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Repost:
From Dr. Edsel Salvana regarding testing for COVID:
(THREAD)
@MedtechMustKnow
PLEASE TAKE TIME TO READ:
Repost:
From Dr. Edsel Salvana regarding testing for COVID:
(THREAD)
@MedtechMustKnow
1. Mass testing is not testing everyone. It is “risk-based” testing. Basically, you test people in increasing circles of risk: test the PUI, then the close contacts, then the community. It is not a shotgun approach because no country can test every single citizen for COVID-19.
So we need to figure out our priorities for testing, and WHAT TEST to use. You CAN’T test 100M people, but you can test the MOST AT RISK.
2. Understand the limitations of testing. No TEST is 100% accurate. There are trade-offs. The probability that a test is positive when the disease is REALLY present is called the SENSITIVITY.
The probability that a negative test actually means the disease is REALLY NOT there is called the SPECIFICITY.
3. A good sensitivity means that a test is able to detect disease MOST of the time if it is PRESENT in a patient. Having a negative test when the disease is PRESENT is called a FALSE NEGATIVE. In other words, the test failed to detect a sick person.
4. A good specificity means that a test is NEGATIVE MOST of the time if there is NO DISEASE in a patient. Having a positive test when the disease is ABSENT is called a FALSE POSITIVE.
5. FALSE NEGATIVES are harmful because you say that someone is COVID-19-free when he actually has COVID-19 so that patient will be free to spread the disease.
6. FALSE POSITIVES are harmful because you will put a patient WITHOUT COVID-19 in the hospital, possibly with REAL COVID-19 patients such that the patient can get COVID, or be isolated needlessly.
7. So how good are the tests? There are two tests we can use for COVID-19 – RT-PCR and antibody tests.
8. RT-PCR is considered the best test for diagnosing ONGOING COVID-19 infection. PCR itself is very sensitive and specific, >90% for both. HOWEVER, the TYPE of specimen and the stage of disease (how many days with symptoms) can affect how often a test is positive.
So for RT-PCR, using a nasopharyngeal swab in a patient WITH disease, the probability of getting a positive test is only 63%. So you will actually MISS 37% of cases. This is why we can do a REPEAT test after 48 hours in a patient who is getting sicker of what looks like COVID,
but was NEGATIVE on the first test. The DANGER of RT-PCR is a FALSE NEGATIVE and you can end up clearing someone who actually has COVID-19. This can happen in UP TO 1/3 OF PATIENTS so its not a perfect test.
9. RT-PCR is also a highly technical process that not only involves having the right machine and kits, BUT also the proper SAFETY INFRASTRUCTURE like a BSL2 laboratory. Many labs and hospitals HAVE RT-PCR machines but they do not have the biosafety infrastructure.
10. Antibody tests include PRNT (Plaque reduction neutralization test, the gold standard), ELISA (enzyme linked immunosorbent assay) and lateral flow IgM/IgG. The first two are LABORATORY based assays and the last is a point of care rapid diagnostic test (POC-RDT).
11. As much as we would like to use rapid lateral flow assays (IgM/IgG) because of convenience, NONE of the lateral flow assays have used the industry standard PRNT assay as a gold standard. In other words, we have NO IDEA how good they are despite their claimed sensitivity and
specificity. The biggest danger is that because it takes 5 to 10 days to make IgM antibody, the test has a high FALSE NEGATIVE rate in those who just started having symptoms. And so you will get a FALSE SENSE OF SECURITY and
end up passing the virus to other people and your family members.
12. The OTHER problem with the lateral flow IgM/IgG is that there are other HUMAN CORONAVIRUSES that cause the common cold, and some antibodies against these viruses may CROSS-REACT with the test, giving you a FALSE POSITIVE, which is bad for the reasons stated.
The BOTTOM LINE is NONE OF THESE TESTS ARE PERFECT. FAR from it. Tests INFORM your response, but they still need to be INTERPRETED in the right context.
To the lay person, they think that a positive is a positive, and a negative is a negative. To us clinicians and scientists, they come with HUGE caveats in management. There are times we WILL NOT believe a test result because it is NOT CONSISTENT / the patient’s clinical picture.
If we let ourselves be mislead by a test result without USING OUR BRAIN, people will DIE. And this also holds for doing public health strategies and mass testing.
I do hope this gives the public a glimpse into how complex these decisions are, and that no matter how much the public clamors for something, it isn’t always in their BEST INTEREST to push the issue over what is SCIENTIFICALLY SOUND. Thank you.
