Once I feed the cats and finish posting this post, I am going to donate blood. I am O+ without CMV which evidently makes the hematologists very happy to schedule me on a regular basis. One part of the e-mail campaign for this donation was a promise that my blood would be tested for COVID-19 antibodies and I would be notified of the results in a week.
I can understand why the blood bank wants to screen for COVID as a means of insuring a safe blood supply. However we can’t think that the numbers will tell us much about a region’s COVID exposure. This is attractive but almost useless for systemic surveillance.
Blood donors are a self-selected population. Before donating blood, a donor has to have answered over forty questions of a basic current health and long term medical history. One of the knock-out questions is if you have had the cold or flu recently. Screening blood for COVID antibodies might allow for a plausible lower boundary of regional infection history. This population is likely to notable underestimate disease prevalence.
The other challenge that we need to be aware of if any blood donor infection and recovered figures are released is the problem of false positives. If we assume that the blood donor population that is screened is likely to have a lower than population average incidence rate, than we also have to assume that a higher proportion of the reported positivity rate is false positives. If we assume that a regional cumulative COVID prevalence rate is 10% and the test returns 1% false positives and no false negatives, than the false positives will be about 8% of the total reported positives. If we assume prevalence is 5% then the false positives are 16% of the reported positives. If we assume prevalence of 2.5%, 29% of the total reported positives are false positives. The Positive Predictive Value of the tests that have Emergency Use Authorizations (EUA) from the FDA are frequently below 90% if we assume a 5% population prevalence.
Since we are assuming that blood donors are less likely than the general population to be infected, the value of the testing result being reported back to the individual donor is going to be low.
And this is problematic. If someone receives notification that they have COVID-19 antibodies, they are likely to act on that information and engage in behaviors that are more likely to increase spread risk. If they are truly immune, this is not a big deal. But if they are not immune, they are more likely to be an unsuspecting spreader during a non-symptomatic phase of a future infection.
Communicating these types of results for a low incidence but high impact disease is tough as the information is likely to change behaviors.