How bad really was the Sars outbreak in Beijing? This is the question starting to trouble World Health Organisation officials after Beijing health authorities admitted they had no accurate record of the number of people infected by the deadly disease in the capital earlier this year. Some 2,521 cases were reported, but the Beijing Health Bureau now says that an 'unknown number' of the infections were likely not Sars, but common pneumonia. Some reports have put the number as high as 900, or 35 per cent of all cases. In response, the World Health Organisation has demanded a re-test of the blood of all Sars patients in Beijing to confirm their diagnosis. 'It's important that the retrospective blood testing be going on, especially with atypical pneumonia,' said WHO spokesman Bob Dietz. 'If we don't, we won't be able to fully come to grips with Sars.' Mr Dietz said the testing is already under way. Knowing the full extent of the Beijing outbreak will be an important step in tracing and studying the disease. If Sars was not as widespread in Beijing as once thought, it also brings into question the drastic control measures taken by city officials, which included forcing people to stay home from work and closing most public venues. By comparison, Guangdong and Hong Kong managed to control Sars without shutting down to nearly the same extent. As is now well known, the reporting of Sars on the mainland was problematic from the start. According to a Beijing Health Bureau official, the complexity of Sars was not fully recognised when it first appeared, which led to initial problems with diagnosis. The situation was further aggravated by official attempts to cover up the outbreak as doctors were told not to report infections. Later, when the full extent of the problem was admitted, Beijing expanded its definition of a Sars patient, causing the number of reported cases to rise dramatically. Officials are now suggesting that the net was cast too widely. 'It is possible that some ordinary pneumonia cases were included mistakenly, because the government and experts were urged to adopt standards that were too strict for fear of missing a single case,' the Beijing Health Bureau official said. A CCTV producer who has researched Sars extensively recently told a public forum that as many as 900 of Beijing's Sars cases were common pneumonia. Health officials agreed that there had been some misdiagnoses, but that the number was not as high as 900.