csh3ll

Which COVID-19 test is relevant for you?

Updated on May 13th, 2021 with summary table on COVID-19 tests and their sources at the bottom.

Executive summary

  • A diagnostic test directly affirms the presence of the COVID-19 virus
  • Rapid (molecular), antigen and antibody tests were deployed for more widespread testing at a lower cost to develop baseline infection rates, especially in outbreak or congregate settings, such as nursing homes and schools. They maybe less accurate than the diagnostic test
  • If you have symptoms or COVID-19 exposure, it is recommended you get a diagnostic test

—

Quite quickly, the different types of COVID-19 tests caused me confusion as to which one I would need and when. In New York City, there is a one-size-fits-all message to get tested. It is recommended by the Test & Trace Corps (published January 6, 2021) to get tested immediately if you have symptoms or (possible) exposure, and periodically if you work outside of your home. However, it is still not clear which test you should get.

The antigen test is recommended (published October 17, 2020) in New York State for use in congregate settings like nursing homes and schools. It is relatively less expensive but less sensitive than a diagnostic test. However, there are recommendations to get the diagnostic test as a confirmation of a positive result. A rapid molecular test was also developed to be used in contexts like nursing homes and schools, similar to the antigen test, and it also recommends a confirmatory diagnostic test for positive results.

New York State uses the antibody test to develop a baseline infection rate. The test developed by the state detects antibodies in the blood, specifically IgG, which is usually developed three to four weeks after infection. It is not recommended to take this test until twenty-one days after a positive diagnostic test or the symptoms of COVID-19 started. Only the diagnostic test itself detects the virus’ genetic makeup.

Updated on May 13th, 2021 with summary table on COVID-19 tests and their sources.

 
Test		 
Nucleic acid amplification (e.g. RT-PCR)		 
Antigen		 
Antibody
TypeDiagnostic, detects virus’ DNADiagnostic, detects spike protein on virus’ surfaceNon-diagnostic, detects antibodies fighting against virus
UseMost accurate for current infectionLess accurate but more affordable for current infectionCheck for prior infection (requires 1-3 weeks after infection)
ScenarioKnown exposure or symptoms; lab version to confirm less accurate test (positive, or symptomatic but negative test from antigen or non-laboratory version)Known exposure or symptoms; high-risk congregate settings; routine screeningSurveillance
Vaccinated alreadyWill not test positiveMay be exempted from routine screeningMay test positive, so cannot assess immunity
Turnaround time1-2 days15 minutes30 minutes (field) to 2 hours (lab)
Cost (government benchmark cash prices)Non-High Throughput: $51.31
High Throughput: $100 ($75 if not done within 2 calendar days)		$45.23 (e.g. ELISA)Multi-step: $42.13
Single-step: $45.23

Sources:

csh3ll

What does the COVID-19 antibody test result mean for me?

Executive summary

  • Your test results could be impacted by the time between when you were infected and get the test
  • You could get a positive result for other reasons besides COVID-19

—

I spent the beginning of my career looking at research studies for healthcare interventions, including pharmaceuticals, diagnostics and devices. However, I have never been close to requiring the information for a personal decision. The COVID-19 pandemic has made this decision close for the mass, so I decided to take a look into the types of tests that one could get in New York City. The New York City website talks about the antibody test, but it mostly focused on providing the public information on where to get it and open times. The New York State website has information on antigen, antibody and molecular testing.

I read the FDA emergency use authorization for the New York State antibody test they developed and found the following notes interesting:

  • Interpretation of results. On the fourth page, there is a table that shows what values were used to translate to categorical results of negative, indeterminate or positive.
  • Cross-reactivity. Although there is only one on page 4-5, the West Nile virus sample came up positive and three others were indeterminate with the test.
  • Clinical performance. On page six, there is a table showing the sensitivity increasing from 17.9% to 79.3% as the days from onset ranges increased (from less than 7 to greater than 20 days – there are five ranges). The results were from over six clinical studies totaling 753 subjects.
  • Limitation. There could be false results for patients taking biotin supplements.

Based on these notes, if I got the antibody test I would like to know my value before the categorical result (to know if I’m borderline) and contextualize the test to when I may have been exposed (if I got a negative result but was potentially exposed less than 20 days ago I may question the result). If I’m also positive, I would consider if I could possibly have the West Nile virus and check if I have biotin in anything I take.