Less Sensitive Covid Testing More Effective

The nasal swab PCR test is most commonly used for identifying individuals with Covid-19, and is an excellent diagnostic test. However as mentioned in a previous article, Advocate for Daily Covid Testing, we need a more effective public health screening tool in the fight against this coronavirus pandemic. It would be an effective test if it had a more rapid turnaround time. Given the discomfort associated with gathering the sample, some have also avoided doing it more frequently. At over $100 per test and with a results turnaround time of up to two weeks, many choose not to test and do not quarantine appropriately because of cost, discomfort, and wait time.

Testing Frequency over Sensitivity

In his New York Times editorial, Dr. Mina describes a simple solution within reach that legislators and community leaders need to advocate: Covid paper strip testing. Several companies and laboratories have developed paper strip tests and other inexpensive, rapid result Covid tests that could be done daily. Though not a perfect test, their cost-effectiveness outweighs the deficit in sensitivity, at $1-5 per day.

The Science of Covid-19 Transmissibility

Scientists are finding that tests detecting the presence of the SARS-CoV-2 virus are not as important as tests that detect when someone is infecting others. Though testing positive by the PCR nasal swab test, some individuals may not be as infectious because the test is much more sensitive at detecting virus particles than is required to identify contagious patients. Looking at the graph below, the Cycle Threshold (Ct) values are inversely correlated to viral load (number of virus particles in a sample). For every whole number Ct reduction, there is a 10x increase in the number of virus particles. A Ct value of 40 would indicate less than 1000 virus particles found. The green line follows the viral load of an infected individual in days, starting with the onset of symptoms.

Threshold of Transmissibility

In South Korea, their national CDC studied Covid positive patients well after symptoms subsided. Household members never got sick despite the positive result of the family member. This suggests that the PCR positive individual was not contagious after a certain point in their recovery.   Scientists now believe the threshold of transmissibility is at a lower Ct value than the more sensitive PCR test, which detects the virus at a higher Ct value. The FDA is using diagnostic standards and applying it to these rapid tests as if they are a diagnostic tool. If we instead think of these rapid tests as a public health screening tool, they need not adhere to the same diagnostic tool standard. What we should be basing the ideal Covid test around is determining who is infective, not who has the viral particle.

Sensitivity vs. Transmissibility

The FDA currently requires tests to detect at a Ct value of 40 and below — so a person may be diagnosed with Covid but not infectious. Scientists and public health experts now believe that a less sensitive test, detecting at a Ct 35 (higher viral load) would only identify infectious Individuals as Covid-19 positive.

A cheaper, less sensitive test like the $1-3 at-home saliva paper strip test could be performed daily and could be used more widely by more individuals. Even if missing a positive result on day 1 for someone with Covid-19, the person testing themselves on subsequent days would provide the positive result and allow the person to begin quarantining and notifying others who may have also been exposed. The yellow line in the graph below indicates the lower sensitivity of the test, but is also within the pink shaded threshold of transmissibility when individuals are infectious. The blue line Nasal Swab PCR test shows the higher sensitivity, even when someone is not infectious (grey shaded area).

The Possibilities of Fast, Affordable Covid Testing for All

Just imagine: individuals could check themselves often, and given a positive result, self quarantine and notify their doctor to confirm the result with a standard swab test. Taken a step further, the government could affordably provide these tests to all citizens. Businesses, schools, and restaurants could more definitively know how to act if each participant took the test and showed their negative result before entering. The public could move about more freely with confidence if these tests were employed as required for certain establishments like offices, classrooms, and restaurants.

Sign the Change.org Petition

Please consider writing a letter to your legislator calling attention to alternative Covid test options, and sign the petition to below. Thank you for spreading the word and working together to put an end to this horrible pandemic.

https://www.change.org/p/nancy-pelosi-provide-daily-rapid-result-covid-testing-for-all

Reference

• RapidTest.org https://www.rapidtest.org
• NEJM https://www.nejm.org/doi/pdf/10.1056/NEJMc2001737
• New York Times https://www.nytimes.com/2020/07/03/opinion/coronavirus-tests.html
• Test Sensitivity is Secondary to Frequency (Dr. Mina) https://www.medrxiv.org/content/10.1101/2020.06.22.20136309v2
• Abbott ID Now https://www.abbott.com/corpnewsroom/product-and-innovation/how-id-now-tackles-covid-19.html
• CNN Test Costs https://www.cnn.com/2020/07/15/health/coronavirus-test-cost/index.html
• Johns Hopkins Tracker https://coronavirus.jhu.edu/map.html
• Live Science https://www.livescience.com/coronavirus-should-schools-reopen-fall.html
• Korean CDC https://www.cdc.go.kr/board/board.es?mid=a30402000000&bid=0030
• Washington Post https://www.washingtonpost.com/health/long-delays-in-getting-test-results-hobble-coronavirus-response/2020/07/12/d32f7fa8-c1fe-11ea-b4f6-cb39cd8940fb_story.html
• Reuters https://www.reuters.com/article/us-questdiagnostics-outlook/quest-says-many-u-s-covid-19-tests-taking-more-than-a-week-idUSKCN24E1CE
• Medcram Covid Possible Breakthrough https://www.medcram.com/courses/take/coronavirus-outbreak-symptoms-treatment/lessons/14699497-update-98-inexpensive-home-covid-19-tests-a-possible-breakthrough