A high number of unreliable rapid antigen tests being conducted in Delhi may be masking a mountain of Covid-19 cases at a time when a 'dangerous cocktail' of pollution, crowding during the festive season and devil-may-care attitudes is leading to record spikes in the daily incidence of the infection in the city, experts said. At the same time, the scaling up of the gold-standard RT-PCR tests and loosening the testing regimen has also significantly helped in diagnosing a large number of infections which would otherwise have gone untraced. Bucking the national trend, the national capital has registered a massive surge in Covid-19 cases over the past few days with 5,891 fresh incidences recorded on October 30, the highest single-day spike till date, even as the positivity rate shot up to nearly 10 per cent. October 30 was the third consecutive day when over 5,000 cases were reported in a day in the city, with infection tally going beyond 3.8 lakh. In contrast, the national number of positive cases has been coming down every day from a high of more than 90,000 at one time to 48,268 on October 31. The numbers tell only part of the story. The bigger issue is that authorities may not even be aware of a large number of positive cases that would have slipped through the cracks because they took the rapid antigen test (RAT), notorious for producing a large percentage of false negatives, meaning many positive patients with no or little symptoms are being declared negative, and therefore safe. The Delhi government has significantly ramped up its testing from a daily average of over 18,000 in August to nearly 56,000 in October. Of these, RT-PCR tests account for about 28 per cent of the total. On August 26, for instance, the number of RT-PCR tests was 5,912 and RAT 13,904. Two months later, on October 29, this went up to 17,029 RT-PCR and 43,095 RAT. According to health department officials, until October 22 this month, the RT-PCR-based positivity rate was 12.32 per cent and corresponding RAT-based figures stood at 3.69 per cent. The overall positivity rate combining the two methods is currently about 10 per cent. Results also depend on whether the sample has been collected properly or not. In RAT, swab is taken from throat, while in RT-PCR it is done from the nose, and in RT-PCR too, a proper jab has to made. Delhi recorded 4,473 cases on September 16, which experts call the 'second major spike' after the June 23 peak of 3,947. Delhi Health Minister Satyendar Jain has said the city may be experiencing a third wave now, but it it is too soon to declare it such until more trends are available.