A few weekends ago, trying to convince my 4 year old son that pants are always necessary for afternoon hikes, my pocket was buzzing with a text that canceled everything.
“COVID19 Community Tracing Collaborative: We have information on the status of your test. We need to talk to you and we will call you back. “
After writing about the efforts to trace people exposed to Covid-19, I knew what that meant. Indeed, a few hours later, a contact tracer called to announce the bad news: I had tested positive. She told me that I had to isolate myself and she asked for the names and phone numbers of those with whom I had been in close contact so that they could be alerted.
It was disturbing of course, but also confusing. I had no symptoms, to my knowledge, I had not been near anyone sick and I always pay attention to wearing a mask, washing hands and social distancing.
I had barely left the house in weeks, in fact. As I explained to the tracker, the only interaction I have had with anyone outside of my family bubble over the past week has been meeting an old friend, but it was outside, both of us wearing masks and staying at least 6 feet apart. My son is in daycare and we are in a “bubble” with another family whose child is also attending. But his school has all sorts of precautions in place, with teachers and parents willingly testing regularly, a new air filtration system and countless cleaning and safe distance protocols. It all seemed like a shocking reminder of the devious nature of the virus.
After a few days of pacing my hotel room, however, I was less sure. At that time, my wife and my son had both received several negative results; my friend and the other family had them too, with about two dozen parents and children in daycare.
I did a second test three days after the first, and the results came back overnight: negative. At my doctor’s suggestion, I took a third, three days later, at a different location. That too came back clearly. As per the guidelines at the time of the Centers for Disease Control and Prevention, I was in quarantine for two weeks. But more and more it felt like something had gone wrong.
I started to wonder what a positive test meant. A Covid the test is not a binary thing. There is no single standard method for detecting the virus; different laboratories set their own thresholds for reporting a positive result. Some experts now believe that the sensitivity of a test and the amount of virus it detects should be factored into behavioral guidelines and public health response.
My first test was at a drive-thru site operated by the city of Cambridge, Massachusetts. A swab of my nose went from there to the Broad Institute, a biomedical research center created by Harvard and MIT that converted its genomics lab to a Covid-19 testing facility in March.
The Broad uses a technique known as polymerase chain reaction (PCR) to detect viral genetic material in a sample. A PCR test typically takes a day or more to produce a result, but it is considered the gold standard for Covid testing because it is so good at detecting tiny fragments of the virus. Rapid tests, which detect specific proteins on the surface of the virus, are cheaper and faster, but they are less accurate than PCR and work better when someone has high levels of the virus.
A Covid PCR test involves preparing a sample using chemical reagents to isolate fragments of RNA and enzymes to generate complementary strands of DNA. The laboratory then amplifies this DNA by adding compatible strands bearing fluorescent markers which detach and activate after binding. This process is repeated over and over again. If the virus is present, the chamber containing the sample should begin to glow.
The number of cycles required to trigger a result is crucial. The more virus a person carries, the fewer cycles it takes; more cycles means the patient probably has only a low level of infection. Labs usually do not disclose the number of cycles it takes to get a result – only whether or not there is one. Broad uses 40 cycles as a limit for its tests, as recommended by the CDC. If the sample does not glow for 40 cycles, the result is considered negative. But some other labs use different thresholds.