I received a slew of phone calls yesterday about antibody testing and did not realize why until I returned home and found the following headlines:
“CDC warns antibody testing still too inaccurate to use for coronavirus-related policy decisions”
“Antibody tests for Covid-19 wrong up to half the time, CDC says”
“CDC: Antibody tests not to be used for decisions on returning to work”
I have been critical of both the CDC and the rushed to market antibody tests, so it is not a surprise that combining the two entities has led to confusion. But what is lost in the CDC’s messaging is that there are good antibody tests out there, and I listed more than a half dozen in yesterday’s post.
The CDC’s publication (link below) drones on and on and summarizes their thinking into to this absolute gem of a sentence: “In the current pandemic, maximizing specificity and thus positive predictive value in a serologic algorithm is preferred in most instances, since the overall prevalence of antibodies in most populations is likely low.”
Got it? Great, then read no further.
[Of course not. That’s gibberish.]
An understandable way to frame these concerns (and maximize the utility of antibody testing) is in the following adage: “right test, right time, right patient.” Follow those three steps, you won’t end up with an inaccurate result.
Right Test: I listed more than a half dozen of serviceable, accurate and available antibody tests in yesterday’s post. Essentially what you are looking for is a test that has a specificity (ability to identify a patient without disease as a true negative result) in the range of 99.5% As a comparison, HIV tests we currently use all report a specificity exceeding 99.5%, and often 100%.
Right Time: Antibody testing for COVID-19 needs to be performed at the right time. Primarily, we are looking for an IgG response as IgG is the most abundant antibody in the blood, accounting for 70 to 75% of the total. In a recent study, the median time to detection of IgG was at day 14 after the start of symptoms. The presence of antibodies was <40% among patients within 1 week! So the timing of antibody testing is critically important.
Right Patient: This is the third key point in antibody testing. For a test to be accurate, it needs to be performed in the right patient. This concept confuses me as well to be honest, because shouldn’t a test be accurate or inaccurate? But context matters in testing, especially in the detection of antibodies. This I evaluate in a case by case basis. For some individuals, there is a high likelihood of a positive test based on exposure or travel history. For others, there is less of a suspicion. But regardless, the test result needs to be matched against their experience to assess accuracy. If the test result matches the patient’s clinical experience, then its probably accurate. If there is a mismatch, either something was missed or the test was wrong.
So all is not lost (although the blaring headlines above would make you think otherwise). We have the Right Tests already. Thoughtful application of them at the Right Time and for the Right Patient is all that needs to be done.