Covid 19 false positives

 Remember that positive predictive value (PPV) varies with disease prevalence when interpreting results from diagnostic tests. PPV is the percent of positive test results that are true positives. As disease prevalence decreases, the percent of test results that are false positives increase.

For example, a test with 98% specificity would have a PPV of just over 80% in a population with 10% prevalence, meaning 20 out of 100 positive results would be false positives.

The same test would only have a PPV of approximately 30% in a population with 1% prevalence, meaning 70 out of 100 positive results would be false positives.  This means that, in a population with 1% prevalence, only 30% of individuals with positive test results actually have the disease.

At 0.1% prevalence, the PPV would only be 4%, meaning that 96 out of 100 positive results would be false positives.

Health care providers should take the local prevalence into consideration when interpreting diagnostic test results.

https://www.fda.gov/medical-devices/letters-health-care-providers/potential-false-positive-results-antigen-tests-rapid-detection-sars-cov-2-letter-clinical-laboratory

https://web.archive.org/web/20201103184558/https://www.fda.gov/medical-devices/letters-health-care-providers/potential-false-positive-results-antigen-tests-rapid-detection-sars-cov-2-letter-clinical-laboratory


But NPR has learned the results of that final quality control test suggested something troubling — it said the kit could fail 33% of the time.

https://www.npr.org/2020/11/06/929078678/cdc-report-officials-knew-coronavirus-test-was-flawed-but-released-it-anyway

http://web.archive.org/web/20211208211434/https://www.npr.org/2020/11/06/929078678/cdc-report-officials-knew-coronavirus-test-was-flawed-but-released-it-anyway

https://www.buzzfeednews.com/article/danvergano/coronavirus-test-new-york-cdc


New York Is Making Its Own Coronavirus Test After The CDC’s Test Has Repeatedly Failed

https://www.buzzfeednews.com/article/danvergano/coronavirus-test-new-york-cdc

http://web.archive.org/web/20200228001555/https://www.buzzfeednews.com/article/danvergano/coronavirus-test-new-york-cdc

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