About 9 per cent of children are diagnosed as having asthma, which is almost twice as high as a decade or more ago. The latest survey of schoolchildren, however, suggests the true prevalence could be as high as 16 per cent.
Derek Williams, a GP from Woking in Surrey, led the research team that tested 237 children aged between 8 and 12 years from mixed socio-economic backgrounds attending a state school in Weybridge.
He found that besides the 22 known asthmatic children in the school, there were another 19 who failed a standard respiratory test for asthma which measures the ability to expel air from the lungs following a bout of intense excercise.
Six of the 22 children known to have asthma also failed the test, indicating they are not being treated 'optimally', Dr Williams added.
Although medical researchers have suspected that asthma is under-reported, with many patients going untreated as a result, the school survey suggests the problem is worse than suspected, according to Professor Ann Tattersfield, a leading asthma expert at Nottingham City Hospital.
'There is a problem sometimes of asthma being missed and untreated, but I think the problem is less now than before. I'm surprised at the number of undiagnosed cases this study appears to have identified,' she said.
Martyn Partridge, chairman of the National Asthma Campaign, said that prevalence figures on childhood asthma varied from 1 per cent in China to 26 per cent in Australia. 'So we are in that sort of range with these figures. However I think there is still underdiagnosis of asthma.'
Dr Williams used the 'free-running asthma screening test' to determine whether the children were asthmatic, when inflammation causes narrowing of the airways to the lungs which can lead to severe breathing difficulties.
Under medical supervision, the children where asked to run as fast as they could for five minutes, followed by a five-minute rest before blowing as hard as they could into a breathing monitor.
'We're very confident that we are picking up undiagnosed asthmatic children,' Dr Williams said. Interviews with them revealed that they were often wrongly treated for respiratory infections rather than asthma and some were losing up to 50 days of schooling a year as a result, he added.
Dr Williams and his colleagues argue in this month's issue of the Archives of Diseases in Childhood that the research shows the importance of using the respiratory test in schools. 'Previously undiagnosed children with asthma could be identified and the control of children with known asthma could be monitored,' they say.
Asthma is one of the fastest growing illnesses in Britain today with severe attacks killing about 2,000 people a year. A range of environmental factors are implicated, ranging from house-dust mites to exhaust emissions.
The National Asthma Campaign yesterday called on the Government to extend its Health of the Nation strategy to include asthma. It said the move would be supported by 70 per cent of MPs.