly identify those who are not infected as negative. This means that a test with lower specificity has more false positives.
The default test used in national laboratories is the polymerase chain reaction (PCR) test. It has a sensitivity rate of more than 93 percent and a specificity rate of more than 95 percent, but it can take up to a day or two for results to appear.
Rapid antigen tests, as the name suggests, are faster with results, taking only 15 to 30 minutes for the results to be ready. These have been used to test before large-scale events and have a lower sensitivity rate of 82 but a specificity rate of 99 percent.
Q: Could the same test sample produce different results?
A: Yes, and for various reasons.
First, due to the varying sensitivity and specificity of each type of Covid-19 test, subjecting the same sample to different testing protocols can give different results. Professor Teo Yik Ying, Dean of the Saw Swee Hock School of Public Health at the National University of Singapore, said it is possible that the testing regimen in some corporate settings may use point-of-care testing or rapid antigen tests, which may provide a faster result but tend to have a higher error rate.
As a result, it is entirely possible that some of the less accurate protocols will produce an erroneous result when the same sample is analyzed using different analysis protocols.
Second, coronavirus enzymes that might be present in a person’s swab sample are susceptible to degradation in the harsh external environment, infectious disease specialist Leong Hoe Nam said. Thus, a sample that previously tested positive it may be negative in subsequent tests.
Q: What are the testing capabilities on board cruise ships and how do they differ from testing in national laboratories?
ANSWER: Michael Goh, Head of International Sales for Genting Cruise Lines and President of Dream Cruises, said last month that his security regime includes a new onboard real-time PCR machine that produces Covid-19 test results in 60 minutes while you can to rule out 22 other respiratory viruses like H1N1 and influenza.
Similarly, Royal Caribbean says on its website that “Sars-CoV-2 rapid tests can be performed directly on board in our medical laboratory which enables fast and accurate RT-PCR tests on site with results in less than one hour, along with a host of other evaluative tests. “
While Dream Cruises and Royal Caribbean did not disclose the sensitivity and specificity rates of the test machines aboard their ships when asked, Goh said Thursday that the PCR machine aboard World Dream was approved by the Authority of Health Sciences and that PCR testing “remains the most accurate test available today.”
Dr Leong said that the accuracy and precision of the machines on cruise ships are likely to be less than those in the laboratories of the Ministry of Health.
Describing machines used in remote station situations as “very optimal” for such situations, Dr. Leong said they are likely to produce “quick and messy” results, and lack the technical expertise of more precise machines, which are larger and difficult to accommodate for boats with space limitations. .
Despite the possible lack of precision of the testing machines compared to regular PCR tests, Dr. Leong said it was still a good idea to have such “stand-alone” machines on board cruise ships for the detection of suspicious patients. of Covid-19.
Q: What does the probability of false positives mean for mass testing as large-scale activities progressively resume?
A: Since no test can be completely accurate, the experts said it is important to learn to deal with the reality of these errors that occur in testing.
Professor Teo said: “The reality is that as testing becomes more frequent, erroneous results will emerge.
“These will have knock-on effects on whether a traveler is incorrectly denied entry or participation, or if someone who is actually infected gets lost and ends up seeding additional cases in the community.
“For observed positive findings, especially when diagnosed with rapid tests or at the point of care, it may be worthwhile to undergo a round of retesting, preferably with a separate testing protocol to avoid systematic errors.
“In this way, we avoid the false scares that can happen, like what happened on the Royal Caribbean ship.”
Reassessment capabilities may not exist aboard ships, Dr. Leong said, as they are typically performed with laboratory experts in consultation with infectious disease experts who have assessed the epidemiological risks of the positive case. As a result, he said the captain’s decision to return Wednesday morning was prudent.
Adding that false positives will continue to occur as testing becomes more widespread, Dr. Leong said patience on the part of patients is important, as repeated swabs and lab tests will be required when they occur.
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