The CDC has released new guidelines on antibody testing that note it’s possible for a test that’s 90-95% accurate to deliver incorrect positive results more than half the time. That sounds shocking, but it can be true—and it’s important to understand why, if you’re considering getting tested or have already gotten a positive result.
In this post I’m talking about coronavirus antibody testing, specifically. Although the caveats we’ll discuss apply to many types of testing, they’re especially important as we try to figure out how many people have been exposed to the new coronavirus.
more likely somewhere around 5% in many locations.
general recommendations to prevent infection with SARS-CoV-2 and otherwise continue with normal activities, including work.
Briefly: if you weren’t sick, you shouldn’t assume you ever were. And if you were sick, you should still follow whatever guidance you were given before. The test changes nothing for you personally.
Antibody tests are useful for large groups of people, so that a mayor or governor or epidemiologist can have an estimate of how common infections truly are. Research on this is ongoing. They’re not useful for decision-making on an individual level. Sorry.
list of currently authorized tests that gives both their sensitivity and specificity. They even do the math for you on both positive predictive value and its counterpart, negative predictive value. Here are a few examples:
This Cellex test picks up 93.8% of positives and 96% of negatives. Under the assumption that 5% of the population has had the virus, if you get a negative test result, there’s roughly a 99.7% chance that you are truly negative. But if you get a positive result, there’s only a 55.2% chance that you actually have the antibodies.
Not all tests have such a low positive predictive value, though. Here’s another:
This is a different type of test, and it’s more accurate. So accurate, in fact, that even if only 5% of the population has had the virus, the positive predictive value is still roughly 92.9%. So your positive result is more likely to be accurate than with the test we discussed in the example, but it’s still no guarantee. (Also notice that the confidence interval is pretty wide—92.9% is an estimate within a wider range.)
Regardless of which test you take, the recommendations still apply: Even if you got one of the more accurate tests, a positive result is no license to go to a party and then cough on your grandma. For more on what antibody tests can and can’t tell you about your COVID-19 status, check out our post on exactly that.