# Liberal Arts Blog — Does a Positive Covid Test Mean What You Think It Means?

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Liberal Arts Blog — Monday is the Joy of Math, Statistics, and Numbers Day

Today’s Topic — Does a Positive Covid Test Mean What You Think It Means?

What are the odds that if you test positive for Covid that you are contagious and should be contact-traced? What’s the math? What’s the science? Who knows both? Who should? Today, the summary of a New York Times article brought to my attention by mathematician Elise Cawley. Thank you, Elise. Experts — please chime in. Correct, elaborate, elucidate.

YES-NO IS NOT THE WAY TO GO. IT’S ALL ABOUT HOW MUCH.

1. It’s not about whether or not you have a trace amount of the virus but how much you have.

2. The current PCR tests are far too sensitive. The vast majority of those who test positive do not have enough to be contagious.

3. “The decision not to test asymptomatic people is just really backward.” (Michael Mina, epidemiologist, Harvard School of public health.)

HOW THE PCR TEST WORKS — THE CONCEPT OF “CYCLE THRESHOLD” (CT)

1. “The PCR test amplifies genetic matter from the virus in cycles; the fewer cycles required, the greater the amount of virus, or viral load in the sample. The greater the viral load, the more likely the patient is to be contagious.”

2. “This number of amplification cycles needed to find the virus, called the virus threshold is never included in the results sent to doctors and coronavirus patients, although it could tell them how infectious the patients are.” You can’t make this up!!!!!

3. “In three sets of testing data that include cycle thresholds, compiled by officials in Massachusetts, New York, and Nevada, up to 90% of people testing positive carried barely any virus, a review by The Times found.” Did you catch that? 90% !!!!!!!!!

WHAT IS THE RIGHT NUMBER OF CYCLES? 40? 37? 30? 26?

1. “Most tests set the limit at 40, a few at 37. This means that you are positive for the coronavirus if the test process required up to 40 cycles, or 37, to detect the virus.”

2. “Test thresholds so high may detect not just live virus but also genetic fragments, leftovers from infection that pose no particular risk — akin to finding a hair in a room long after a person has left, Dr. Mina said. Awesome analogy!

3. “I’m shocked that people would think that 40 could represent a positive.” (Juliet Morrison, virologist, University of California, Riverside)

NB: “Dr. Mina said he would set the figure at 30, or even less. Those changes would mean the amount of genetic material in a patient’s sample would have to be 100-fold to 1000-fold that of the current standard for the test to return a positive result — at least one worth acting on.” Did you catch that? 100 fold to 1000-fold?

Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be.

What to Know About COVID-19 Tests, From PCR to Antigen to Antibody

COVID-19 testing

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