MUMBAI: A doctor who developed a dry cough and fever was relieved to test negative for Covid-19 at the BMC-run Kasturba Hospital recently. However, a couple of days later, he tested posi-tive at a private hospital he drove himself to, due to worsening of his symptoms.False negative reports are clearly emerging as an additional challenge amid a raging epidemic. A false negative result means a patient ’s test results are “negative” even though he harbours the virus. Such patients get sent back from hospitals or, worse, they get a false sense of relief, drop their guard, and spread the infection at home and within the community.Studies from Wuhan , where the pandemic originated, found false negative reports could be as high as 30%.Experts listed a series of reasons — the nature of the virus, the incubation period of the infection, improper collection of samples, types of kits available in the market and so on. “There needs to be an adequate amount of viruses in the body for a test to detect it. This is called limit of detection by equipment,’’ said Dr Jayanti Shastri , head of the civic-run Kasturba Hospital’s Molecular Diagnostic Laboratory.Mumbai doctors have found another “mischievous” fact about the novel coronavirus—its presence in the patient’s body waxes and wanes every day, contributing to the false negative tests. “A patient may test negative today, but will test positive a few days later,” said Shastri, adding that is the reason BMC discharges a patient only after two consecutive tests taken 72 hours apart are negative.Moreover, there are several Covid testing kits available in the market. Each kit may have different sensitivity and show different r