Steroids can have side effects
Testing the breath and phlegm of children with severe asthma might spare them potentially damaging oral steroid drugs, say scientists.
The tests could help spot early the signs of worsening asthma, allowing it to be brought back under control.
Research by Imperial College London suggested the tests cut the use of oral steroids by nearly a third over a three-year period.
A national guideline already recommends the use of sputum tests in adults.
One in 10 children has asthma, making it the most common long-term medical condition of childhood.
Many asthmatic children have a steroid inhaler to help them control the asthma, but if this is not working, then higher doses of steroid are given in tablet or medicine form to get it back under control, and reduce the risk of serious attacks or hospitalisation.
Doctors try to avoid using these "rescue steroids" unless they have to, because if used too frequently, side effects can include growth problems.
The Imperial team's tests look for signs of increasing lung inflammation - the presence of cells called eosinophils in the sputum, and higher than usual levels of nitric oxide gas in the breath.
These signs could appear before the symptoms of asthma themselves changed.
They compared the progress of children assessed using symptoms and lung function tests - the conventional way - with a group given the sputum and breath tests.
In either case, if doctors judged that the asthma was likely to be worsening, they increased the dose of the steroid inhalers used by the children.
If this failed to improve matters, oral steroids were used.
They found that, on average, there was a 29% reduction in the number of courses of oral steroids used by the children in the sputum and breath test group.
A third of the children in that group did not need any courses of oral steroids, compared with 12.5% of those in the conventionally monitored group.
Professor Andrew Bush, who led the research, said: "It's a much more efficient way of making sure children with severe asthma get the treatment they need."
His colleague Dr Louise Fleming said that the aim was not only to reduce the number of severe attacks, but to make sure that children were not on too high a dose of inhaled steroids all the time.
Professor Chris Brightling, from the Institute of Lung Health at Leicester University, said a recent guideline had suggested these techniques be used to help control severe asthma in adults.
He said it was likely that children with severe asthma would also benefit.
"These are a measure of the control of inflammation, and this measurement can be an 'early warning sign' of an exacerbation of asthma."
Dame Helena Shovelton, from the British Lung Foundation, which funded the research, said that "targeted treatment" was "good news" for children with asthma.