Experts have developed a way of accurately testing lung function in young children with breathing problems.
A child with cystic fibrosis using the lung test
Very young children cannot manage the special breathing technique needed for conventional function tests.
But Great Ormond Street Hospital doctors have developed a method that can pick up problems in children under five even if they have no symptoms.
The research was unveiled at the launch of the British Lung Foundation's 'Lungs are for Life' campaign.
Respiratory disease is the most commonly reported long-term illness in children - 34% of weekly GP consultations and 15% of hospital admissions of children are due to breathing difficulties.
But for children under the age of five years with chronic respiratory problems, doctors usually have to rely on a chest examination and parental reports of symptoms to assess the nature and severity of lung disease.
With the Multiple Breath Washout technique the child breathes quietly through a face mask while watching a favourite video for about 10 to 15 minutes.
The face mask is connected to a flow meter which measures how much air goes in and out of the lungs with each breath and a gas analyser.
The mask delivers a special air mix containing a normal amount of oxygen and a tiny amount of a 'tracer' gas called sulphur hexafluoride, which has no effect on the body but which helps doctors to measure how efficiently the lungs are working.
Unlike many other lung function tests, the method is very sensitive to narrowing of the small airways, where disease often starts.
Professor Janet Stocks and colleagues at University College London, Institute of Child Health and Great Ormond St Hospital tested their new method in 40 pre-school children with cystic fibrosis and around 40 children with healthy lungs.
The test detected abnormalities in 73% of the young children with cystic fibrosis, even though many of them had no chest symptoms at the time of measurement and normal results with other tests.
Although tested in children with cystic fibrosis, the results are equally applicable to other lung diseases such as asthma and chronic lung disease in children who were born prematurely.
"It means that early lung disease can be detected before the symptoms are apparent," said Professor Stocks, who is professor of respiratory physiology at the Institute of Child Health.
"The children can then be given appropriate treatment before irreversible lung damage has occurred.
"It also means that we can undertake continuous assessments of lung growth and development from infancy, which will give us invaluable information on the progression of lung disease and the effect of different types of medication during early childhood."
Helena Shovelton, chief executive of the British Lung Foundation, said: "There is increasing evidence that much adult lung disease has its origins in early life.
"The development of better ways of detecting and treating lung disease in early childhood not only brings relief to the children who suffer from such diseases and those who care for them but could have life-long benefits."
Dr John Moore-Gillon, spokesperson for the British Thoracic Society and a consultant in respiratory medicine, said the technique was going to be "extremely useful" for specialists.
"With young children with significant lung disease it's always been difficult to take measurements of lung function.
"Relying on a child to say they're not feeling well is not a good measure. This will indicate deterioration before the child themselves is aware of it."