![]() The results suggested that the PCR tests missed 48 (36%) out of 134 infected close contacts. In one recent study, Chinese researchers attempted to contact all close contacts of individuals who had tested positive for COVID-19, but who had tested negative themselves, and invited them to perform an antibody test. However, studies performed under real-life conditions suggest false negatives may be more commonplace. When performed under laboratory conditions, the UK Government estimates that PCR tests should never show more than 5% false positives or 5% false negatives. How carefully material was collected from the nose and/or throat could also influence the likelihood of an infected person testing negative (a false negative) – with home tests likely to be less accurate than those done by professionals. A PCR test done to confirm a coronavirus infection in someone who has recently developed COVID-19-like symptoms, is more likely to detect the presence of viral RNA, than a test performed on someone who only became infected the day before, or who became infected several weeks ago. ![]() False negativesĮven though PCR tests are widely regarded as the gold-standard for detecting SARS-CoV-2, they are not perfect, and their probability of detecting an infection will vary depending on when that test is performed. Less sensitive lateral flow or rapid-antigen tests require a higher viral load to record a positive result, which is why they often only identify people during their most infectious period. In general, asymptomatic people may test positive for 1-2 weeks, while those with mild-to moderate disease often continue to test positive for a week or more after this. From this point, the amount of virus gradually declines, until it can no longer be detected by PCR. Studies have suggested that PCR tests start to detect RNA from SARS-CoV-2, roughly 1-3 days before the onset of symptoms – similar to when people start to become infectious – with the highest viral loads observed during the first few days of symptoms (assuming the person is symptomatic).
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