We’ve been getting so many questions from patients on the subject of antibody testing that we decided it warranted a post devoted to the subject.
Tell me about these antibody tests I keep hearing about.
There are two main types of antibody tests: qualitative and quantitative. Both are blood tests drawn after a person has recovered from illness.
A qualitative test tells us whether or not a person has antibodies for a particular virus in the blood, which is helpful for epidemiologists to monitor the spread of a disease in a population. Unfortunately, the results are too error-prone to be reliable on a case-by-case, individual basis.
A quantitative test tells us whether antibody levels are sufficient to protect a person from becoming reinfected or transmitting a virus to others. However, when it comes to COVID-19, the World Health Organization warns that “there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection.” In other words, we do not know enough about the development of immunity after a COVID-19 infection (or even if immunity develops at all) for antibody levels to be meaningful.
Wouldn’t it still be helpful for me to know whether I have antibodies to SARS-CoV-2?
Not really, no. Not until more people have had COVID-19, and we determine what quantitative antibody test result offers immunity and how long that immunity lasts. However, if you are contacted about participating in an epidemiological survey to measure the spread of SARS-CoV-2 (the qualitative test), we would recommend you do so. Research studies recruiting participants can be found here. As it is, the likelihood of getting an error from a qualitative test is too high for it to be useful to you, but it can help us study the virus.
This is why these tests are problematic right now
Epidemiologists estimate that there have been about 10,400 total infections in Oregon. In a population of 4.2 million Oregonians, this means that only a little over two out of every thousand Oregonians have had COVID-19 so far. When the actual number of people who have had the disease is so low, like it is in Oregon, your chance of getting a false positive (your test wrongly indicates you’ve had the disease, when you haven’t) can be far greater than your chance of getting an accurate test result.Lucas Waldron from ProPublica did a fantastic job of explaining this, so (with permission) we’re borrowing from this resource.
Let’s say you’re running an antibody study among 1,000 people and only 4% of the population is actually infected. Presume the test correctly identifies positives 100% of the time, meaning it is 100% “sensitive” in scientific parlance.
With a 4% infection rate, the test would accurately identify those 40 people who are positive.
But say the test is 95% “specific,” meaning that it returns false positives 5% of the time. Then among the 960 people who are truly negative, 48 people would get a false positive.
In this scenario, more people would get a false positive result than a true positive.
Based on Oregon’s 0.2% infection rate and the best test available in the state, we’d produce about 209,480 false positives if we tested everyone with an antibody test. This means that if your antibody test is positive, there is only about a 4% chance that you actually have the antibodies.
If your antibody test is negative, it’s also no guarantee. Per UCSF infectious disease specialist Chaz Langelier, MD, PhD, “early during infection, a negative antibody test does not mean someone is uninfected and […] it takes time to develop an antibody response.” There are also reports of patients who had symptoms and positive PCR tests, but subsequently only negative antibody tests. You could test negative for SARS-CoV-2, but actually be at risk of spreading the coronavirus in your family and community.
My friend’s doctor ordered an antibody test, why aren’t you?
The guidelines from the World Health Organization, Centers for Disease Control, Infectious Disease Society of America, large research centers, and the Oregon Health Authority all state that coronavirus antibody tests are not recommended for the general public. Johns Hopkins scientists are hopeful that antibody tests will be useful in the future, but caution that we’re just not there yet, stating that “while [antibody] testing has the potential to provide valuable information to individuals and to public health authorities, there are significant areas of uncertainty that will need to be addressed in the coming weeks and months.”
At the end of the day, our best advice for staying safe will not change based on the result of an antibody test. If you read all this and you still feel absolutely sure you want an antibody test, please let us know, so we can work with you. Whether an antibody test result is positive or negative, we recommend that you stay home as much as possible, wash your hands frequently, keep your hands off your face and your mask, stay 6-10 feet away from other people, and wear a mask when you go out. An antibody test result will not permit increased travel, more socialization, a change in work restrictions, or reopening the economy.