What does a positive COVID-19 test result mean?
I would like to provide context on what a positive (or negative) COVID-19 test result means.
In short, for low prevalence diseases, there are high false positive rates.
Without getting too much into the weeds, I came across this great interactive website demonstrating the biostatistics behind COVID19 testing.
The fact is, we do not know the prevalence (how many people have COVID-19 in the population at any moment). How many people have the virus changes how we interpret positive and negative test results.
The higher the prevalence, the more people who have COVID-19 at any one time, the more likely a positive test result is a true positive and not a false positive. This is called Positive Predictive Value (PPV).
Using published sensitivity and specificity from a real COVID-19 Rapid Antigen test, I calculated the PPV for 4 scenarios with different prevalence:
- Testing in Florida has consisted of those with symptoms, exposure to people with COVID-19, retesting to assure recovery, and those with concerns for COVID-19. This increases the likelihood of a positive result. The positive rate of this population is 8%. I used this as a high prevalence rate.
- The estimated COVID-19 prevalence in Miami is 6% – the highest in the state. This is a more realistic high prevalence rate.
- I also ran 3% and 1% prevalence rate scenarios as I suspect those are closer to real numbers for the state.
What I found:
- If you have a negative COVID-19 test result in any of these scenarios, you can be more than 99.99% sure you do not have COVID-19!
- If you live in Miami and have a positive COVID-19 test result, you only have a 50.3% chance of truly having COVID-19! There is a 49% False Positive rate for that estimated prevalence!
- For the state of Florida, I think the overall prevalence is 1-3%. Anyone in Florida with a positive COVID-19 test, the overall likelihood you really are infected by COVID-19 is somewhere between 13-33%!