By Edsel Maurice Salvana via Facebook
Let's be rational when doing testing.
What is the risk of COVID-19 for someone who goes out everyday and interacts with people for work and business? It depends on the ongoing community transmission rate. It's an imperfect measure, but the positivity rate of a test in a locality can give an idea of the pretest probability of infection.
For a doctor in an ER in NCR who sees patients daily, the pretest probability is about 3 to 5%, less the mitigating effect of PPE. Still reasonable to test periodically. Same for a factory worker (if they don't stay in a factory bubble), jeepney driver, or security guard in NCR. Having symptoms increases the yield, being asymptomatic decreases it.
But for a person in Batanes who didn't travel anywhere or come in contact with anyone from outside the island and doesn't have any symptoms, the pretest probability is near ZERO. If you decide to test that person because of "mass testing," chances are its going to be negative nearly 100% of the time. There is also a tiny chance it returns a false POSITIVE and that will just make everyone panic.
Same for a person who lives in Manila but has not gone out in the last nine months and meticulously follows no contact deliveries and does not allow visitors. Near zero pretest probability, and there is a risk of a false positive if you insist on "mass testing" just because it's catchy.
So doing "mass testing" without considering pretest probability is a brain dead exercise and may actually be harmful because it may yield some false positives. People with exposure OR symptoms - go ahead and test. People with NO RISK, DO NOT TEST. Waste of resources and risk of false positive. I hope this helps explain why we sometimes do not recommend testing. It isn't for everyone. Stay safe.
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