“The numbers that contribute to inconclusive results are very low. Professor Vardas says such results aren’t a frequent occurrence and they don’t contribute to a backlog in testing in any way. Either too early or too late to detect the virus properly and it is recommended in all international literature that a re-test should be done.”Īs the number of coronavirus cases continues to rise at a rapid rate, the question then lies as to how many of the test results are coming back invalid or insufficient. It doesn’t mean it’s an invalid result, what it does mean is that perhaps the timing of the test was at the wrong time. But of course-impossible to know for sure.Clinical virologist at Lancet Laboratories, Professor Eftyxia Vardas says, “Receiving an inconclusive result for a COVID-19 PCR, does not mean that there has been a laboratory error. He says my follow-up negative tests could indicate I was in the final stages of defeating the virus. “Your example is exactly why we need to be thinking about” the number of cycles to trigger a result, Mina says. It doesn’t quite solve the case, but it’s probably as close as I’m going to get. If the viral load were dropping, Lennon says, it’s also slightly more likely that one or both of the follow-up tests might miss the last traces. Each of the experts I spoke to said this indicates a low viral load, consistent with the early or, more likely, late stages of an infection. He says using the number of cycles needed to trigger a positive result to inform clinical decisions is fraught, partly because it is unclear how accurate a representation of viral load this really is.Īlthough the Broad does not tell people the number of cycles involved in their test, Lennon agreed to look up mine-35.5. To get to the bottom of my own test, I spoke to Niall Lennon, a leader of the Broad testing program. Matthew Binnicker, a professor at the Mayo Clinic who studies the diagnosis and management of viral diseases, says this means we should not test everyone, but only people who are symptomatic or in high-risk groups. The chances of a true false positive might be one in many thousand.īut across millions of tests, that means many people who are told they tested positive may in fact not have had the virus. Then there are so-called false positives, when a sample is contaminated because of human error or misread by a machine. Her data shows significant variation in the minimum amount of virus that a test can nearly always detect-from as few as 10 copies of the virus per milliliter to 10,000 copies per milliliter. Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security, has been collecting details on Covid-19 tests to help people judge their reliability. “And by the time you got the first potentially positive PCR result, you were right on the verge of becoming negative again,” he says. Mina says I may have been exposed and infected without realizing it. The way a patient’s viral load rises and falls over the course of a few days means that the virus can be missed if someone is tested too early or too late. 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 best when someone has high levels of the virus. A PCR test typically takes a day or more to produce a result, but it is considered the gold standard of Covid testing, because it’s so good at picking up minuscule fragments of the virus. The Broad uses a technique known as polymerase chain reaction (PCR) to detect viral genetic material in a sample. A swab from my nose went from there to the Broad Institute, a biomedical research center created by Harvard and MIT that converted its genomics lab into a Covid-19 testing facility in March. My initial test was at a drive-through site run by the city of Cambridge, Massachusetts. Some experts now think that the sensitivity of a test, and how much virus it detects, should be factored into behavioral guidelines and the public health response. There is no single, standard way to detect the virus different labs set their own thresholds for signaling a positive result. I began to wonder what it means to test positive.
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